Reusing 18 AWG Cable for a 12 VDC Electric Strike
A county access-control retrofit in Charlotte reuses an existing 18 AWG stranded copper pair from the IDF to a 12 VDC electric strike. The one-way cable distance is 180 ft. The strike draws 1.0 A when energized and is rated 12 VDC ±10%. The power supply at the IDF measures 12.4 VDC open-circuit and is dedicated to the strike. The installer wants to connect the existing pair because pulling new cable through a finished corridor would require firestopping and a change order. The owner reports that a test with a temporary 12 V battery at the door worked, but the permanent supply is at the IDF. You are asked to approve the reuse. The panel's 12 VDC output is not adjustable. The lock manufacturer lists a minimum operating voltage of 10.8 V.
1. Calculate the voltage at the strike using 18 AWG copper resistance, showing the loop length. Is the reuse acceptable?
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Use the total loop length, not the one-way length. The circuit is 180 ft one-way, so the loop is 360 ft. Chapter 9 Table 8 gives approximately 6.51 ohms per 1,000 ft for 18 AWG stranded copper. Loop resistance is 0.360 × 6.51 = 2.34 ohms. At 1.0 A, the drop is 1.0 × 2.34 = 2.34 V. With the measured 12.4 V supply, the strike sees about 10.06 V. At a nominal 12 V supply, it sees about 9.66 V. Both are below the manufacturer's 10.8 V minimum. The temporary battery test at the door does not prove the permanent circuit because it had almost no cable drop. The reuse is not acceptable.
2. Recommend the minimum practical solution and any contract or code considerations before the work proceeds.
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The minimum practical fix is to pull a larger pair or relocate the power supply closer to the door. With 14 AWG, the loop resistance is about 0.360 × 2.58 = 0.93 ohm, so the drop at 1.0 A is about 0.93 V. At 12.4 V the strike would see about 11.47 V, and at 12 V it would see about 11.07 V, both above 10.8 V. A 16 AWG pair is marginal or below minimum: loop resistance about 1.47 ohms gives about 10.93 V at 12.4 V and about 10.53 V at 12 V. The circuit is a Class 2 circuit under Article 725, and the new cable route needs firestopping and support. Because the existing pair was part of the approved scope, the larger cable or relocated supply should be handled by a written change order before installation.
3. What field test would you require after installation, and why is an open-circuit voltage measurement misleading?
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I would require a loaded voltage measurement at the strike terminals while the strike is energized, at both normal supply voltage and the lowest expected supply voltage. Record the voltage and current, and verify that the strike remains above 10.8 V at the device terminals, not at the power supply. Open-circuit voltage is misleading because it does not include the voltage drop across the loop resistance under load. A reading of 12.4 V at the IDF can fall below the lock's minimum once the 1.0 A load is applied. The test should also confirm polarity, tight terminations, and that the circuit is not being used to power other loads.