Low Voltage Systems (C-7) · Lesson 1 of 4

Voltage Drop on Low-Voltage DC Runs (Locks, Cameras, Sounders)

"Voltage Drop on Low-Voltage DC Runs (Locks, Cameras, Sounders)" is lesson 1 of 4 in the Low Voltage Systems (C-7) Learning Hub. It opens with "Why Low Voltage Has So Little Room to Lose" and works through 5 short steps, then a worked example you can check yourself against.

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Steps in this lesson

  1. Why Low Voltage Has So Little Room to Lose
  2. Measure the Whole Loop, Not One Way
  3. Look Up the Resistance in CEC Chapter 9 Table 8
  4. Apply Ohm's Law to the Full Load on That Pair
  5. Compare to the Device Minimum, Then Fix What Fails

The worked example in the course applies this lesson to a realistic job in 5 steps, from "Add the load current" through "Resistance and drop for each gauge".

What you'll be able to do

  • Explain the main idea of "Voltage Drop on Low-Voltage DC Runs (Locks, Cameras, Sounders)" in plain language you can use on the exam.
  • Apply this step with confidence: "Look Up the Resistance in CEC Chapter 9 Table 8".
  • Apply this step with confidence: "Apply Ohm's Law to the Full Load on That Pair".
  • Apply this step with confidence: "Compare to the Device Minimum, Then Fix What Fails".
  • Test yourself on the worked example and the quick-check quiz inside the classroom.
Voltage Drop on a 24 VDC Lock Circuit 24 VDC supply 2 maglocks 1.0 A total out: 250 ft return: 250 ft 250 ft one way 500 ft loop (2 x 250) Current flows out AND back 14 AWG = 3.14 ohm per 1,000 ft Loop = 2 x 250 = 500 ft R = 3.14 x 0.500 = 1.570 ohm Drop = I x R = 1.0 A x 1.570 ohm Drop = 1.57 V At lock = 24.0 - 1.57 = 22.43 V Minimum = 24 x 0.90 = 21.6 V 18 AWG: 20.03 V (fails) 16 AWG: 21.51 V (fails, 0.09 V short) 22.43 V at the lock ≥ 21.6 V minimum operating voltage 14 AWG PASSES - 0.83 V of margin (16 AWG would not) Resistance: 2025 CEC Chapter 9 Table 8, uncoated copper, 7 strands, 75 C (ohm per 1,000 ft) CC
The diagram for this lesson, exactly as it appears in the Learning Hub.

The first steps of this lesson

Step 1: Why Low Voltage Has So Little Room to Lose

Magnetic locks, cameras, strobes and card readers run on 12 or 24 volts, so even a couple of volts lost in the wire is a big share of what the device needs. A 2.4 V drop is 10 percent of a 24 V supply but only 2 percent of a 120 V circuit. Too little voltage means a maglock that holds weakly, a camera that reboots, or a notification appliance that will not sound. The California code books on disk do not print a percentage limit for this drop on a Class 2 circuit. The limit you design to is the minimum operating voltage on the device's own nameplate or installation sheet, and the lesson below uses 24 VDC plus or minus 10 percent as a stated example.

Step 2: Measure the Whole Loop, Not One Way

Direct current has to travel out on one conductor and come back on the other, so the wire that matters is twice the one-way distance. A lock 250 feet from its power supply is a 500-foot circuit. Forgetting to double the distance is the most common voltage-drop mistake and it makes every answer come out half as bad as the real jobsite.

The remaining 3 steps, the worked example and the quiz are in the course. This is a preview. The full lesson, with the step-by-step explanations, a worked example and a quick-check quiz, is inside the classroom.

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