Voltage Drop Calculator
Find how many volts a wire loses over its length, or the smallest wire size that keeps voltage drop under your limit.
How it works
Pick single- or three-phase, the source voltage and your load, in amps, or in watts, kilowatts or horsepower with a power factor.
Choose copper or aluminum, a wire size from 14 AWG to 500 kcmil, and the ONE-WAY length from the panel to the load. The return path is already included in the formula.
Results are color-coded: green at 3% or less, orange up to 5%, red above, following the NEC informational notes (3% branch circuit, 5% total). Switch to “Find wire size” to get the minimum conductor for any limit.
Formula
Single-phase: VD
Where:
- K
- = Resistivity: 12.9 copper, 21.2 aluminum (Ω·cmil/ft at 75 °C)
- I
- = Load current in amps
- L
- = One-way length in feet
- CM
- = Conductor area in circular mils
Worked example
A 120 V, 20 A single-phase circuit runs 100 ft (one way) on 12 AWG copper (6,530 cmil).
VD = 2 × 12.9 × 20 × 100 ÷ 6,530 = 7.90 V, which is 6.58%, red. Upsizing to 8 AWG drops it to about 2.6%.
Practical Tips for Minimizing Resistance
When planning a long electrical run, verify the actual source voltage at the panel under load rather than assuming a nominal 120V or 240V. Small fluctuations at the start can amplify the effects of voltage drop at the far end. If dealing with significant distances, consider moving the transformer closer to the load or increasing the system voltage if equipment allows, as higher voltages carry power at lower amperage, reducing required conductor size.
Choose a wire gauge that accounts for future expansion. If a calculation puts you at the 3% limit, upsizing now is cheaper than trenching a second line later. For outdoor runs, ensure you use the correct insulation type like THWN-2 and account for ambient temperature; the NEC provides correction factors that alter a wire's effective resistance and ampacity.
Common Mistakes in Conductor Sizing
A frequent error is doubling the length of the run in the formula. Most modern voltage drop formulas already incorporate the return path multiplier into the equation. Entering the 'round-trip' distance instead of the 'one-way' distance results in an oversized wire recommendation that adds unnecessary cost to your project.
Ignore the physical limitations of terminals at your own peril. You might calculate that a 4/0 AWG wire is needed, but if your breaker is only rated for 2/0 AWG, you cannot safely terminate it without proper adapters. Always defer to the NEC and local building codes, as they prioritize safety and fire prevention over efficiency. Consult a licensed electrician for any high-current installations.
Frequently asked questions
The NEC recommends (in informational notes, not hard rules) no more than 3% on a branch circuit and 5% total for feeder plus branch. Some local codes and equipment makers require it.
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For informational purposes only. This calculator does not provide professional engineering advice. Consult a qualified professional before making decisions based on these results.