Mag-Lock Power and Egress at a Honolulu Mid-Rise
At a 12-story affordable housing project in Honolulu, a lobby door uses a 24 V DC mag lock rated 600 lb holding force and 0.45 A. The lock is fed from an IDF power supply 260 ft away through 18 AWG stranded copper. At the lock, you measure 19.1 V while the supply terminals read 23.5 V. The lock chatters and sometimes releases when the building A/C compressor starts. A request-to-exit (REX) motion sensor unlocks the door, and a fire alarm relay is available but not yet connected. The door is a required means of egress. The owner wants a 4-hour standby battery and no delay on egress. You must determine the voltage-drop cause, size the power supply and battery, and coordinate the fire alarm release.
1. Calculate the expected voltage drop for the 18 AWG run at 0.45 A and compare it with the measured 4.4 V drop. What does the difference indicate, and what would you check first?
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18 AWG copper is about 6.385 ohms per 1,000 ft. The loop is 2 x 260 = 520 ft, so R = 6.385 x 0.520 = 3.32 ohms. At 0.45 A, expected drop = 1.49 V, and the lock should see about 23.5 - 1.49 = 22.0 V. The measured drop is 23.5 - 19.1 = 4.4 V, about 2.9 V more than the wire alone explains. That points to bad splices, a long coil of cable, undersized or aluminum conductor, corroded terminals, a poor plug-in connector, or a supply that folds back under load. Check the actual lock current with a clamp meter, inspect every splice and terminal, measure voltage at both ends under load, and verify the conductor gauge and length. Do not fix this by raising the supply voltage until the cause is found.
2. Select a power supply and battery configuration for the lock, REX, and reader. The reader draws 0.15 A at 24 V and the REX draws 0.03 A. Assume 4 h standby and a 20% aging margin. Show the load and battery Ah calculation.
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Load = lock 0.45 A + reader 0.15 A + REX 0.03 A = 0.63 A at 24 V. Four-hour standby = 0.63 x 4 = 2.52 Ah. Add 20% aging margin: 2.52 x 1.2 = 3.02 Ah. A 24 V battery is two 12 V batteries in series, so use two 12 V, 7 Ah batteries in series or a listed 24 V 7 Ah pack; the 4-hour rate of a 7 Ah battery is below its 20-hour rating, but it still covers 3.02 Ah with margin. Select a 24 V DC power supply with a listed charger, enough continuous current for the lock inrush plus reader and REX, and a fire alarm release input. Mount the supply in the IDF, protect the output with a fuse or breaker, and calculate voltage drop again after the final conductor is chosen. Use 16 AWG or larger for the lock if the run stays at 260 ft.
3. How must the mag lock, REX, and fire alarm relay be wired and tested to satisfy egress and fire release? Include fail-safe behavior and what happens on power loss.
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The mag lock must be fail-safe: it releases on loss of power and on fire alarm. Wire the fire alarm relay to interrupt the lock power, not just the reader, and use a listed relay and power supply. The REX must unlock the door on the egress side without a delay, and the door must remain mechanically openable. Do not rely on software or a network command for release. Test by simulating fire alarm, removing AC power, removing battery, and actuating the REX; verify the lock releases every time and the door opens with normal egress force. Verify the fire alarm relay is supervised or monitored as required, label the circuit, and have the AHJ witness the release test. If the lock has a bond sensor, confirm it reports locked and unlocked correctly after each test.