Voltage Drop Calculator

Pick a wire size and see the voltage drop, the voltage at the load and the power lost, then see the smallest wire that would meet your limit.

Example values are shown. Change any field to calculate your own circuit.

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Enter your circuit

All fields are required unless marked optional.

Load 40 A

Amps Watts

Load current (A)

Use the amps on the nameplate or breaker. Choose Watts above to enter power instead.

Run 150 ft one-way

One-way length

Enter the distance from the source to the load, not the round trip.

Length unit

Circuit AC single-phase · 240 V · Copper · 3% max drop

Electrical system

Voltage (V)

Nominal supply voltage: 120 or 240 V for AC homes, 12 V for most vehicles.

Wire material

Maximum voltage drop (%)

3% is a common design target; use 1–2% for sensitive electronics.

Wire operating temperature

Hotter wire has more resistance. 75 °C is the conservative NEC reference.

Your wire 8 AWG

Wire size you have

Advanced settings

Terminal temperature rating (optional)

Continuous load (3 hours or more): add 125% (optional)

Based on NEC Table 310.16 ampacities and Chapter 9 Table 8 resistance. No temperature or bundling derating. Check with a licensed electrician. How this is calculated

On this page

What voltage drop is (and why it is also called voltage loss)
The formulas
Worked example
How much voltage drop is acceptable?
Assumptions and limits

What voltage drop is (and why it is also called voltage loss)

Every wire has resistance. When current flows, some of the source voltage is used up in the wire itself, so the load receives less than the source supplies. That difference is the voltage drop, sometimes called voltage loss. It is lost as heat in the wire, which is why the calculator also reports the power lost in watts.

A drop that is too large makes motors run hot, dims lights, makes electronics reset, and on 12 V systems can cut charging current to a battery. Making the wire thicker lowers resistance and therefore the drop.

The formulas

DC and single-phase AC: Vdrop = 2 × I × R × L ÷ 1000
Three-phase AC: Vdrop = √3 × I × R × L ÷ 1000
Percent drop: Vdrop ÷ source voltage × 100

I is the current in amps, L is the one-way length in feet, and R is the resistance of the wire in ohms per 1,000 ft. Metric users can enter metres; the calculator converts for you.

Worked example

A 240 V single-phase load draws 40 A and sits 150 ft from the panel. Copper wire at the NEC reference temperature of 75 °C is assumed.

8 AWG: R ≈ 0.779 Ω per 1,000 ft, so Vdrop = 2 × 40 × 0.779 × 150 ÷ 1000 ≈ 9.35 V, which is about 3.9% of 240 V. That is above a 3% target.
6 AWG: R ≈ 0.490 Ω per 1,000 ft, so Vdrop ≈ 5.88 V, about 2.5%. That meets the 3% target.

8 AWG carries 40 A comfortably, but the length decides the answer here, so the 6 AWG wire is the right choice if you want to stay under 3%.

How much voltage drop is acceptable?

The NEC does not set a mandatory limit for most circuits. Its informational notes suggest keeping branch-circuit drop to about 3% and the combined feeder and branch drop to about 5%. These are recommendations. Your equipment may need less: LED drivers, sensitive electronics and battery chargers often work best at 1–2%, while some motors tolerate more during running but struggle at start-up.

Assumptions and limits

Resistance only; reactance is ignored. Use NEC Chapter 9 Table 9 impedance values for large AC conductors on long runs.
Stranded copper and aluminum conductors, with resistance corrected to the temperature you choose. The default 75 °C is the NEC Table 8 reference and is slightly conservative for lightly loaded wire.
The suggested wire size also checks ampacity from NEC Table 310.16 with no ambient or bundling derating.
Always confirm with a licensed electrician and your local code. See the disclaimer.

Looking for the wire size for a particular load instead? Use the wire size calculator, or the 12V wire size calculator for RV, van, boat and solar wiring.