Voltage Drop Calculations Explained (NEC Circular-Mils Method)

Voltage drop is found with VD = (2 × K × I × L) ÷ CM for single-phase and VD = (1.732 × K × I × L) ÷ CM for three-phase, where K is 12.9 for copper or 21.2 for aluminum, I is the load current, L is the one-way length in feet, and CM is the conductor's circular mils. The NEC recommends keeping a branch circuit under 3%.

What each term means

• K — the resistivity constant: 12.9 for copper, 21.2 for aluminum. Memorize both.
• I — the load current in amps.
• L — the one-way distance in feet (the formula's multiplier already accounts for the return path).
• CM — circular mils, from NEC Chapter 9, Table 8. A #10 copper is 10,380 CM; #8 is 16,510 CM.
• Multiplier — 2 for single-phase, √3 (1.732) for three-phase.

The 3% rule

The NEC doesn't require a voltage-drop limit, but two Informational Notes recommend one: 3% on a branch circuit (210.19(A)) and 5% total for feeder plus branch combined (215.2(A)). Exam questions almost always test the 3% branch-circuit figure.

A worked example (single-phase)

A 20 A load on #10 copper, 100 ft away, on a 120 V circuit:

• VD = (2 × 12.9 × 20 × 100) ÷ 10,380
• VD = 51,600 ÷ 10,380 = 4.97 V
• Percent: 4.97 ÷ 120 = 4.1%

That's over 3% — on the exam you'd size up to #8 copper (16,510 CM), which drops it to about 3.1 V (2.6%), back under the limit.

How to fix excessive drop

Four levers: bigger conductor (more circular mils), shorter run, less load, or copper instead of aluminum. On long runs, upsizing the wire is the usual answer.

Run the numbers free

Plug any scenario into our Voltage Drop Calculator — single- or three-phase, copper or aluminum — and it shows the formula and the percentage against the 3% rule. Then take the free 8-question diagnostic to see if voltage drop and the other calcs are exam-ready or need work.