Voltage Drop

Single-phase or three-phase voltage drop over length.

Run the calculator

Example

You enter

Phase single
Material copper
AWG 10
Length one-way (ft) 150
Current (A) 20
Source voltage (V) 240

You get

Voltage drop 7.45 V
Percent drop 3.11 %

Details, formula, and sources

VD = 2 * I * R * L for single-phase; VD = √3 * I * R * L for three-phase. R from NEC Chapter 9 Table 8 (DC ohm/kFT) with temperature correction; reactance per IEEE 141 for long runs.

NEC 2023 (NFPA 70). Chapter 9 Tables 8 and 9; IEEE 141 by name.

Free read-only access at nfpa.org/freeaccess. IEEE 141 is licensed; principles free at university OCW and IEEE-USA outreach.

Estimate. AHJ and licensed electrician govern. Verify against the NEC edition adopted in your jurisdiction.

Field names used by the API: phase, material, awg, length_ft, current_A, source_voltage_V, drop_V, percent

Conductor temperature 75°C unless user suppliesNEC 310.15(B)(1)
Power factor 0.85 for AC unless user suppliesengineering practice for general lighting / receptacle loads
Voltage-drop target 3% branch + 5% feeder = 5% totalNEC informational notes 210.19(A)(1) FPN 4 / 215.2(A)(1) FPN 2