{"id":83481,"date":"2026-09-28T17:34:34","date_gmt":"2026-09-28T20:34:34","guid":{"rendered":"https:\/\/a3aengenharia.com\/?post_type=articles&#038;p=83481"},"modified":"2026-09-28T17:34:34","modified_gmt":"2026-09-28T20:34:34","slug":"electrical-cable-sizing-current-capacity-voltage-drop-nbr-5410","status":"publish","type":"articles","link":"https:\/\/a3aengenharia.com\/en-us\/content\/technical-articles\/electrical-cable-sizing-current-capacity-voltage-drop-nbr-5410\/","title":{"rendered":"Electrical Cable Sizing: Current-Carrying Capacity, Voltage Drop, and NBR 5410"},"content":{"rendered":"<p class=\"wp-block-paragraph\"><strong>Electrical cable sizing<\/strong> is the process of defining conductor cross-sections so they carry the expected current, remain protected against overcurrents, comply with voltage-drop criteria, and withstand the thermal effects of a short circuit.<\/p><p class=\"wp-block-paragraph\">The direct answer is: <strong>cable size should not be selected solely from load power or a generic current table<\/strong>. The calculation must consider design current, installation method, temperature, grouping, conductor material, insulation, protective device, circuit length, and short-circuit conditions.<\/p><p class=\"wp-block-paragraph\">Therefore, two installations with the same power can require different cross-sections. A cable installed alone on a tray can have a different thermal condition from the same cable grouped with other circuits in conduit. Likewise, a long feeder can be limited by voltage drop even when its current-carrying capacity is sufficient.<\/p><h2 class=\"wp-block-heading\">What is electrical cable sizing?<\/h2><p class=\"wp-block-paragraph\">Sizing an electrical cable means determining the nominal cross-section of phase, neutral, and protective conductors from the actual circuit conditions.<\/p><p class=\"wp-block-paragraph\">The result is not merely a \u201cwire size.\u201d It is a specification that should indicate, among other aspects:<\/p><ul class=\"wp-block-list\"><li>conductor material;<\/li><li>nominal cross-section in mm\u00b2;<\/li><li>stranding class;<\/li><li>insulation and sheath type;<\/li><li>insulation voltage rating;<\/li><li>number of loaded conductors;<\/li><li>installation method;<\/li><li>reference temperature;<\/li><li>correction factors;<\/li><li>corrected current-carrying capacity;<\/li><li>calculated voltage drop;<\/li><li>associated protection;<\/li><li>short-circuit withstand capability.<\/li><\/ul><p class=\"wp-block-paragraph\">In everyday usage, expressions such as <strong>wire-size calculation<\/strong>, <strong>electrical cable calculation<\/strong>, and <strong>conductor sizing<\/strong> often refer to the same process. Technically, however, the nominal conductor cross-section and the complete installation conditions should be used.<\/p>\n<h2 class=\"wp-block-heading\">Why is load current alone not enough to select the cable?<\/h2><p class=\"wp-block-paragraph\">Load current is the starting point, not the final result. It allows the design current to be determined, but it does not yet indicate how much heat the cable can dissipate, what the voltage drop will be, or whether it can withstand the thermal energy of a fault.<\/p><p class=\"wp-block-paragraph\">The main criteria are distinct:<\/p><figure class=\"wp-block-table\"><table><tbody><tr><td>Criterion<\/td><td>What it verifies<\/td><td>Practical consequence<\/td><\/tr><tr><td>Design current<\/td><td>expected operating current<\/td><td>defines the circuit&#8217;s initial requirement<\/td><\/tr><tr><td>Current-carrying capacity<\/td><td>steady-state cable heating<\/td><td>limits permissible current according to installation<\/td><\/tr><tr><td>Voltage drop<\/td><td>voltage reduction along the circuit<\/td><td>may require a larger cross-section in long circuits<\/td><\/tr><tr><td>Overload protection<\/td><td>coordination among load, cable, and device<\/td><td>prevents the conductor from operating above its permissible condition<\/td><\/tr><tr><td>Short circuit<\/td><td>thermal effect of fault current<\/td><td>checks whether the cross-section withstands the energy until protection operates<\/td><\/tr><tr><td>Minimum cross-section<\/td><td>mechanical strength and application requirements<\/td><td>prevents inadequate cross-sections even at low current<\/td><\/tr><\/tbody><\/table><\/figure><p class=\"wp-block-paragraph\">A table that directly associates power with \u201cwire size\u201d usually ignores at least some of these criteria. It can be useful as a preliminary reference, but it does not replace sizing.<\/p><div class=\"wp-block-a3a-destaque\"><p class=\"wp-block-paragraph\"><strong>Sizing is a sequence of verifications<\/strong><\/p><p class=\"wp-block-paragraph\">Current-carrying capacity, voltage drop, protection, and short circuit must converge on a single final cross-section. To understand how these criteria fit into the complete system, see <a href=\"\/conteudo\/artigos-tecnicos\/nbr-5410\/\">NBR 5410: LV electrical installations, grounding, SPDs, and compliance<\/a>.<\/p><\/div>\n<h2 class=\"wp-block-heading\">What data is required to calculate cable cross-section?<\/h2><p class=\"wp-block-paragraph\">Before consulting current-carrying-capacity tables, the circuit data must be gathered.<\/p><h3 class=\"wp-block-heading\">Load data<\/h3><ul class=\"wp-block-list\"><li>active power, apparent power, or rated current;<\/li><li>supply voltage;<\/li><li>single-phase, two-phase, or three-phase system;<\/li><li>power factor;<\/li><li>efficiency;<\/li><li>operating duty;<\/li><li>starting current, where applicable;<\/li><li>presence of harmonics;<\/li><li>load simultaneity.