Voltage Drop Calculator
Find the voltage drop and percent drop for a wire run, and the smallest copper or aluminum wire that keeps drop under your target.
- Voltage drop
- Voltage at the load
- Wire area
- Wire resistance
- Longest run for target
- Smallest Wire for Target
Saved Runs
Items you add are saved in this browser.
| Run | Circuit | Wire | Length | Amps | Drop | Min. Size | Remove |
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How to Calculate Voltage Drop
Every wire has resistance, so some voltage is lost along the run and the load gets a little less than the panel supplies. The longer the run, the higher the current and the thinner the wire, the bigger the drop. This calculator uses the K method, a standard way electricians estimate drop from the wire's cross-section in circular mils.
K is the resistance of a circular-mil-foot of wire: about 12.9 ohms for copper and 21.2 for aluminum at 75 °C. I is the current in amps, D the one-way length in feet, and CM the wire's area in circular mils from NEC Chapter 9, Table 8. To find the smallest wire for a target drop, solve for CM and pick the next size up.
Worked Example
Wire Sizes in Circular Mils
| Size | Circular Mils | Max One-Way Ft. for 3% at 120 V, 15 A (Cu) |
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Tips
An informational note in the NEC suggests keeping drop to 3% on a branch circuit and 5% overall from the service to the farthest outlet. That's a recommendation for performance, not a code requirement, but big drops make motors run hot and lights dim. Size the wire for ampacity first, then upsize for voltage drop on long runs. This method ignores reactance, which matters for large conductors in steel conduit.