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Voltage drop calculator

Check voltage drop for a given wire size or find the minimum AWG to stay within NEC-recommended limits. Single-phase and three-phase.

Your voltage drop will appear here, please add:

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How to calculate voltage drop

Voltage drop is the loss of voltage between the panel and the load as current travels through the wire's resistance. Too much drop means motors overheat, electronics misbehave, and runs waste energy as heat. The NEC recommends keeping drop at or below 3% on branch circuits and 5% total (branch + feeder combined).

  1. Measure the one-way distance — from the panel to the load. Do not double it yourself; the calculator handles the round-trip for single-phase and the √3 factor for three-phase.
  2. Confirm the load current in amps — from the breaker size or equipment nameplate. If you only have the wattage, switch the load unit to watts and the tool will convert.
  3. Pick wire size and material — copper by default. Aluminum is cheaper for long service runs but typically needs to go up one or two AWG sizes to match copper's performance.
  4. Calculate — the tool uses Vd = 2·L·I·R / 1000 for single-phase and Vd = √3·L·I·R / 1000 for three-phase. Resistance values come from NEC Chapter 9 Table 8.
  5. Compare to the NEC recommendation — ≤3% branch, ≤5% total. Borderline or excessive? Switch to "Find a wire size" to see the minimum AWG that meets your target.

Sizing a whole service before pulling wire? Use the electrical load calculator.

Frequently asked questions

What is the NEC voltage drop limit?

NEC 210.19 recommends a maximum of 3% voltage drop on branch circuits and 5% total (branch + feeder). These are recommendations in an Informational Note, not mandatory code, but most jurisdictions and good field practice follow them.

What happens if voltage drop is too high?

Motors overheat and pull more current, incandescent lighting dims, sensitive electronics misbehave, and the wire itself runs hotter than designed. Over time, excessive drop shortens equipment life and wastes energy.

Should I use copper or aluminum?

Copper has lower resistance per AWG and drops less voltage for the same size, so it is the default for most branch circuits. Aluminum is cheaper and lighter for long service runs (100 ft and up), but you typically need to go one or two AWG sizes larger to match copper's performance.

Does wire length or wire size matter more?

They move together in the formula. Doubling the length doubles the drop; jumping one AWG size (for example 10 → 8) drops resistance by about 37%. For long runs where shortening is not an option, upsize the wire.

Does this calculator work for three-phase circuits?

Yes. Set phase to three-phase and the calculator switches from Vd = 2·L·I·R / 1000 to Vd = √3·L·I·R / 1000. Enter line-to-line voltage (208 V, 480 V, etc.) and the current per phase.