<\/li><\/ul><h3 class=\"wp-block-heading\">Circuit data<\/h3><ul class=\"wp-block-list\"><li>length between source and load;<\/li><li>conductor material;<\/li><li>insulation type;<\/li><li>number of loaded conductors;<\/li><li>installation method;<\/li><li>ambient or soil temperature;<\/li><li>number of grouped circuits;<\/li><li>parallel cables;<\/li><li>available voltage-drop margin;<\/li><li>prospective short-circuit current.<\/li><\/ul><h3 class=\"wp-block-heading\">Protection data<\/h3><ul class=\"wp-block-list\"><li>rated or adjusted circuit-breaker current;<\/li><li>trip curve or trip unit;<\/li><li>interrupting capacity;<\/li><li>operating time;<\/li><li>selectivity and coordination with other devices.<\/li><\/ul><p class=\"wp-block-paragraph\">This information must be consistent with the <a href=\"\/conteudo\/artigos-tecnicos\/projetos-eletricos-baixa-tensao-etapas-normas-recomendacoes-2\/\">load schedule, single-line diagram, and calculation report<\/a>. When any of these documents changes, cable sizing must be reviewed.<\/p>\n<h2 class=\"wp-block-heading\">How is design current calculated?<\/h2><p class=\"wp-block-paragraph\">Design current, normally represented by <strong>Ib<\/strong>, is the current expected in the circuit under the conditions considered.<\/p><h3 class=\"wp-block-heading\">Single-phase or two-phase load<\/h3><p class=\"wp-block-paragraph\">For a load supplied between two active conductors, a common way to determine current is:<\/p><p class=\"wp-block-paragraph\"><code>Ib = P \/ (V \u00d7 cos \u03c6 \u00d7 \u03b7)<\/code><\/p><p class=\"wp-block-paragraph\">Where:<\/p><ul class=\"wp-block-list\"><li><strong>P<\/strong> is active power;<\/li><li><strong>V<\/strong> is the voltage applied to the load;<\/li><li><strong>cos \u03c6<\/strong> is power factor;<\/li><li><strong>\u03b7<\/strong> is efficiency.<\/li><\/ul><p class=\"wp-block-paragraph\">For purely resistive loads, power factor and efficiency may be close to 1, depending on the application.<\/p><h3 class=\"wp-block-heading\">Three-phase load<\/h3><p class=\"wp-block-paragraph\">For a balanced three-phase load:<\/p><p class=\"wp-block-paragraph\"><code>Ib = P \/ (\u221a3 \u00d7 V \u00d7 cos \u03c6 \u00d7 \u03b7)<\/code><\/p><p class=\"wp-block-paragraph\">The voltage used must correspond to line-to-line voltage when the formula is applied in this form.<\/p><h3 class=\"wp-block-heading\">Apparent power<\/h3><p class=\"wp-block-paragraph\">When the load is specified in volt-amperes or kilovolt-amperes, current can be calculated directly from apparent power without applying the power factor again.<\/p><p class=\"wp-block-paragraph\">The most common error at this stage is mixing active power, apparent power, phase voltage, line voltage, efficiency, and power factor. Before calculating, the quantities must be correctly identified.<\/p>\n<h2 class=\"wp-block-heading\">What is current-carrying capacity?<\/h2><p class=\"wp-block-paragraph\"><strong>Current-carrying capacity<\/strong> is the maximum current a conductor can carry continuously, under specified conditions, without exceeding the permissible temperature for its insulation and installation.<\/p><p class=\"wp-block-paragraph\">It is not a fixed property of cross-section. A 10 mm\u00b2 cable, for example, does not have one permissible current valid for every situation. Capacity changes according to:<\/p><ul class=\"wp-block-list\"><li>conductor material;<\/li><li>insulation;<\/li><li>maximum permissible temperature;<\/li><li>installation method;<\/li><li>number of loaded conductors;<\/li><li>ambient temperature;<\/li><li>grouping;<\/li><li>ventilation;<\/li><li>installation in soil;<\/li><li>soil thermal resistivity.<\/li><\/ul><p class=\"wp-block-paragraph\">Therefore, questions such as \u201chow many amperes can a 4 mm\u00b2 cable carry?\u201d do not have a complete technical answer without knowing the installation conditions.<\/p><p class=\"wp-block-paragraph\">The capacity obtained under the table&#8217;s reference condition can be represented by <strong>It<\/strong>. After correction factors are applied, the conductor&#8217;s effective capacity, normally represented by <strong>Iz<\/strong>, is obtained.<\/p><h2 class=\"wp-block-heading\">How does the installation method affect sizing?<\/h2><p class=\"wp-block-paragraph\">The installation method determines how heat generated by the cable is dissipated.<\/p><p class=\"wp-block-paragraph\">Cables installed in free air, on trays, on ladder racks, in embedded conduit, in trunking, inside thermally insulating walls, or directly buried have different thermal conditions.<\/p><p class=\"wp-block-paragraph\">This difference directly affects current-carrying capacity. A cable with ventilation around it tends to dissipate heat differently from a cable confined in conduit with several circuits.<\/p><p class=\"wp-block-paragraph\">The reference method should represent the actual condition or the most unfavorable condition along the section. When the same circuit passes through different environments, each section may need to be checked and the limiting criterion adopted.<\/p><p class=\"wp-block-paragraph\">It is also not enough to draw a generic route on a plan. Infrastructure must be coordinated with architecture, structure, HVAC, telecommunications, and other disciplines. Route changes during construction can alter length, grouping, temperature, and voltage drop.