How to size an access control power supply: current, power, locks, PoE, battery, backup runtime, supervision, protection, and testing.
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An access control power supply should not be sized solely from the lock’s nominal voltage or from a simplified sum of catalog power consumption. In a professional system, the power architecture must support controllers, readers, locks, relays, sensors, interfaces, and peripherals across their different operating states; account for inrush or peak current where applicable; voltage drop in circuits; required backup runtime; battery recharging; fault supervision; and system behavior during a mains power failure.
The same logic applies when selecting a power supply for a lock. The lock is only one of the loads at an access point. The design must assess whether it is fail-safe or fail-secure, how it behaves when energized and de-energized, the current it requires in steady state and during transitions, the distance to the power supply, and what must remain operational when primary power is lost. Power supplies, batteries, and backup runtime are therefore part of the access control architecture, not accessories selected at the end of installation.
Power should be treated as an access control subsystem
An access point combines loads with different characteristics. The controller processes logic and communications; readers and OSDP devices depend on stable power; locks may require significantly different currents depending on their technology; sensors and inputs draw little current but must remain available so the platform can know the actual state of the door.
When everything is connected to a single power supply without proper segregation, supervision, or reserve capacity, a simple electrical failure can take several doors out of service at once. The design should therefore define a power architecture by controller, group of doors, or zone, considering criticality and the impact of failure.
| Load | What to verify | Common design error |
| Controller | voltage, power, PoE/DC, maximum consumption | considering only typical consumption |
| Reader | voltage, current, quantity per door | ignoring the sum of peripheral loads |
| Lock | current at rest, during actuation, and in safe state | selecting the power supply only by voltage |
| Relays and interfaces | coil and output current | powering everything from the controller |
| Sensors | voltage and supervision | losing monitoring during a failure |
| Battery | capacity, charging, temperature, service life | calculating runtime only from nominal capacity |
Power and current must be evaluated by operating scenario
The power architecture should be based on the load schedule and operating scenarios. Power supply, battery, backup runtime, and protection must be defined before installation.
The starting point is to identify all loads and their possible states. Total current should not be calculated only by summing average values: sizing must consider the most demanding simultaneous condition that can plausibly occur in the system.
A door with an electromagnetic lock, for example, may draw current continuously while remaining secure, whereas an electric strike or motorized mechanism may have a different consumption profile during actuation. Controllers and readers add permanent load. Auxiliary modules, fire-system interfaces, converters, and relays must also be included in the load calculation.
In simplified terms, the electrical power of each DC load can be expressed as P = V × I. However, selecting the power supply also requires operating margin and verification of manufacturer characteristics. A power supply should not be sized to operate continuously at its nominal limit.
Peak current and simultaneity affect sizing
Some devices draw more current during actuation, startup, or movement. If several doors are commanded simultaneously, the power supply must support the expected combination without excessive voltage drop or controller resets.
Engineering should define simultaneity scenarios: normal operation, planned collective release, power restoration, restart, actuation of multiple barriers, and other relevant conditions. In systems with many doors, distributing power can reduce the impact of peaks and limit fault propagation.
Voltage drop can compromise a correctly sized power supply
A power supply capable of delivering the total current can still fail in the field if the voltage reaching the device falls below its permitted range. Circuit length, conductor cross-section, current, and connections determine voltage drop.
This is particularly relevant for locks, electromagnetic locks, and low-voltage devices. Long circuits and high currents increase voltage drop. Instead of compensating by arbitrarily raising the output voltage, the design should size conductors, topology, and power-supply locations according to the load and the equipment’s allowable voltage range.
The calculation record should verify the highest-current condition and account for connections, protection, and distribution. The criterion must be compatible with the limits specified by equipment manufacturers.
PoE and local power are not equivalent in every architecture
Network door controllers may accept PoE, dedicated DC power, or both. This provides architectural alternatives, but it does not mean that the entire door can necessarily be powered from the switch.
The PoE power budget must account for the controller and the loads it can actually support at its outputs. Higher-power locks or multiple peripherals may require a local power supply. The design must also verify behavior when the switch, telecommunications UPS, or data network loses power.
The decision should compare availability, distance, fault segregation, maintenance, backup capability, and door requirements. In critical applications, supervised local power may reduce dependencies; in other cases, centralized PoE protected by a UPS may simplify the infrastructure.
Backup batteries must meet the required runtime, not a generic value
Battery capacity must be derived from a backup runtime requirement. The design must establish how long the access control system should remain operational after loss of primary power and which loads must remain energized during that period.
A basic estimate starts with the total current during backup mode multiplied by the required time. In practice, however, nominal battery capacity should not be treated as fully usable. Temperature, aging, discharge rate, efficiency, allowable end voltage, and design margin affect effective capacity.
For this reason, a simplified relationship such as Ah ≈ I × t is only a conceptual starting point. Final sizing should apply the factors recommended by the battery/charger manufacturer and the adopted engineering criterion. It is also necessary to verify that the charger can restore the battery within the required time without exceeding its limitations.
Backup runtime must account for lock behavior
The backup requirement varies significantly with barrier technology. A lock that requires continuous power to remain in its secure state creates a different demand from a lock that mainly consumes power while changing state.
In addition, total power loss may change the physical state of the door according to the fail-safe or fail-secure principle. This should not be confused with runtime. The battery keeps energy available; fail-safe/fail-secure defines what happens when that energy is no longer available.
The design should show the complete sequence: normal mains available, mains unavailable with battery available, low-battery condition, and total loss of power. At each stage, it should be clear which functions remain available and what physical state is expected at the door.
The power supply should provide supervision appropriate to criticality
In a higher-level system, the power supply is not an “invisible box.” Mains failure, low battery, missing battery, charger failure, or an open protection device may need to be reported to the security platform.
