Voltage Drop Calculator

The formula the calculator uses

For a two-wire run, current flows out on one conductor and back on the other, so the resistance is that of twice the one-way length:

R = ρ × (2 × L) ÷ A, then VD = I × R, % drop = VD ÷ V × 100, V at load = V − VD

where ρ is resistivity in Ω·mm²/m, L the one-way length in meters (feet × 0.3048), A the cross-section in mm², I the current in amperes and V the source voltage. The calculator starts from ρ = 0.0172 for copper and 0.0282 for aluminum at 20 °C and corrects it to the conductor temperature T: ρT = ρ20 × (1 + α × (T − 20)), with α = 0.00393 per °C for copper and 0.00403 for aluminum. AWG sizes are converted with the ASTM B258 circular-mil areas (1 circular mil = 0.0005067 mm²). That copper figure is the conventional value for annealed conductor copper; pure copper is slightly lower, 1.68 × 10⁻⁸ Ω·m (0.0168 Ω·mm²/m) in Georgia State University's HyperPhysics table. Aluminum's figure is about 1.64 times copper's, which is why aluminum feeders are usually sized larger.

The model is DC resistance only. It ignores conductor reactance and power factor, which add to the drop on large AC conductors and motor loads. For a balanced three-phase circuit, the multiplier is √3 (1.732) instead of 2, so you can multiply this calculator's single-phase result by 0.866 to estimate the line-to-line drop for the same per-phase current and length.

AWG sizes, areas and resistance

Copper, per conductor, as this calculator computes it. Areas are the ASTM B258 nominal values; the 75 °C column matches NEC Chapter 9, Table 8 for solid uncoated copper (stranded wire runs about 2–3% higher, e.g. 1.98 Ω for 12 AWG).

Why 75 °C results come out 20–25% higher

Conductor resistance rises with temperature. HyperPhysics gives copper's temperature coefficient as 0.00386 per °C, so warming from 20 °C to 75 °C adds roughly 21% before stranding effects. US practice uses the DC resistances in NEC Chapter 9, Table 8, which are stated at 75 °C. The International Association of Electrical Inspectors (IAEI) expresses them as a K factor for the classic formula VD = 2 × K × I × L ÷ CM (L in feet, CM in circular mils): K = 12.9 for uncoated copper and 21.2 for aluminum.

This calculator applies that correction itself. At the default 75 °C its copper resistance is about 21.6% above the 20 °C value, equivalent to K ≈ 12.6 for solid copper (the IAEI's 12.9 reflects stranded conductors); set the temperature to 20 °C to reproduce a cool-conductor calculation. The assumed temperature is the main reason two voltage drop calculators can disagree on the same circuit: one is modeling a cool conductor, the other a conductor at its 75 °C rating.

What the NEC actually says about 3% and 5%

The NEC's voltage drop percentages sit in informational notes. The note to the branch-circuit conductor rules (210.19) says conductors sized to prevent a drop exceeding 3% at the farthest outlet, with a maximum total of 5% for feeders and branch circuits combined, provide reasonable efficiency of operation; the feeder rules (215.2) carry a parallel note. Section 90.5 states that informational notes are explanatory and not enforceable as requirements, so a circuit at 4% does not by itself violate the NEC.

Mandatory limits do exist in specific places:

• Fire pumps (695.7): the voltage at the controller line terminals must not drop more than 15% during motor starting, and the voltage at the motor terminals must not drop more than 5% while running at 115% of full-load current.
• Sensitive electronic equipment (Article 647): sets a branch-circuit limit as low as 1.5%.
• Phase converters (455.6): require a voltage drop check as part of conductor sizing.
• Federal military construction: UFC 3-501-01 (2024) makes the limits mandatory for its projects: feeders and service conductors 3%, branch circuits 3%, and no more than 5% total, measured from the point of government ownership.

Local amendments and energy codes may also adopt limits, so the controlling number is whatever your authority having jurisdiction enforces.

Bringing an excessive drop down

The drop scales with current and length and inversely with conductor area, which gives four levers. UFC 3-501-01 lists the same ones: increase conductor size, reduce the load, shorten the run, lower conductor temperature and improve power factor.

Upsizing for voltage drop does not change the ampacity rules: the wire still has to meet the NEC ampacity and termination requirements for its overcurrent device, and, per UFC 3-501-01, an equipment grounding conductor is increased in proportion when phase conductors are upsized for voltage drop (NEC 250.122).

Limits of this estimate

The calculator gives the steady-state resistive drop of one run. It does not include upstream feeder drop (add the percentages to check the 5% total), transformer regulation, motor inrush, reactance or connection resistance, and it assumes the whole run is at the temperature you enter. For anything that will be inspected, use NEC Chapter 9 values or ask a licensed electrician.