<\/p>\n<h2 class=\"wp-block-heading\">How are correction factors applied?<\/h2><p class=\"wp-block-paragraph\">Current-carrying-capacity tables normally assume reference conditions. When the actual installation differs, correction factors are applied.<\/p><h3 class=\"wp-block-heading\">Temperature factor<\/h3><p class=\"wp-block-paragraph\">Higher temperatures reduce heat-dissipation capability. The applicable factor depends on cable insulation and ambient or soil temperature.<\/p><h3 class=\"wp-block-heading\">Grouping factor<\/h3><p class=\"wp-block-paragraph\">When several circuits or loaded cables are installed close together, heating from one affects the others. The factor depends on the number of circuits, arrangement, and installation method.<\/p><h3 class=\"wp-block-heading\">Combined application<\/h3><p class=\"wp-block-paragraph\">When more than one factor applies, they are applied together. In simplified form:<\/p><p class=\"wp-block-paragraph\"><code>Iz = It \u00d7 k1 \u00d7 k2 \u00d7 ... \u00d7 kn<\/code><\/p><p class=\"wp-block-paragraph\">Where:<\/p><ul class=\"wp-block-list\"><li><strong>It<\/strong> is capacity under the reference condition;<\/li><li><strong>k1, k2 &#8230; kn<\/strong> are the applicable factors;<\/li><li><strong>Iz<\/strong> is corrected capacity.<\/li><\/ul><p class=\"wp-block-paragraph\">Another way to use the relationship is to determine the required tabulated capacity:<\/p><p class=\"wp-block-paragraph\"><code>Required It \u2265 Ib \/ (k1 \u00d7 k2 \u00d7 ... \u00d7 kn)<\/code><\/p><p class=\"wp-block-paragraph\">The lower the correction factors, the greater the required reference capacity and, normally, cable cross-section.<\/p>\n<h2 class=\"wp-block-heading\">How are cable and circuit breaker coordinated?<\/h2><p class=\"wp-block-paragraph\">The cable and protective device must be selected together.<\/p><p class=\"wp-block-paragraph\">For overload protection, a basic relationship is:<\/p><p class=\"wp-block-paragraph\"><code>Ib \u2264 In \u2264 Iz<\/code><\/p><p class=\"wp-block-paragraph\">Where:<\/p><ul class=\"wp-block-list\"><li><strong>Ib<\/strong> is design current;<\/li><li><strong>In<\/strong> is the rated or adjusted current of the protective device;<\/li><li><strong>Iz<\/strong> is the conductor&#8217;s corrected current-carrying capacity.<\/li><\/ul><p class=\"wp-block-paragraph\">This means the circuit breaker must allow normal load operation but cannot permit the cable to operate continuously above its permissible capacity.<\/p><p class=\"wp-block-paragraph\">The device&#8217;s conventional operating current must also be checked against conductor capacity. Selection does not end with comparison of three nominal values.<\/p><p class=\"wp-block-paragraph\">Selecting a larger circuit breaker to avoid trips without recalculating the cable can eliminate overload protection. Likewise, selecting a cable from load current and then installing an incompatible device makes the sizing inconsistent.<\/p><p class=\"wp-block-paragraph\">The article <a href=\"\/conteudo\/artigos-tecnicos\/como-dimensionar-disjuntores-baixa-tensao-corrente-curva-ka-seletividade\/\">How to size low-voltage circuit breakers<\/a> covers rated current, trip curve, interrupting capacity, and selectivity in greater depth.<\/p><div class=\"wp-block-a3a-destaque\"><p class=\"wp-block-paragraph\"><strong>Cable and protection cannot be specified separately<\/strong><\/p><p class=\"wp-block-paragraph\">The circuit breaker&#8217;s rated current, curve, interrupting capacity, and selectivity must remain compatible with the conductor. See the method in the whitepaper <a href=\"\/conteudo\/whitepapers\/whitepaper-metodo-especificacao-dimensionamento-disjuntores-baixa-tensao\/\">Circuit-Breaker Specification and Sizing Method<\/a>.<\/p><\/div>\n<h2 class=\"wp-block-heading\">How does voltage drop affect cable size?<\/h2><p class=\"wp-block-paragraph\">Every conductor has impedance. When current flows, a difference arises between voltage at the source and voltage available at the load.<\/p><p class=\"wp-block-paragraph\">Voltage drop depends on:<\/p><ul class=\"wp-block-list\"><li>current;<\/li><li>length;<\/li><li>cross-section;<\/li><li>conductor material;<\/li><li>temperature;<\/li><li>resistance;<\/li><li>reactance;<\/li><li>power factor;<\/li><li>single-phase or three-phase system.<\/li><\/ul><p class=\"wp-block-paragraph\">A cable can satisfy current-carrying capacity and still produce a voltage drop incompatible with the circuit. This occurs mainly in long feeders, high-current loads, motors, sensitive equipment, and circuits supplied by alternative sources.<\/p><p class=\"wp-block-paragraph\">Increasing cross-section reduces resistance and therefore voltage drop. However, the analysis must consider the complete route from the installation source to the equipment, not only the final section.<\/p><p class=\"wp-block-paragraph\">It is also important to distinguish steady-state voltage drop from transient variations, such as those associated with motor starting. Steady-state calculation and starting assessment have different purposes.<\/p>\n<h2 class=\"wp-block-heading\">How is short-circuit withstand verified?<\/h2><p class=\"wp-block-paragraph\">During a short circuit, the conductor may be subjected to current far above its operating current. Until the protective device interrupts the fault, this current produces intense heating.