Voltage Drop Calculations for Electricians (and the Exam)
Voltage drop is one of the few exam calculations that matters every day in the field. Long runs to a garage, a well pump or a parking lot light all need it. Here's the formula, what the NEC actually says about it, and a worked example.
What the NEC says
For general branch circuits and feeders, the NEC's voltage drop guidance is in informational notes. They suggest about 3% on a branch circuit and 5% total for feeder plus branch circuit. Informational notes aren't enforceable on their own, but exam questions often ask you to size for them, and good practice follows them. A few specific articles do make voltage drop mandatory, so read the question.
The formulas
Single-phase: VD = (2 × K × I × L) ÷ CM
Three-phase: VD = (1.732 × K × I × L) ÷ CM
- K = resistance constant, about 12.9 for copper and 21.2 for aluminum (ohms per circular-mil-foot)
- I = load current in amps
- L = one-way length in feet
- CM = conductor area in circular mils, from Chapter 9, Table 8
| Size (AWG) | Circular mils |
|---|---|
| 14 | 4,110 |
| 12 | 6,530 |
| 10 | 10,380 |
| 8 | 16,510 |
| 6 | 26,240 |
Worked example
A 120 V, 16 A load, 100 feet one way, on 12 AWG copper.
VD = (2 × 12.9 × 16 × 100) ÷ 6,530 = 6.3 V. 6.3 ÷ 120 = 5.3%, over the 3% suggestion.
Try 10 AWG: (2 × 12.9 × 16 × 100) ÷ 10,380 = 4.0 V = 3.3%. Still a little high. 8 AWG gives about 2.5 V, or 2.1%.
Solving for wire size
Flip the formula: CM = (2 × K × I × L) ÷ allowable VD. With 3% of 120 V = 3.6 V, the example needs at least 11,467 CM, so 8 AWG (16,510 CM) is the next size up that works.
Don't forget
- L is one-way length. The 2 in the single-phase formula already covers the round trip.
- If you upsize the conductors for voltage drop, upsize the equipment grounding conductor in proportion too (250.122(B)). See grounding conductor sizing.
- Upsized conductors still have to fit the terminals and the box. See box fill.
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More guides: How to navigate the NEC · 2026 NEC changes for exam takers · Massachusetts exam: 527 CMR and the NEC · How to tab and mark your NEC · 30-day journeyman study plan · Box fill calculations (314.16) · Conductor ampacity and derating · Grounding conductor sizing (250.66 vs 250.122) · Voltage drop calculations