Voltage drop is the voltage lost along a conductor because of its resistance. Too much drop makes lights dim, motors run hot and electronics misbehave, even when the wire is within its ampacity rating.
The formula
For single-phase circuits, VD = 2 × K × I × L ÷ CM. For balanced three-phase circuits, replace 2 with 1.732. K is about 12.9 for copper and 21.2 for aluminum at 75°C, I is amps, L is the one-way length in feet and CM is the conductor's circular mils from NEC Chapter 9, Table 8.
What are the limits?
The informational notes in NEC 210.19(A) and 215.2(A) recommend a maximum of 3% on a branch circuit or feeder and 5% for the feeder and branch circuit combined. They are recommendations, not enforceable requirements, but plans, specs and some other rules may require them. The CEC (Rule 8-102) uses the same 3% and 5% figures.
Worked example
A 20 A load on a 120 V circuit 100 ft from the panel, using 12 AWG copper (6,530 cmil): VD = 2 × 12.9 × 20 × 100 ÷ 6,530 = 7.9 V, or 6.6%. That is too much. With 10 AWG (10,380 cmil) the drop is 5.0 V (4.1%), and with 8 AWG (16,510 cmil) it is 3.1 V (2.6%), which passes.
A more precise method
Our voltage drop calculator uses the AC resistance and reactance values in NEC Chapter 9, Table 9 with the load power factor: Z = R·cosθ + X·sinθ per 1,000 ft, then VD = 2 × I × L × Z ÷ 1,000 (1.732 instead of 2 for three-phase). For the 12 AWG example above it gives 8.0 V (6.7%), close to the simple formula. The difference grows on large conductors in steel conduit.
How to reduce voltage drop
- Increase the conductor size.
- Shorten the run or move the panel closer.
- Use a higher voltage where the equipment allows it; 240 V halves the percentage drop of the same load at 120 V.
- Use copper instead of aluminum, or parallel conductors on large feeders.
Motors and long runs
Motor starting current can be several times the running current, so temporary drop is larger. Large conductors in steel conduit also have reactance that this simple formula ignores; use the impedance values in Chapter 9, Table 9 for big feeders.
Try the calculators
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