<\/p><p class=\"wp-block-paragraph\">A frequently used adiabatic relationship is:<\/p><p class=\"wp-block-paragraph\"><code>I\u00b2 \u00d7 t \u2264 k\u00b2 \u00d7 S\u00b2<\/code><\/p><p class=\"wp-block-paragraph\">Or equivalently:<\/p><p class=\"wp-block-paragraph\"><code>S \u2265 I \u00d7 \u221at \/ k<\/code><\/p><p class=\"wp-block-paragraph\">Where:<\/p><ul class=\"wp-block-list\"><li><strong>I<\/strong> is RMS short-circuit current;<\/li><li><strong>t<\/strong> is protection operating time;<\/li><li><strong>S<\/strong> is conductor cross-section;<\/li><li><strong>k<\/strong> depends on material, insulation, and the temperatures considered.<\/li><\/ul><p class=\"wp-block-paragraph\">The verification demonstrates whether the conductor thermally withstands the energy until interruption. Under some conditions, protection may limit current and let-through energy, requiring manufacturer data.<\/p><p class=\"wp-block-paragraph\">Maximum short-circuit current is relevant to withstand and <a href=\"\/conteudo\/artigos-tecnicos\/capacidade-interrupcao-disjuntores-icu-ics-curto-circuito-presumido\/\">circuit-breaker interrupting capacity<\/a>. Minimum short-circuit current must also be considered to confirm that the device operates at the most unfavorable point of the circuit.<\/p><div class=\"wp-block-a3a-destaque\"><p class=\"wp-block-paragraph\"><strong>Short-circuit current defines more than the kA rating<\/strong><\/p><p class=\"wp-block-paragraph\">In addition to circuit-breaker interrupting capacity, let-through energy, operating time, and conductor withstand must be verified. For systems with several distribution levels, see the <a href=\"\/servicos\/servicos-transversais\/estudo-curto-circuito-seletividade-coordenacao-protecoes\/\">Short-Circuit, Selectivity, and Protection Coordination Study<\/a>.<\/p><\/div>\n<h2 class=\"wp-block-heading\">Must minimum cross-section also be verified?<\/h2><p class=\"wp-block-paragraph\">Yes. Even when thermal calculations result in a small cross-section, the installation may be subject to minimum cross-sections for mechanical, construction, functional, or standards-related reasons.<\/p><p class=\"wp-block-paragraph\">Minimum cross-section depends on circuit purpose, material, installation type, and conductor function. It must be checked separately from current-carrying capacity.<\/p><p class=\"wp-block-paragraph\">Terminals, connectors, bending radius, installation stress, mechanical strength, and equipment compatibility must also be considered.<\/p><h2 class=\"wp-block-heading\">How is the neutral conductor sized?<\/h2><p class=\"wp-block-paragraph\">The neutral conductor should not be automatically reduced.<\/p><p class=\"wp-block-paragraph\">Its sizing depends on:<\/p><ul class=\"wp-block-list\"><li>phase balance;<\/li><li>presence of single-phase loads;<\/li><li>expected neutral current;<\/li><li>triplen harmonics;<\/li><li>load type;<\/li><li>possibility of unbalanced operation;<\/li><li>applicable protection and isolation.<\/li><\/ul><p class=\"wp-block-paragraph\">In installations with many switch-mode power supplies, information-technology equipment, electronic lighting, UPS systems, and other nonlinear loads, neutral current may be significant even when the phases appear balanced.<\/p><p class=\"wp-block-paragraph\">Cross-section reduction may only be adopted when technical conditions are demonstrated. In many systems, the neutral must have the same cross-section as the phases or be specifically assessed.<\/p>\n<h2 class=\"wp-block-heading\">How is the protective conductor sized?<\/h2><p class=\"wp-block-paragraph\">The protective conductor, or PE, forms part of the fault-current path and equipotential bonding of exposed conductive parts.<\/p><p class=\"wp-block-paragraph\">Its cross-section may be defined by standards-based criteria associated with the phase conductors or by adiabatic calculation, depending on the situation. The result must be compatible with:<\/p><ul class=\"wp-block-list\"><li>fault current;<\/li><li>operating time;<\/li><li>material;<\/li><li>insulation;<\/li><li>installation method;<\/li><li>mechanical protection;<\/li><li>electrical continuity.<\/li><\/ul><p class=\"wp-block-paragraph\">The PE must not be confused with the neutral. Their functions differ, although certain system configurations may use a combined conductor in part of the installation.<\/p><p class=\"wp-block-paragraph\">The design must clearly show bars, conductors, separations, and equipotential-bonding points. The <a href=\"\/solucoes\/engenharia-eletrica\/aterramento-eletrico\/\">Electrical Grounding<\/a> solution covers this integration in greater depth.<\/p><h2 class=\"wp-block-heading\">Are copper and aluminum cables sized in the same way?<\/h2><p class=\"wp-block-paragraph\">The general criteria are similar, but copper and aluminum have different properties.<\/p><p class=\"wp-block-paragraph\">Selection must consider:<\/p><ul class=\"wp-block-list\"><li>electrical conductivity;<\/li><li>available cross-sections;<\/li><li>mass;<\/li><li>terminals and connectors;<\/li><li>connection preparation;<\/li><li>thermal expansion;<\/li><li>corrosion protection;<\/li><li>compatibility between metals;<\/li><li>space and bending radius;<\/li><li>maintenance requirements.<\/li><\/ul><p class=\"wp-block-paragraph\">An aluminum conductor normally requires a different cross-section from a copper conductor for the same application. Substitution should not be based only on an approximate current value.<\/p><p class=\"wp-block-paragraph\">Terminals, busbars, and devices must be declared compatible with the material used.