Free tool · NEC circular-mils method

Voltage Drop Calculator

Calculate single- or three-phase voltage drop with the NEC circular-mils formula. Enter material, wire size, load, distance, and voltage to get the drop and the percent against the 3% recommendation — formula shown.

Voltage drop

4.97V

Formula2 × 12.9 × I × L ÷ CM
K (copper)12.9
Circular mils (#10)10,380
Voltage drop4.97 V (4.14%)

4.14% drop — above the recommended 3%. Size up the conductor or shorten the run.

How to calculate voltage drop (NEC)

  1. 1

    Pick the formula for your phase

    Single-phase: VD = (2 × K × I × L) ÷ CM. Three-phase: VD = (1.732 × K × I × L) ÷ CM. The multiplier accounts for the conductors carrying current.

  2. 2

    Plug in K and circular mils

    K is the resistivity constant: 12.9 for copper, 21.2 for aluminum. CM is the conductor's circular-mil area from NEC Chapter 9, Table 8 (e.g. #10 copper = 10,380 CM).

  3. 3

    Solve and check against 3%

    I is the load current, L is the one-way length in feet. The result is the voltage drop in volts; divide by system voltage for the percent. The NEC recommends keeping a branch circuit under 3% (210.19(A) Informational Note).

Remember the constants: K = 12.9 (copper) or 21.2 (aluminum), multiplier = 2 for single-phase or √3 (1.732) for three-phase, and circular mils come from NEC Chapter 9, Table 8. Keep a branch circuit under 3%.

Voltage drop — frequently asked questions

What is the formula for voltage drop?

Single-phase: VD = (2 × K × I × L) ÷ CM. Three-phase: VD = (1.732 × K × I × L) ÷ CM. K is 12.9 for copper or 21.2 for aluminum, I is the current in amps, L is the one-way distance in feet, and CM is the conductor's circular mils from NEC Chapter 9, Table 8.

What is the maximum allowable voltage drop per the NEC?

The NEC does not require a hard limit, but it recommends a maximum of 3% on a branch circuit and 5% total for feeders plus branch circuits combined (210.19(A) and 215.2(A) Informational Notes). Most exam questions test the 3% branch-circuit figure.

Is voltage drop on the journeyman electrician exam?

Yes. Voltage drop using the circular-mils formula is a common calculation question. Knowing K (12.9 copper / 21.2 aluminum) and where to find circular mils (Chapter 9, Table 8) lets you solve it fast on the open-book exam.

How do you reduce voltage drop?

Increase the conductor size (more circular mils), shorten the run, reduce the load, or use copper instead of aluminum. Increasing wire size is the most common fix on long runs.

Could you pass the journeyman exam today?

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