Voltage Drop Calculator — NEC 3%/5% for DC, Single & Three-Phase

Every conductor resists current, and that resistance steals volts between panel and load. This voltage drop calculator turns that physics into a field answer: enter load and one-way distance, pick a wire, and read volts lost, percent drop, and volts at the load. Three modes cover the three questions electricians actually ask — does this wire pass, what is the smallest wire that passes, and how far can this wire run. The math follows NEC Chapter 9 throughout, with a fast K-method default and an Advanced Table 9 mode for motors, steel conduit, and large feeders.

NEC voltage drop rules: 3% branch, 5% total

NEC voltage drop guidance lives in two informational notes: 210.19(A) Note 4 recommends no more than 3% voltage drop on branch circuits, and 215.2(A) Note 2 recommends no more than 5% combined across feeder plus branch to the farthest outlet. These are design recommendations rather than hard code — fire pumps and a few special cases excepted — but most inspectors and engineers treat the 3% voltage drop target as the pass/fail line for branch work. Sensitive electronics, EV chargers, and motors often deserve an even tighter target, while a feeder-plus-branch total under 5% keeps the whole path honest. This page scores every result against both lines: green at or under 3%, yellow between 3% and 5%, red above 5%.

The voltage drop formula: K-method and circular mils

The voltage drop formula behind the Basic mode is the K-method from NEC Chapter 9 Table 8 notes: VD = 2 × K × I × L ÷ CM for DC and single-phase runs, and VD = √3 × K × I × L ÷ CM for three-phase, where I is load amps, L is one-way length in feet, and CM is the conductor area in circular mils. K is the resistance of a one-circular-mil, one-foot conductor: 12.9 for copper and 21.2 for aluminum at 75°C. This K method voltage drop approach is the fast default for resistive loads and quick field checks. The circular mils voltage drop math uses Table 8 areas directly, so changing one AWG size changes CM — and therefore drop — by a predictable step. For the full derivation with worked numbers, see the K vs Table 9 formula reference and the voltage drop calculator NEC Table 8/9 reference for raw R and X values.

Calculator modes: check drop, wire size for voltage drop, max distance

Check-drop mode answers “does this wire pass” for a conductor already on the truck. Min-size mode reverses the formula to find the smallest wire size for voltage drop at your target percentage — the right wire size for voltage drop on long runs where ampacity alone would undersize the circuit. Max distance mode reverses it again: given wire, load, and voltage, it reports how far the run can go before crossing 3%. Copper and aluminum, AWG through kcmil, and parallel sets are all supported, with distance in feet or metres. For precomputed answers, open the max distance chart covering common sizes, loads, and voltages.

DC voltage drop calculator: solar, battery and LED

This DC voltage drop calculator mode covers 12V, 24V, and 48V solar, battery, RV, marine, and LED runs, where percent math is brutal: losing 1V on 120V is 0.8%, but on 12V it is 8.3%. Size low-voltage runs for 2–3% with short distances and fat wire — a 10A 30 ft 12V run on #10 copper already loses 6.2% and needs #6 to reach 2.5% — or step the system up to 24V or 48V to quarter the percentage. Enter watts if you only know the load: amps = watts ÷ volts. For the full solar walkthrough, see worked voltage drop examples.

Single phase voltage drop: homes, RV and EV chargers

Single phase voltage drop covers 120V branch circuits, 240V appliances, RV hookups, and EV chargers — the runs where homeowners and apprentices meet this math first. The formula doubles the one-way length (×2) to account for the hot-plus-neutral round trip, which is why a 20A 120V kitchen run on #12 copper fails 3% at 100 ft (6.6%) and needs #8 minimum (2.6%). Higher voltage forgives distance: the same load at 240V halves the percentage, so EV chargers and dryers reach far further per AWG. Always enter one-way distance here; the ×2 is handled for you.

Three phase voltage drop: feeders and motors

Three phase voltage drop covers 208V and 480V feeders, motors, and commercial panels, where current returns through the other phases instead of a neutral. The formula therefore uses √3 instead of 2 — about 13% less drop than single-phase for the same amps, feet, and copper. Motors add two wrinkles: running power factor of 0.8–0.9 raises effective impedance, and inrush current at startup dwarfs running drop, so size for running amps and check starting conditions separately. For 480V feeders above #4, or any run in steel conduit, switch to Advanced Table 9 rather than trusting K-method alone.

Basic K-method versus Advanced Table 9

Basic K-method is the honest answer for resistive loads at PF 1.0 in PVC conduit: heaters, lighting, and small branch circuits. Advanced Table 9 computes effective impedance as Zeff = R × PF + X × sinθ, then VD = 2 × I × Zeff × L ÷ 1000 (√3 for three-phase), using the exact R and X for your wire size, conduit type, and power factor. Steel and EMT conduit add reactance that PVC does not, and low power factor stretches Zeff further — together they can add 10–30% more drop than K alone on large feeders. The Advanced panel shows every R, X, and Zeff number plus an mV/A/m cross-check, so two calculators never have to disagree about the basis again. When fill passes but the run is long, confirm conductor ampacity in the wire size and ampacity calculator and raceway space in the conduit fill calculator.

Related voltage drop resources

For distances, use the max distance chart. For method, use the voltage drop formula reference and NEC Table 8/9 reference. For the full collection, browse all electrical calculators.