<\/p>\n<h2 class=\"wp-block-heading\">How does insulation affect cable capacity?<\/h2><p class=\"wp-block-paragraph\">Insulation determines, among other aspects, the maximum permissible conductor temperature in steady state and during a short circuit.<\/p><p class=\"wp-block-paragraph\">Cables with different insulation compounds may have different capacities for the same cross-section and installation method. However, insulation with a higher permissible temperature does not eliminate limitations of terminals, equipment, environment, or infrastructure.<\/p><p class=\"wp-block-paragraph\">The specification must identify the complete cable type. Stating only \u201c25 mm\u00b2 cable\u201d does not define material, insulation, voltage rating, sheath, fire behavior, or application.<\/p><h2 class=\"wp-block-heading\">How are cables sized for motors?<\/h2><p class=\"wp-block-paragraph\">Motors require additional attention because they have starting current and specific protection characteristics.<\/p><p class=\"wp-block-paragraph\">Sizing must consider:<\/p><ul class=\"wp-block-list\"><li>mechanical power;<\/li><li>efficiency;<\/li><li>power factor;<\/li><li>rated current;<\/li><li>duty;<\/li><li>starting method;<\/li><li>starting current and duration;<\/li><li>voltage drop during starting;<\/li><li>overload protection;<\/li><li>short-circuit protection;<\/li><li>feeder length.<\/li><\/ul><p class=\"wp-block-paragraph\">Starting current does not necessarily mean that the cable must be sized to carry it continuously. However, the circuit must withstand the event without excessive voltage drop, nuisance operation, or incompatible heating.<\/p><p class=\"wp-block-paragraph\">The motor circuit breaker, overload relay, contactor, and short-circuit protective device perform different functions. The article <a href=\"\/conteudo\/artigos-tecnicos\/disjuntor-motor-o-que-e-como-funciona-quando-usar\/\">Motor Circuit Breaker: What It Is, How It Works, and When to Use It<\/a> details this application.<\/p>\n<h2 class=\"wp-block-heading\">How do harmonics affect sizing?<\/h2><p class=\"wp-block-paragraph\">Nonlinear loads distort current and can produce harmonics. Possible effects include:<\/p><ul class=\"wp-block-list\"><li>increased RMS current;<\/li><li>additional conductor heating;<\/li><li>increased neutral current;<\/li><li>additional losses;<\/li><li>heating of transformers and busbars;<\/li><li>improper protection operation.<\/li><\/ul><p class=\"wp-block-paragraph\">Sizing based only on fundamental-frequency power may be insufficient. Installations with UPS systems, inverters, switch-mode power supplies, electronic lighting, data centers, and high concentrations of electronic equipment should assess the load spectrum and operating conditions.<\/p><h2 class=\"wp-block-heading\">When should parallel cables be used?<\/h2><p class=\"wp-block-paragraph\">Parallel cables can be used in high-current feeders provided current sharing is technically controlled.<\/p><p class=\"wp-block-paragraph\">For this purpose, parallel conductors should have equivalent conditions of:<\/p><ul class=\"wp-block-list\"><li>material;<\/li><li>cross-section;<\/li><li>length;<\/li><li>route;<\/li><li>impedance;<\/li><li>termination;<\/li><li>physical arrangement;<\/li><li>loading.<\/li><\/ul><p class=\"wp-block-paragraph\">Differences in route or connection can cause unequal current sharing. Phase arrangement also affects impedance and heating.<\/p><p class=\"wp-block-paragraph\">Parallel cables must be represented in the diagram, calculation report, lists, and installation details.<\/p>\n<h2 class=\"wp-block-heading\">Three-phase cable-sizing example<\/h2><p class=\"wp-block-paragraph\">As an illustrative example, consider a three-phase load with the following assumptions:<\/p><ul class=\"wp-block-list\"><li>active power: 30 kW;<\/li><li>line-to-line voltage: 380 V;<\/li><li>power factor: 0.90;<\/li><li>efficiency: 0.92;<\/li><li>length: 70 m;<\/li><li>copper conductors;<\/li><li>installation grouped with other circuits;<\/li><li>planned circuit breaker: 63 A.<\/li><\/ul><p class=\"wp-block-paragraph\">Design current is:<\/p><p class=\"wp-block-paragraph\"><code>Ib = 30,000 \/ (\u221a3 \u00d7 380 \u00d7 0.90 \u00d7 0.92)<\/code><\/p><p class=\"wp-block-paragraph\">The result is approximately:<\/p><p class=\"wp-block-paragraph\"><code>Ib \u2248 55 A<\/code><\/p><p class=\"wp-block-paragraph\">Assume that the first cross-section analyzed has a reference capacity of 76 A. For the installation condition, consider illustrative factors of 0.91 for temperature and 0.80 for grouping:<\/p><p class=\"wp-block-paragraph\"><code>Iz = 76 \u00d7 0.91 \u00d7 0.80<\/code><\/p><p class=\"wp-block-paragraph\"><code>Iz \u2248 55.3 A<\/code><\/p><p class=\"wp-block-paragraph\">Although the corrected value is slightly higher than design current, it is not compatible with a 63 A circuit breaker because the relationship <code>In \u2264 Iz<\/code> would not be satisfied.<\/p><p class=\"wp-block-paragraph\">The next cross-section, with a hypothetical reference capacity of 96 A, would result in:<\/p><p class=\"wp-block-paragraph\"><code>Iz = 96 \u00d7 0.91 \u00d7 0.80<\/code><\/p><p class=\"wp-block-paragraph\"><code>Iz \u2248 69.9 A<\/code><\/p><p class=\"wp-block-paragraph\">This alternative permits the preliminary coordination <code>55 A \u2264 63 A \u2264 69.9 A<\/code>.