Every wire has resistance, so some of the source voltage is lost along the way before it reaches the load. Over a short run the loss is negligible, but on a long run to a garage, well pump, EV charger or outbuilding it can leave motors struggling and lights dim. This calculator works out the voltage drop for DC, single-phase and three-phase circuits, the percentage of the supply that represents, the voltage left at the load and the power wasted as heat — and it finds the smallest wire size that meets your target.
How to use the voltage drop calculator
- Choose DC, AC single-phase or AC three-phase.
- Enter the source voltage (line-to-line for three-phase) and the load current in amps.
- Enter the one-way length from the source to the load, in feet or meters. The calculator accounts for the return path.
- Select copper or aluminum, the wire-size system (AWG / kcmil or mm²) and the size.
- Set the conductor temperature — 75 °C matches the resistance tables in NEC Chapter 9 — and your target maximum drop, normally 3%.
Voltage drop formula
Here L is the one-way length and r the conductor’s resistance per unit length at the operating temperature, from ρ ÷ A with copper at 1.724 × 10⁻⁸ Ω·m and aluminum at 2.82 × 10⁻⁸ Ω·m at 20 °C. In a single-phase or DC circuit the current flows out on one conductor and back on the other, hence the factor of 2. In a balanced three-phase circuit the line-to-line drop is √3 times the drop in one conductor.
Worked example
120 V, 15 A, 100 ft of 12 AWG copper (the default)
12 AWG copper at 75 °C has 1.931 Ω per 1,000 ft (0.006337 Ω/m).
Vdrop = 2 × 30.48 m × 0.006337 Ω/m × 15 A = 5.794 V, or 5.794 ÷ 120 = 4.83%, leaving 114.2 V at the load. The wire wastes 86.9 W as heat.
10 AWG gives 3.04% — just over the target — so the smallest size that meets 3% is 8 AWG at 1.91%. With 12 AWG, the run could be no longer than about 62 ft.
A three-phase feeder. At 480 V, 100 A over 300 ft of 2/0 aluminum, the drop is √3 × 91.44 m × 0.000511 Ω/m × 100 A = 8.09 V, only 1.69%. Higher voltage keeps the percentage low even on long runs.
Reducing voltage drop
- Use a larger conductor. Each three-gauge increase roughly halves the resistance and the drop.
- Raise the voltage. Moving a 3,600 W load from 120 V to 240 V halves the current and cuts the percentage drop to a quarter.
- Shorten the run. Relocating a subpanel closer to the loads shortens every branch circuit.
- Use copper for long, small circuits and reserve aluminum for larger feeders where its lower cost pays off.
Limits of this method
The calculator uses the DC resistance of the conductor, which is accurate for DC and for AC circuits with small conductors and loads near unity power factor. For AC conductors of 1/0 AWG and larger, skin effect and the inductive reactance of the run add to the drop, especially when the power factor is low; NEC Chapter 9, Table 9 gives effective impedance values for those cases. Motor starting current, which can be six times the running current, also causes momentary dips not shown here.
Wire geometry and resistance for any size are in the wire gauge calculator, and the watts to amps calculator converts a load’s wattage into the current to enter here.
Calculations are for estimation only. Conductor size is also set by ampacity, insulation, terminations and local amendments. Follow your local electrical code and have a licensed electrician design and install circuits.
Frequently asked questions
How do you calculate voltage drop?
For DC and single-phase circuits, the current flows out and back, so Vdrop = 2 × L × R × I, where L is the one-way length and R the resistance per unit length. For a balanced three-phase circuit, Vdrop = √3 × L × R × I. Percent drop is Vdrop ÷ source voltage × 100.
What is the maximum allowable voltage drop?
The National Electrical Code does not set a hard limit for most circuits, but its informational notes recommend no more than 3% on a branch circuit or feeder and 5% for the feeder and branch circuit combined. Some equipment, such as fire pumps and sensitive electronics, has stricter requirements.
How far can I run 12 gauge wire on a 20 amp circuit?
At 120 V with copper at 75 °C, 12 AWG stays within 3% for a one-way run of about 47 ft at a full 20 A, or about 62 ft at 15 A. Longer runs need 10 AWG or larger, even though 12 AWG has enough ampacity.
Does voltage drop depend on the voltage?
The volts lost depend only on current and wire resistance, but the percentage does not. The same 5.8 V drop is 4.8% of 120 V but only 2.4% of 240 V. That is why long runs and large loads favor higher voltages: they need less current for the same power.
Why does aluminum wire need to be larger?
Aluminum's resistivity is about 1.64 times copper's, so an aluminum conductor needs roughly 1.6 times the cross-sectional area, about two AWG sizes larger, for the same voltage drop. It is lighter and cheaper, which is why it is common for large feeders and service conductors.