Electrical (C-10) · Lesson 3 of 6

Voltage Drop with CEC Chapter 9 Table 8

"Voltage Drop with CEC Chapter 9 Table 8" is lesson 3 of 6 in the Electrical (C-10) Learning Hub. It opens with "What the Energy Code limits" and works through 5 short steps, then a worked example you can check yourself against.

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Steps in this lesson

  1. What the Energy Code limits
  2. Where the wire resistance comes from
  3. The math (Ohm's law)
  4. Adjusting for temperature
  5. Pick the smallest wire that fits the budget

The worked example in the course applies this lesson to a realistic job in 6 steps, from "Find the voltage budget" through "Check the stranded value".

What you'll be able to do

  • Explain the main idea of "Voltage Drop with CEC Chapter 9 Table 8" in plain language you can use on the exam.
  • Apply this step with confidence: "The math (Ohm's law)".
  • Apply this step with confidence: "Adjusting for temperature".
  • Apply this step with confidence: "Pick the smallest wire that fits the budget".
  • Test yourself on the worked example and the quick-check quiz inside the classroom.
Voltage Drop: 20 A at 150 ft on 120 VTOTAL DROP vs 6.00 V LIMIT12 AWG12.78 V10 AWG8.46 V8 AWG5.784 Vlimit 6.00 Vfeeder 1.20 Vbranch: failsbranch: passesCEC Table 8, uncoated Cu, solidLoop R = 2 x 150 ft x R / 100012 AWG 1.93: 0.579 ohm x 20 A = 11.58 V10 AWG 1.21: 0.363 ohm x 20 A = 7.26 V8 AWG 0.764: 0.2292 ohm x 20 A = 4.584 VAdd 1.20 V feeder drop, divide by 120 V:12 AWG 12.78 V = 10.65% FAIL10 AWG 8.46 V = 7.05% FAIL8 AWG 5.784 V = 4.82% PASSLimit: 5% x 120 V = 6.00 V(Energy Code §130.5(c))8 AWG is the smallest size that holds total drop to 4.82% (limit 5%)This checks voltage drop only; ampacity is a separate check.PANEL20 A150 ft one way, 120 VSources: 2025 CEC Chapter 9 Table 8; 2025 California Energy Code §130.5(c)CC
The diagram for this lesson, exactly as it appears in the Learning Hub.

The first steps of this lesson

Step 1: What the Energy Code limits

2025 California Energy Code §130.5(c) (nonresidential and hotel/motel buildings): the maximum combined voltage drop on both installed feeder conductors and branch circuit conductors to the farthest connected load or outlet shall not exceed 5 percent. Multifamily buildings carry the same 5 percent limit in §160.6(c). The exception is voltage drop permitted by California Electrical Code Sections 647.4, 695.6 and 695.7. The limit is combined: whatever the feeder uses up leaves less for the branch circuit.

Step 2: Where the wire resistance comes from

2025 CEC Chapter 9, Table 8 "Conductor Properties" prints direct-current resistance at 75°C (167°F) in ohms per kilometer and ohms per 1000 feet (ohm/kFT). For uncoated copper: 14 AWG solid 3.07; 12 AWG solid 1.93; 10 AWG solid 1.21; 8 AWG solid 0.764; and for stranded 6 AWG 0.491, 4 AWG 0.308, 2 AWG 0.194, 1/0 AWG 0.122. Stranded 12, 10 and 8 AWG are slightly higher (1.98, 1.24 and 0.778). Table 8 Note 1 says the values are valid only for the parameters given.

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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