<\/p><p class=\"wp-block-paragraph\">The process is not yet complete. The following must still be checked:<\/p><ol class=\"wp-block-list\"><li>voltage drop over 70 m;<\/li><li>thermal short-circuit withstand;<\/li><li>applicable minimum cross-section;<\/li><li>terminal compatibility;<\/li><li>neutral conductor, if present;<\/li><li>protective conductor;<\/li><li>load starting, if it is a motor;<\/li><li>selectivity and interrupting capacity.<\/li><\/ol><p class=\"wp-block-paragraph\">The capacity values used in this example are deliberately illustrative. Final nominal cross-section must be obtained from the applicable tables and technical data for the selected cable according to the actual installation method.<\/p><div class=\"wp-block-a3a-destaque\"><p class=\"wp-block-paragraph\"><strong>The result depends on circuit assumptions<\/strong><\/p><p class=\"wp-block-paragraph\">The example demonstrates the method but does not replace the tables and data for the selected cable. Installation method, temperature, grouping, and protection must represent actual design conditions.<\/p><\/div>\n<h2 class=\"wp-block-heading\">Common cable-sizing errors<\/h2><p class=\"wp-block-paragraph\">Common errors include:<\/p><ul class=\"wp-block-list\"><li>selecting cross-section solely from power;<\/li><li>using a universal wire-size table;<\/li><li>confusing diameter with cross-section in mm\u00b2;<\/li><li>ignoring power factor and efficiency;<\/li><li>using the wrong voltage in the formula;<\/li><li>disregarding installation method;<\/li><li>not applying temperature and grouping factors;<\/li><li>selecting the circuit breaker before checking the cable;<\/li><li>increasing circuit-breaker rating to avoid trips;<\/li><li>not checking voltage drop;<\/li><li>ignoring starting current;<\/li><li>automatically reducing the neutral;<\/li><li>disregarding harmonics;<\/li><li>not checking minimum and maximum short-circuit current;<\/li><li>omitting PE cross-section;<\/li><li>replacing copper with aluminum without recalculation;<\/li><li>not updating diagrams and calculation reports after changes.<\/li><\/ul><p class=\"wp-block-paragraph\">These problems can cause heating, losses, trips, starting failures, inadequate equipment voltage, and incomplete protection.<\/p>\n<h2 class=\"wp-block-heading\">How should cable sizing be documented in the electrical design?<\/h2><p class=\"wp-block-paragraph\">The calculation report should make the assumptions traceable and the result reproducible.<\/p><p class=\"wp-block-paragraph\">For each relevant circuit, it is advisable to record:<\/p><ul class=\"wp-block-list\"><li>source and destination;<\/li><li>identification on the single-line diagram;<\/li><li>load and operating duty;<\/li><li>design current;<\/li><li>installation method;<\/li><li>number of loaded conductors;<\/li><li>reference capacity;<\/li><li>correction factors;<\/li><li>corrected capacity;<\/li><li>associated circuit breaker;<\/li><li>voltage drop;<\/li><li>short-circuit current;<\/li><li>thermal verification;<\/li><li>phase, neutral, and PE cross-sections;<\/li><li>installation notes.<\/li><\/ul><p class=\"wp-block-paragraph\">The load schedule, diagram, plans, and calculation report must present the same data. Changing cross-section in only one document creates inconsistencies for estimating, construction, and maintenance.<\/p><p class=\"wp-block-paragraph\">The <a href=\"\/servicos\/planejamento\/projeto-eletrico-baixa-tensao\/\">Low-Voltage Electrical Design<\/a> service integrates surveys, loads, calculations, protection, diagrams, schedules, and specifications.<\/p><div class=\"wp-block-a3a-destaque\"><p class=\"wp-block-paragraph\"><strong>Turn the calculation into executable documentation<\/strong><\/p><p class=\"wp-block-paragraph\">A3A Engenharia develops low-voltage electrical designs with calculation reports, diagrams, load schedules, specifications, and verification criteria. Learn about the <a href=\"\/servicos\/planejamento\/projeto-eletrico-baixa-tensao\/\">Low-Voltage Electrical Design<\/a> service.<\/p><\/div>\n<h2 class=\"wp-block-heading\">When should specialized engineering be engaged?<\/h2><p class=\"wp-block-paragraph\">Specialized analysis is recommended when the installation has:<\/p><ul class=\"wp-block-list\"><li>LVMSB and several distribution levels;<\/li><li>an on-site transformer;<\/li><li>generators or UPS systems;<\/li><li>long feeders;<\/li><li>motors and drives;<\/li><li>critical loads;<\/li><li>parallel cables;<\/li><li>aluminum conductors;<\/li><li>high concentration of nonlinear loads;<\/li><li>load expansion;<\/li><li>recurring trips or overheating;<\/li><li>lack of reliable diagrams;<\/li><li>need for short-circuit and selectivity studies.<\/li><\/ul><p class=\"wp-block-paragraph\">Under these conditions, sizing one circuit cannot be separated from system architecture. Operating modes, alternative sources, short-circuit levels, selectivity, grounding, and service continuity must be analyzed together.<\/p><p class=\"wp-block-paragraph\">The <a href=\"\/solucoes\/engenharia-eletrica\/infraestrutura-eletrica-de-baixa-tensao\/\">Low-Voltage Electrical Installations<\/a> solution organizes this integrated approach from diagnosis through design, upgrading, inspection, and commissioning.<\/p><div class=\"wp-block-a3a-destaque\"><p class=\"wp-block-paragraph\"><strong>Cable sizing should be treated as part of the electrical system<\/strong><\/p><p class=\"wp-block-paragraph\">Feeders, switchboards, sources, protection, grounding, and operating modes must be coordinated. See the <a href=\"\/solucoes\/engenharia-eletrica\/infraestrutura-eletrica-de-baixa-tensao\/\">Low-Voltage Electrical Installations<\/a> solution.