Voltage Drop Calculator

Every wire has some resistance, so when current flows through it a little of the supply voltage is used up driving the current along the wire itself. That loss is the voltage drop. Over a short run of adequately sized wire it is negligible, but on long runs, with thin wire or heavy loads, it can leave equipment underpowered: motors run hot and struggle to start, lights dim, and electronics may reset. The energy lost is turned into heat in the cable.

The US National Electrical Code (NEC) does not set a mandatory limit for most circuits, but an informational note recommends sizing conductors so the drop is no more than 3% on a branch circuit or feeder and no more than 5% in total from the service to the farthest outlet. On a 120 V circuit, 3% is 3.6 V.

Voltage drop calculation

For direct current and single-phase AC, current travels out on one conductor and back on another, so the length of wire carrying the current is twice the one-way distance. Three-phase circuits share the return path among the phases, which replaces the factor of 2 with √3, about 1.732.

For AC circuits in conduit, NEC Chapter 9, Table 9 lists both the resistance and the inductive reactance of each wire size. The calculator combines them into an effective impedance using the load's power factor, which is what the NEC recommends for realistic results. The wire-size option instead estimates resistance from the conductor's cross-section and the resistivity of the metal, and the other option lets you type a value from a manufacturer's data sheet.

Single phase or DC: VD = 2 × I × R × L ÷ n · Three phase: VD = √3 × I × R × L ÷ n

I

load current in amperes

R

resistance (or effective impedance) per unit length of one conductor

L

one-way length of the run

n

number of parallel conductor sets

With NEC data, R is replaced by the effective impedance Z = R × PF + X × sin(arccos PF).

Typical AWG wire sizes

American Wire Gauge (AWG) numbers run backward: the larger the number, the thinner the wire. Each step of three gauges roughly halves the cross-sectional area and doubles the resistance, and every six gauges halves the diameter. Sizes larger than 1 AWG are written 1/0, 2/0, 3/0 and 4/0 (one aught to four aught), and beyond that wires are named by their area in thousands of circular mils (kcmil), where one circular mil is the area of a circle one thousandth of an inch across.

The table below is calculated from the AWG definition, in which 36 AWG is 0.005 inch and 4/0 is 0.46 inch, with 39 equal ratio steps between them. Resistance is for solid annealed copper at 20 °C; aluminum's is about 64% higher for the same size, which is why aluminum feeders are usually two sizes larger than copper ones. Stranded wire of the same gauge has slightly higher resistance, and all metals' resistance rises with temperature, by about 0.4% per °C for copper.

Typical uses in US homes: 14 AWG for 15 A lighting circuits, 12 AWG for 20 A outlet circuits, 10 AWG for 30 A dryers and water heaters, 6 AWG for 50 A ranges and EV chargers, and 4/0 aluminum or 2/0 copper for 200 A service entrances. Always confirm sizes against the current NEC ampacity tables and local code, which also account for insulation type and installation conditions.

Frequently asked questions