Supervision allows the team to act before a fault becomes door downtime. In cabinets with multiple outputs, it is also desirable to identify faults by circuit or load group when the architecture supports it.
The design should define which states will be monitored, how they will be presented to the operator, which events trigger maintenance, and how the information will be recorded. A battery installed without diagnostics can remain degraded for months and only be discovered during a power outage.
Electrical protection must limit fault propagation
A short circuit or peripheral failure should not, whenever the architecture allows, take the entire group of doors offline. Fuses, protected outputs, circuit-based distribution, and protection coordination help limit the impact.
Controller and power-supply documentation must be followed. Some controllers require a listed power supply for a specific application, defined power limits, and external protection on backup inputs. The specification should incorporate these requirements without assuming that any power supply with the same voltage is equivalent.
Power supply, battery, and fire systems must be coordinated
When power, fire protection, network infrastructure, and access control are treated separately, an unengineered interface can compromise door response.
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When the fire system affects controlled doors, the power architecture becomes part of the functional behavior. Depending on the architecture, the fire signal may act on the controller, a relay, the lock power supply, or a combination of these interfaces.
The design must ensure that the intended emergency condition is reached even if part of the access control infrastructure is unavailable. The command path, priority, and effect of backup power must be analyzed together with door logic and life-safety requirements.
Centralizing or distributing power supplies is an architectural decision
Centralized power supplies simplify battery concentration, supervision, and maintenance, but can increase circuit lengths and amplify the impact of a common failure. Distributed power supplies reduce distances and can isolate faults, but increase the number of maintenance points.
There is no single answer. The design should consider the number of doors, building geometry, electrical availability, redundancy, supervision, maintenance, and criticality. In large facilities, a hybrid solution by floor or zone is often more coherent than a single power supply for the entire system.
How to specify an access control power supply without locking in a manufacturer
A performance-based specification should define verifiable requirements such as:
- output voltage and range compatible with the loads;
- required continuous capacity and peak condition;
- number and protection of outputs;
- battery charging and management capability;
- minimum system backup runtime by scenario;
- mains, battery, and fault supervision when required;
- short-circuit and overload protection;
- enclosure and environmental conditions compatible with the installation;
- interfaces required by the fire or supervision system;
- capacity documentation and calculation record;
- evidence of performance during FAT and SAT.
The objective is not to copy the power rating of a reference power supply. Capacity should be derived from the actual loads and operating scenarios.
FAT, SAT, and commissioning should test the power system
Testing only credentials and doors does not demonstrate the reliability of the power system. FAT can validate power-supply capacity, supervision, and alarm logic. SAT should verify voltage at the points of use, load behavior, voltage drop, transition to backup, fault indication, and recovery.
When contracted backup runtime is relevant, the test plan should define how it will be demonstrated without unnecessarily compromising battery service life. The result must generate evidence: measured values, power-supply status, recorded events, and door behavior.
When power must be addressed in the Access Control System Design
Whenever the system depends on electromechanical or electronic devices to control a barrier, power must be part of the design. In small systems this may result in a simple architecture; in corporate, industrial, data center, or campus installations, it often requires a power diagram, load schedule, runtime criteria, protection, and supervision by zone.
The Access Control System Design should coordinate power, network, locks, controllers, doors, fire-system interfaces, and testing. This integrated view prevents the common situation in which the software logic is correct but the system fails because the electrical infrastructure was sized only during installation.
Final considerations
An access control power supply should be selected from the system architecture and loads, not only from the voltage printed on the lock. Current, peaks, voltage drop, PoE, battery, backup runtime, supervision, protection, fault behavior, and fire-system integration form a single engineering problem.
When these requirements are defined in the design and verified during commissioning, power stops being a hidden point of weakness and becomes part of the system’s measurable reliability.
Commissioning must demonstrate transition to backup, supervision, voltages, and door behavior — not merely that the power supply is energized.
Technical references
[1] AXIS COMMUNICATIONS. AXIS A1601 Network Door Controller — DC power and backup battery input. Available at: https://help.axis.com/pt-br/axis-a1601
[2] AXIS COMMUNICATIONS. Network door controllers — solutions with integrated power, battery charging, and battery backup. Available at: https://www.axis.com/pt-br/products/network-door-controllers
[3] AXIS COMMUNICATIONS. AXIS TA1203 Enclosure with Power Unit — 250 W power system and battery management. Available at: https://www.axis.com/pt-br/products/axis-ta1203-enclosure-with-power-unit
[4] UL SOLUTIONS. Access Control System Testing and Certification — UL 294. Available at: https://www.ul.com/services/access-control-system-testing-and-certification
[5] ALTRONIX. AL1024ULX Power Supply/Charger — power, battery charging, and transfer to backup. Available at: https://www.altronix.com/products/AL1024ULX
Frequently asked questions
All loads must be identified, considering maximum current and peaks, simultaneity, voltage drop, operating margin, battery backup runtime, supervision, and fault behavior. Simply adding nominal currents is not sufficient.
It can in some architectures, provided the power supply and outputs have the capacity, protection, and compatibility required by all loads. In other designs, separating controller and lock power improves availability and limits faults.
The basic relationship starts from current during backup multiplied by time, but final sizing must account for effective capacity, aging, temperature, efficiency, discharge rate, and charger requirements.
Not necessarily. The controller may be powered by PoE, but locks and peripherals must fit within the available power budget. The architecture must also verify switch backup and fault behavior.
Yes. SAT should verify voltages, transition to backup, fault alarms, door behavior, and recovery, while providing evidence of the runtime and supervision requirements defined in the design.