<\/p><\/div>\n<h2 class=\"wp-block-heading\">Conclusion<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">Electrical cable sizing starts with design current but does not end there. The cross-section must simultaneously satisfy current-carrying capacity, overload protection, voltage drop, short-circuit withstand, and the application&#8217;s minimum requirements.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Installation method, temperature, grouping, material, insulation, length, and load behavior can change the result. Therefore, the same power does not automatically lead to the same cable size.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Technically documented sizing connects the load, cable, circuit breaker, infrastructure, and operating conditions. This consistency makes it possible to construct, verify, and maintain the installation safely and traceably.<\/p>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary>Technical references<\/summary>\n<p class=\"wp-block-paragraph\">[1] ASSOCIA\u00c7\u00c3O BRASILEIRA DE NORMAS T\u00c9CNICAS. <a href=\"https:\/\/www.abntcatalogo.com.br\/pnm.aspx\" target=\"_blank\" rel=\"noopener noreferrer\">ABNT NBR 5410:2004 \u2014 Low-voltage electrical installations<\/a>. Rio de Janeiro: ABNT, 2004.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">[2] INTERNATIONAL ELECTROTECHNICAL COMMISSION. <a href=\"https:\/\/webstore.iec.ch\/publication\/1878\" target=\"_blank\" rel=\"noopener noreferrer\">IEC 60364-5-52 \u2014 Low-voltage electrical installations \u2014 Part 5-52: Selection and erection of electrical equipment \u2014 Wiring systems<\/a>.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">[3] INTERNATIONAL ELECTROTECHNICAL COMMISSION. IEC 60949 \u2014 Calculation of thermally permissible short-circuit currents, taking into account non-adiabatic heating effects.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">[4] SCHNEIDER ELECTRIC. <a href=\"https:\/\/www.electrical-installation.org\/enwiki\/Sizing_and_protection_of_conductors\" target=\"_blank\" rel=\"noopener noreferrer\">Electrical Installation Guide \u2014 Sizing and protection of conductors<\/a>. Technical guide.<\/p>\n<\/details>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary>Frequently asked questions<\/summary>\n<div class=\"schema-faq wp-block-yoast-faq-block\"><div class=\"schema-faq-section\" id=\"faq-question-como-calcular-a-bitola-de-um-cabo-el-trico-508c7630\"><strong class=\"schema-faq-question\">How do you calculate electrical cable size?<\/strong> <p class=\"schema-faq-answer\">First determine design current. Then verify installation method, current-carrying capacity, correction factors, protection coordination, voltage drop, short circuit, and minimum cross-section.<\/p><\/div><div class=\"schema-faq-section\" id=\"faq-question-o-que-capacidade-de-condu-o-de-corrente-4c5b3abf\"><strong class=\"schema-faq-question\">What is current-carrying capacity?<\/strong> <p class=\"schema-faq-answer\">It is the maximum current the conductor can carry continuously under specified conditions without exceeding the permissible temperature.<\/p><\/div><div class=\"schema-faq-section\" id=\"faq-question-a-pot-ncia-da-carga-suficiente-para-escolher-o-c-28b98099\"><strong class=\"schema-faq-question\">Is load power sufficient to select the cable?<\/strong> <p class=\"schema-faq-answer\">No. Power allows current to be calculated, but sizing also depends on installation, temperature, grouping, length, protection, and short circuit.<\/p><\/div><div class=\"schema-faq-section\" id=\"faq-question-quantos-amp-res-suporta-um-cabo-de-4-mm-13d365fe\"><strong class=\"schema-faq-question\">How many amperes can a 4 mm\u00b2 cable carry?<\/strong> <p class=\"schema-faq-answer\">There is no single value valid for every installation. Capacity depends on material, insulation, installation method, temperature, grouping, and number of loaded conductors.<\/p><\/div><div class=\"schema-faq-section\" id=\"faq-question-qual-a-diferen-a-entre-bitola-e-se-o-do-cabo-80e11f4e\"><strong class=\"schema-faq-question\">What is the difference between wire size and cable cross-section?<\/strong> <p class=\"schema-faq-answer\">Wire size is a common term. Technically, the conductor&#8217;s nominal cross-section, expressed in square millimeters, is used.<\/p><\/div><div class=\"schema-faq-section\" id=\"faq-question-como-o-agrupamento-altera-a-bitola-do-cabo-6a5d28cc\"><strong class=\"schema-faq-question\">How does grouping affect cable size?<\/strong> <p class=\"schema-faq-answer\">Grouped circuits heat one another, reducing current-carrying capacity. A correction factor may require a larger cross-section.<\/p><\/div><div class=\"schema-faq-section\" id=\"faq-question-como-a-temperatura-afeta-a-capacidade-do-cabo-3de763f3\"><strong class=\"schema-faq-question\">How does temperature affect cable capacity?<\/strong> <p class=\"schema-faq-answer\">Higher ambient temperatures reduce heat dissipation and normally decrease the conductor&#8217;s permissible current.<\/p><\/div><div class=\"schema-faq-section\" id=\"faq-question-o-disjuntor-deve-ser-escolhido-antes-do-cabo-066951fd\"><strong class=\"schema-faq-question\">Should the circuit breaker be selected before the cable?<\/strong> <p class=\"schema-faq-answer\">Cable and circuit breaker must be coordinated. Design current, corrected cable capacity, and device current must satisfy overload-protection relationships.