Access-control failures are often blamed on software, credentials or readers when the underlying problem is electrical. A lock that receives adequate voltage at the power supply can still behave unpredictably at the door because cable resistance, startup current and shared loads reduce the voltage available under real operating conditions.
Reliable design begins with the lock manufacturer’s requirements, not a generic current allowance. The calculation must account for continuous load, momentary inrush, cable length, conductor size, relay and connector losses, ambient conditions and the behavior expected during a mains-power failure.
Start with the Complete Door Load
List every powered device on each circuit: magnetic locks, electric strikes, electrified panic hardware, request-to-exit devices, readers, sounders, door controllers and auxiliary relays. Record both nominal and peak current where the manufacturer provides them. Some locking devices draw a substantially higher current while moving or energizing than they require once settled.
A supply selected only from steady-state totals may reset, current-limit or produce an incomplete unlock during simultaneous demand. Designers should apply the manufacturer’s sizing method and retain reasonable engineering margin rather than treating the nameplate capacity as continuously available under every condition.
Voltage Drop Is a Circuit Calculation
The relevant distance is the complete current path, including the outbound and return conductors. Longer runs, smaller conductors and higher current increase voltage drop. The correct test point is therefore the device terminals while the lock is operating, not the unloaded supply output.
Moving from 12 VDC to 24 VDC can reduce current for an equivalent power requirement and make long runs easier to manage, but only when every connected device is approved for that voltage. Mixed-voltage peripherals require deliberate separation or regulated conversion; improvised sharing creates difficult faults and may invalidate a listing.
Design for Inrush and Simultaneous Events
Emergency release, scheduled unlocking and a fire-alarm interface can command many doors at once. The worst case is not necessarily one door opening during normal use. Commissioning should test the largest credible simultaneous event while monitoring supply voltage and fault outputs.
Distributed power can reduce long cable runs and localize faults, but it adds batteries, enclosures and inspection points. Centralized power simplifies maintenance oversight but may require larger conductors and careful circuit segregation. The right architecture depends on building size, survivability goals and maintenance capability.
Fail-Safe and Fail-Secure Are Operational Choices
Power loss behavior must follow the approved life-safety and security design. Fail-safe hardware unlocks when power is removed; fail-secure hardware remains mechanically secure while normally allowing free egress from the protected side. The correct choice is governed by door function, code requirements, fire strategy and the authority having jurisdiction.
A fire-alarm release circuit should be documented, supervised where required and tested end to end. It is not enough to confirm that an input changes state at the access controller; the test must verify the intended mechanical result at every affected opening.
Battery Backup Needs a Defined Mission
Backup duration should come from a stated operating objective: maintain normal access, keep selected secure doors powered, preserve controller communications, or support an orderly transition. Calculate battery capacity from the actual standby and alarm loads, then account for aging, temperature and allowable discharge according to the approved equipment instructions.
Battery supervision and periodic load testing are as important as initial capacity. A system can report healthy mains power for years while degraded batteries remain invisible until an outage. Maintenance records should identify replacement dates and measured performance rather than relying solely on visual inspection.
Commissioning Checklist
- Measure voltage at each lock during its highest-current operating state.
- Test simultaneous unlock and emergency-release scenarios.
- Verify circuit protection, fault reporting and battery supervision.
- Confirm fire-alarm interfaces and egress behavior with the approved sequence.
- Record conductor sizes, distances, loads and supply settings in as-built documents.
- Repeat critical tests after battery replacement or hardware changes.
Power design is one layer of a complete access system. For the broader architecture, see SectechMedia’s access control and identity technologies guide.
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