Voltage drop calculator
Choose conductor size and material, enter the load current and one-way length, and see the drop in volts and percent, the voltage left at the load, and the smallest wire that keeps you under your target.
5.3% is over the NEC's 5% total recommendation. Motors run hot, lights dim and electronics can misbehave. Smallest copper size for 3%: 8 AWG (2.07%).
Where the volts go
Every foot of wire is a small resistor, so the voltage slides down steadily from the panel to the load. Slide along the run to read the voltage at any point. Change the size or current above and the slope changes with it. The dashed lines mark 3% and 5% below the source.
The drop happens in both conductors: half in the hot on the way out, half in the neutral on the way back. That is where the 2 in the formula comes from.
How the calculation works
VD = 2 × I × R × L ÷ 1000 (DC, single-phase)VD = 1.732 × I × R × L ÷ 1000 (three-phase)I is load current in amps, L the one-way length in feet, and R the conductor's DC resistance in ohms per 1,000 ft from NEC Chapter 9, Table 8 at 75 °C (stranded, uncoated copper, or aluminum). The 2 accounts for the current going out and coming back; in a balanced three-phase circuit the factor is √3.
Worked example
A 20 A load on 12 AWG copper, 100 ft from the panel on 120 V: VD = 2 × 20 × 1.98 × 100 ÷ 1000 = 7.92 V, or 6.6%. Moving to 10 AWG (1.24 Ω/kft) gives 4.96 V (4.1%); 8 AWG (0.778) gives 3.1 V (2.6%).
How much drop is allowed?
The NEC's limits are recommendations in Informational Notes, not rules: 3% for a branch circuit (210.19) or feeder (215.2), and 5% for feeder plus branch combined. Some jurisdictions and energy codes make them mandatory, and some equipment (fire pumps, sensitive electronics) has its own limits. Check with your inspector.
Limits of this calculator
- It uses DC resistance. For AC on conductors of about 1/0 and larger, or in steel conduit, reactance adds to the drop; NEC Chapter 9 Table 9 gives AC impedance.
- Resistance is at 75 °C, a conservative operating temperature. Lightly loaded conductors run cooler and drop slightly less.
- Motor starting current is several times running current, so the dip at start-up is much larger than this steady-state figure.
Questions people ask
What is the voltage drop formula?
Single-phase and DC: VD = 2 × I × R × L ÷ 1000. Three-phase: VD = 1.732 × I × R × L ÷ 1000. I is current in amps, R is ohms per 1,000 ft from NEC Chapter 9 Table 8, and L is one-way length in feet.
How much voltage drop does the NEC allow?
The NEC recommends (it does not require) no more than 3% on a branch circuit or feeder and 5% total, in Informational Notes to 210.19 and 215.2. Some local codes and energy codes make limits mandatory, and sensitive equipment may need less.
Why use 75 °C resistance values?
Chapter 9 Table 8 lists DC resistance at 75 °C, a conductor running warm under load. That is conservative compared with the 20 °C figures on many spec sheets, which run about 20% lower.
Does this include AC reactance?
No. This uses DC resistance, which is accurate for DC and a close estimate for AC on small conductors at unity power factor. For large conductors (about 1/0 and up) or in steel conduit, NEC Chapter 9 Table 9 impedance values give a more precise AC result.
How do I reduce voltage drop?
Use a larger conductor, shorten the run, raise the circuit voltage (240 V instead of 120 V halves the current for the same load), or split the load across circuits.