<\/p><\/div><div class=\"schema-faq-section\" id=\"faq-question-o-cabo-pode-atender-corrente-e-reprovar-por-qued-40da83f0\"><strong class=\"schema-faq-question\">Can a cable satisfy current capacity but fail the voltage-drop criterion?<\/strong> <p class=\"schema-faq-answer\">Yes. In long circuits, the cross-section may carry current safely from a thermal standpoint and still produce excessive voltage drop.<\/p><\/div><div class=\"schema-faq-section\" id=\"faq-question-como-o-curto-circuito-influencia-a-se-o-do-cabo-7be04c6f\"><strong class=\"schema-faq-question\">How does short circuit affect cable cross-section?<\/strong> <p class=\"schema-faq-answer\">The cross-section must withstand the thermal energy produced by fault current until the protective device operates.<\/p><\/div><div class=\"schema-faq-section\" id=\"faq-question-o-neutro-pode-ter-se-o-menor-que-as-fases-457b5316\"><strong class=\"schema-faq-question\">Can the neutral have a smaller cross-section than the phase conductors?<\/strong> <p class=\"schema-faq-answer\">Only when technical conditions permit. Unbalance, single-phase loads, and harmonics may require a neutral of equal or specifically calculated cross-section.<\/p><\/div><div class=\"schema-faq-section\" id=\"faq-question-como-dimensionar-o-condutor-de-prote-o-5a80911e\"><strong class=\"schema-faq-question\">How is the protective conductor sized?<\/strong> <p class=\"schema-faq-answer\">PE cross-section may be defined by standards-based criteria related to the phase conductors or by thermal calculation, considering fault current and operating time.<\/p><\/div><div class=\"schema-faq-section\" id=\"faq-question-cabos-de-cobre-e-alum-nio-usam-a-mesma-se-o-e598ca8e\"><strong class=\"schema-faq-question\">Do copper and aluminum cables use the same cross-section?<\/strong> <p class=\"schema-faq-answer\">Not necessarily. The materials have different conductivity and connection characteristics, requiring recalculation and compatible components.<\/p><\/div><div class=\"schema-faq-section\" id=\"faq-question-como-dimensionar-cabos-para-motores-d31bdb36\"><strong class=\"schema-faq-question\">How are cables sized for motors?<\/strong> <p class=\"schema-faq-answer\">Rated current, efficiency, power factor, duty, starting, voltage drop, length, and protection coordination must be considered.<\/p><\/div><div class=\"schema-faq-section\" id=\"faq-question-poss-vel-usar-uma-tabela-universal-para-dimensio-0850db9a\"><strong class=\"schema-faq-question\">Can a universal table be used to size cables?<\/strong> <p class=\"schema-faq-answer\">Not in a technically complete way. Tables are valid only for specific conditions and do not replace installation, protection, voltage, and short-circuit verifications.<\/p><\/div><div class=\"schema-faq-section\" id=\"faq-question-quem-deve-dimensionar-cabos-el-tricos-a86ea6a9\"><strong class=\"schema-faq-question\">Who should size electrical cables?<\/strong> <p class=\"schema-faq-answer\">Installation sizing should be performed and documented by a qualified professional according to the applicable complexity and technical responsibilities.<\/p><\/div><\/div>\n<\/details>\n\n\n\n<details class=\"wp-block-details is-layout-flow wp-block-details-is-layout-flow\"><summary>Complementary technical materials<\/summary>\n\n\n\n<h2 class=\"wp-block-heading\">Related solutions<\/h2>\n\n\n\n<ul class=\"wp-block-list\"><li><a href=\"\/solucoes\/engenharia-eletrica\/infraestrutura-eletrica-de-baixa-tensao\/\">Low-Voltage Electrical Installations<\/a><\/li><li><a 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circuit.<\/p>\n","protected":false},"author":1,"featured_media":78678,"parent":0,"template":"","meta":{"_a3a_global_related_solutions":[],"_a3a_global_related_services":[],"_a3a_global_related_materials":[],"_a3a_post_lang":"en-us","_a3a_translation_group_id":"2672a5b7-4636-4625-93d9-e2c2853f883b","_a3a_i18n_canonical_slug":"electrical-cable-sizing-current-capacity-voltage-drop-nbr-5410","_a3a_prod_post_id":"","_a3a_lang_url_en-us":"","_a3a_lang_url_es-es":""},"categories":[],"segments":[],"mercados":[],"etapas":[],"class_list":["post-83481","articles","type-articles","status-publish","has-post-thumbnail","hentry"],"_links":{"self":[{"href":"https:\/\/a3aengenharia.com\/en-us\/wp-json\/wp\/v2\/articles\/83481","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/a3aengenharia.com\/en-us\/wp-json\/wp\/v2\/articles"}],"about":[{"href":"https:\/\/a3aengenharia.com\/en-us\/wp-json\/wp\/v2\/types\/articles"}],"author":[{"embeddable":true,"href":"https:\/\/a3aengenharia.com\/en-us\/wp-json\/wp\/v2\/users\/1"}],"version-history":[{"count":1,"href":"https:\/\/a3aengenharia.com\/en-us\/wp-json\/wp\/v2\/articles\/83481\/revisions"}],"predecessor-version":[{"id":83482,"href":"https:\/\/a3aengenharia.com\/en-us\/wp-json\/wp\/v2\/articles\/83481\/revisions\/83482"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/a3aengenharia.com\/en-us\/wp-json\/wp\/v2\/media\/78678"}],"wp:attachment":[{"href":"https:\/\/a3aengenharia.com\/en-us\/wp-json\/wp\/v2\/media?parent=83481"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/a3aengenharia.com\/en-us\/wp-json\/wp\/v2\/categories?post=83481"},{"taxonomy":"segments","embeddable":true,"href":"https:\/\/a3aengenharia.com\/en-us\/wp-json\/wp\/v2\/segments?post=83481"},{"taxonomy":"mercados","embeddable":true,"href":"https:\/\/a3aengenharia.com\/en-us\/wp-json\/wp\/v2\/mercados?post=83481"},{"taxonomy":"etapas","embeddable":true,"href":"https:\/\/a3aengenharia.com\/en-us\/wp-json\/wp\/v2\/etapas?post=83481"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}