Wire Voltage Drop Calculator (NEC Method)
Voltage drop in electrical wiring wastes energy and can cause equipment to malfunction or run hot. The National Electrical Code (NFPA 70) recommends keeping branch circuit voltage drop below 3% of supply voltage, and total feeder-plus-branch drop below 5%. The NEC voltage drop formula is Vd = 2 * L * I * R / 1,000, where L is the one-way wire length in feet, I is the load current in amps, and R is the resistance per 1,000 feet of the selected conductor. Select your wire gauge from the dropdown or enter a custom resistance value; the calculator pre-loads NEC table values for copper conductors.
NEC voltage drop formula
Vd = 2 * L * I * R / 1,000
% drop = Vd / Vs * 100
L is one-way length in feet, I is current in amps, R is resistance per 1,000 feet from NEC Table 9 (copper, 75 degrees C). The factor of 2 accounts for the round-trip path. NEC recommends Vd/Vs less than 3% for branch circuits.
Reducing voltage drop
- Use a larger wire gauge (lower AWG number) to reduce resistance and voltage drop on long runs.
- Shorten the wire run where possible by locating panels closer to loads.
- For 240 V loads, voltage drop is a smaller percentage of supply voltage than for 120 V loads, for the same wire and current.
- The NEC 3% recommendation applies to single branch circuits; for feeders powering many circuits, keep feeder drop under 2% to leave headroom for branch circuit drop.
Frequently asked questions
What voltage drop does the NEC recommend?
NFPA 70 (NEC) recommends (but does not require) that voltage drop on any branch circuit not exceed 3%, and the combined drop on feeders and branch circuits not exceed 5%. Voltage drop beyond these levels causes equipment to run hot and inefficiently.
What is the NEC voltage drop formula?
Vd = (2 * L * I * R) / 1000, where L is the one-way wire length in feet, I is current in amps, and R is resistance per 1,000 feet in ohms. The factor of 2 accounts for the full round-trip current path (hot conductor and neutral/return).
What is the resistance of common wire gauges?
For solid copper wire: 14 AWG is 3.07 ohms per 1,000 ft; 12 AWG is 1.93 ohms/1,000 ft; 10 AWG is 1.21 ohms/1,000 ft; 8 AWG is 0.764 ohms/1,000 ft; 6 AWG is 0.491 ohms/1,000 ft. Larger AWG numbers mean smaller, higher-resistance wire.
Does temperature affect wire resistance?
Yes. Copper resistance increases about 0.393% per degree Celsius above 20 degrees C. At 75 degrees C (NEC rated temperature for most conductors), resistance is roughly 21% higher than at 20 degrees C. NEC tables use 75 degrees C ampacity values.
Does this formula apply to AC circuits?
The resistive voltage drop formula is a good approximation for AC circuits at power frequencies (60 Hz) for most residential and commercial wiring. For high-current or large-gauge conductors, skin effect and inductive reactance may require more detailed calculation.
Official sources
- NFPA 70 (National Electrical Code): NFPA 70, Table 9 and Section 210.19.
- IEEE: IEEE Standards Association.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.