Guide
How to size an access control power supply
Short answer: add the current (amps) of every lock, reader and device on the supply at its voltage, add 20–25 % headroom, size the battery for the standby time you need, and check the voltage drop on the long runs.
1. Add up the current
Use the amps from each data sheet at the voltage you will run (12 V or 24 V). A lock draws about half the current at 24 V that it draws at 12 V.
| Device | Typical current at 12 V DC | At 24 V DC |
|---|---|---|
| Maglock 600 lb | 0.5 A | 0.25 A |
| Electric strike | 0.25–0.45 A | 0.12–0.25 A |
| Card reader | 0.1–0.25 A | – |
| Request-to-exit motion sensor | 0.03 A | 0.03 A |
These are examples – always use the data sheet of the exact model.
2. Add headroom
Keep the load at about 75–80 % of the supply’s output, or of each output fuse on a distributed supply (e.g. Altronix or LifeSafety Power boards). Locks draw more when they are cold or when several release at once.
3. Size the battery
Battery (Ah) = load (A) × hours of standby ÷ 0.8. The 0.8 allows for ageing and temperature. Example: 1.65 A for 4 hours needs 1.65 × 4 ÷ 0.8 = 8.25 Ah – use two 7 Ah batteries in parallel, or a 12 Ah battery. Some codes and fire-alarm interfaces require longer standby – check what applies to the job.
4. Check the voltage drop
Voltage drop = 2 × length (ft) × wire resistance (Ω per 1,000 ft) ÷ 1,000 × current (A).
| Wire | Ω per 1,000 ft (copper) |
|---|---|
| 22 AWG | 16.1 |
| 18 AWG | 6.39 |
| 16 AWG | 4.02 |
| 14 AWG | 2.53 |
Example: a maglock at 0.5 A, 200 ft away on 18/2: 2 × 200 × 6.39 ÷ 1,000 × 0.5 = 1.28 V – about 10.7 % of 12 V. Too much for many locks: use 24 V, 16 AWG or a supply closer to the door.
Fail-safe or fail-secure
Maglocks and fail-safe strikes unlock when power is lost – they are often required on egress doors and tied to the fire alarm. Fail-secure strikes stay locked. Decide per door, following the code and the authority having jurisdiction.
Questions
Can readers share the lock supply?
Often yes, but many designers power readers from the panel and locks from a separate supply so a lock fault does not take the reader down.
What about PoE locks?
PoE door controllers power the reader and a lock from one PoE+ port – check the lock current against what the controller can deliver.
Related
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