How do I size a power supply for an access control system?

Add the continuous current of every device on the supply — maglocks and readers draw constantly — then add 25% headroom. Four maglocks at 500mA plus readers and a controller typically calls for a 4A supply, not the 2.85A the raw math suggests.

Do maglocks fail safe or fail secure on power loss?

Maglocks are inherently fail-safe: no power means the door is unlocked. That's why battery backup sizing matters — the battery is what keeps doors secured during an outage. Electric strikes can be ordered fail-safe or fail-secure.

How many hours of battery backup does access control need?

Four hours of standby is a common commercial baseline in Canada and the US, but requirements vary by code, occupancy and AHJ. Life-safety egress rules always take precedence over lock power — verify with your local authority.

Access Control Power Supply & Battery Calculator

Add every locking device, reader and controller on the supply. Get the power supply amperage you should install and the battery Ah needed to hold the doors through an outage.

Devices on this power supply

Device	Qty	mA each

System voltage

Required standby (hours)

Verify the requirement with your AHJ / monitoring station.

Power & battery sizing

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continuous load at 12 V

Recommended PSU rating (25% headroom)—

Standard PSU size to install—

Battery capacity for 4 h standby—

Suggested battery configuration—

Load in watts—

Battery sizing includes a 25% aging/derating factor. Maglocks draw continuously — they define your battery load.

What do you need next?

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Typical access control device current draw

Device	12 VDC draw	Notes
Maglock (600 lb)	~500 mA	Continuous; ~250 mA at 24V
Maglock (1200 lb)	~500–600 mA	Continuous
Electric strike	~240 mA	Intermittent (fail-secure) or continuous (fail-safe)
Prox/smart card reader	~100–150 mA	Continuous
Door controller (per door)	~250 mA	Continuous
REX motion sensor	~25 mA	Continuous
Electrified lever/mortise	~300 mA inrush	Check datasheet

The fail-safe problem

Maglocks release the instant power fails — that's a life-safety feature, but it means a power outage unlocks your building. The standby battery, not the power supply, is what keeps doors secured through an outage. Size the battery for the full continuous load including every maglock, and remember egress requirements (REX, push-to-exit, fire alarm tie-in) always override lock power.

Frequently asked questions

How do I size the power supply?

Sum every device's continuous draw, add 25% headroom, and round up to a standard supply size (2A, 4A, 6A, 10A). Running a PSU at its limit shortens its life and leaves nothing for door additions.

Fail-safe vs fail-secure?

Fail-safe unlocks on power loss (maglocks, always); fail-secure stays locked (typical strikes). Egress code drives the choice — when in doubt, ask your AHJ.

How many hours of standby?

4 hours is a common commercial baseline; monitored ULC installations and specific occupancies may require more. Verify locally.

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