Free Voltage Drop Calculator
Find the voltage drop on any run from system voltage, distance, current, and wire gauge, with NEC 3% and 5% guidance flags, or work backwards to find the wire size you need. Built for electricians and DIY planners.
This free voltage drop calculator uses VD = 2 x K x I x L / CM to estimate voltage drop in volts and percent for copper or aluminum conductors from 14 AWG to 4/0, flags results against the NEC 3% branch and 5% feeder-plus-branch guidance, and can work backwards to the smallest wire gauge that meets your target drop.
Nominal system voltage: 120 or 240 for homes, 208 or 480 for commercial.
Measure from the panel to the load, one way. The formula doubles it for the round trip.
The current the load actually draws, not the breaker size.
Measure from the panel to the load, one way.
The current the load actually draws, not the breaker size.
The maximum drop you will accept. 3% matches the NEC branch circuit guidance.
Estimates for planning only, not wiring instructions. See Terms of Use.
NEC 3% and 5% voltage-drop guidance
The National Electrical Code does not hard-require voltage drop limits for most installations, but it does publish recommended maximums: 3% for a branch circuit and 5% for the combined feeder and branch circuit. These come from NEC Article 210.19(A)(1) Fine Print Note No. 4 (and a parallel note for feeders in 215.2(A)(1)). They are recommendations, not enforceable rules, but inspectors, engineers, and utilities treat them as the standard.
A branch circuit runs from the last breaker to the outlets or equipment it serves. If the drop along that run stays under 3%, everything downstream is close to nominal voltage. A feeder runs from the service panel to a subpanel. The 5% figure is the total from the service to the final outlet: feeder drop plus branch drop. So if your feeder already drops 2%, your branch circuit gets only 3% before you exceed the recommended combined maximum.
When does it actually matter? Voltage drop turns into heat in the wire, and equipment sees less than its rated voltage. Motors suffer first: low voltage raises motor current, which shortens motor life and can trip breakers on startup. Long runs to sheds, barns, wells, or outdoor lighting are the classic residential case. A 150-foot run at 120 volts on 14 AWG loses nearly 8%, which is why upsizing to 12 or 10 AWG is standard practice.
Size the wire for the current it must carry first (ampacity per NEC 310.16), then check voltage drop with this calculator. If the drop exceeds 3% on a branch circuit, go up one or more wire sizes and recalculate. For anything beyond basic circuits, consult a licensed electrician before you buy wire.
Voltage drop calculator questions
How do you calculate voltage drop?
Use the formula VD = 2 x K x I x L / CM. K is 12.9 for copper and 21.2 for aluminum, I is the load current in amps, L is the one-way length in feet, and CM is the wire's circular mil area from the AWG table. Divide the volts lost by the system voltage and multiply by 100 for the percentage.
What do the NEC 3% and 5% voltage drop numbers mean?
The National Electrical Code recommends a maximum 3% voltage drop on a branch circuit (breaker to outlet) and a maximum 5% drop for the feeder and branch circuit combined (service to outlet). They appear as Fine Print Notes, not enforceable rules, but inspectors and engineers treat them as the standard.
How does the "what gauge do I need" mode work?
Enter the system voltage, one-way distance, load current, conductor material, and your target drop percent. The calculator tests each AWG size from 14 to 4/0 and returns the smallest gauge that keeps the drop at or under your target. Always check ampacity separately: the correct current-carrying size from NEC 310.16 still applies.
Should I use copper or aluminum wire?
Copper has lower resistance, so aluminum needs a larger gauge to hold the same voltage drop (K = 21.2 versus 12.9). Aluminum is common and economical for large feeders, but it needs compatible terminations and proper installation. For anything beyond basic branch circuits, consult a licensed electrician.