Voltage drop calculator
Enter the circuit voltage, load current, one-way length and conductor size. The calculator gives the voltage drop, the percent of supply voltage and, if the drop is over 3%, the smallest larger wire size that brings it under.
How voltage drop is calculated
This calculator uses the standard formula found in exam prep and NEC references:
Single-phase: VD = 2 × K × I × L ÷ CMThree-phase: VD = 1.732 × K × I × L ÷ CM
- K is the resistivity constant: 12.9 for copper, 21.2 for aluminum.
- I is the load current in amps.
- L is the one-way length of the circuit in feet.
- CM is the conductor area in circular mils, from NEC Chapter 9, Table 8.
Percent voltage drop is the drop divided by the supply voltage, times 100.
Worked example
A 120-volt, 20 A load is 100 ft from the panel on 12 AWG copper.
VD = 2 × 12.9 × 20 × 100 ÷ 6,530 = 7.9 V, which is 6.6% of 120 V. Moving up to 8 AWG (16,510 CM) cuts it to 3.1 V, or 2.6%.
How much voltage drop is allowed?
The NEC doesn't set a hard limit for most circuits. Informational notes recommend sizing branch circuits for no more than 3% voltage drop, and feeders plus branch circuits together for no more than 5%. Some articles, such as those for fire pumps and sensitive electronic equipment, do set mandatory limits. Exam questions usually ask you to stay at or under 3% for a branch circuit.
Frequently asked questions
Is the length one way or round trip?
One way. The 2 in the single-phase formula already accounts for the current going out and back.
Why does three-phase use 1.732 instead of 2?
In a balanced three-phase circuit the return current is shared between the phases, so the multiplier is the square root of 3 (1.732) rather than 2.
Does this calculator account for power factor or conductor temperature?
No. It uses the common exam method with fixed K values, which assumes a resistive load. For real installations with long runs or large motors, use the conductor resistance and reactance values in NEC Chapter 9, Table 9.