Voltage Drop Calculator
Running power to a shed, a well pump, or an EV charger at the far end of the house? Wire has resistance, and long runs eat voltage. Enter the conductor, gauge, one-way distance, and load to see exactly how many volts arrive — and what gauge keeps the run inside the NEC's recommended 3%.
panel to load, one direction
Voltage drop
7.90 V
Percent drop
6.6%
Voltage at load
112.1 V
✕ 6.6% drop — exceeds the NEC-recommended 5% maximum. Upsize the wire, shorten the run, or raise the voltage. Smallest copper gauge that keeps this run ≤ 3%: 8 AWG (2.6% drop).
Uses the single-phase/DC formula Vdrop = 2 × K × I × L ÷ CM with K ≈ 12.9 (copper) or 21.2 (aluminum) — the NEC Chapter 9 convention for conductors at 75°C. Enter the one-way run length; the formula already doubles it for the round trip. Low-voltage (12/24 V) circuits are hit hardest because the same volts lost are a much bigger share of the supply.
Why Long Runs Need Thicker Wire
The single-phase formula is Vdrop = 2 × K × I × L ÷ CM, where K ≈ 12.9 for copper (21.2 for aluminum) — the NEC Chapter 9 convention for conductors at 75°C — I is the load current, L is the one-way length in feet, and CM is the wire's area in circular mils (see our wire gauge reference chart). A 20 A load on 12 AWG copper at 100 ft drops 2 × 12.9 × 20 × 100 ÷ 6,530 = 7.90 V — that's 6.6% of a 120 V supply, more than double the recommendation. Stepping up to 8 AWG (16,510 CM) cuts the drop to 3.13 V, or 2.6%, which passes.
Formula & worked example
Vdrop = 2 × K × I × L ÷ CM (single-phase or DC) K = 12.9 copper / 21.2 aluminum (NEC Ch. 9, 75°C) I = load current (A), L = one-way length (ft) CM = conductor area in circular mils 12 AWG copper, 20 A, 100 ft, 120 V: 2 × 12.9 × 20 × 100 ÷ 6,530 = 7.90 V = 6.6% ✕ fails 8 AWG copper, same run: 2 × 12.9 × 20 × 100 ÷ 16,510 = 3.13 V = 2.6% ✓ passes NEC guideline: ≤ 3% branch circuit, ≤ 5% total
Frequently Asked Questions
What wire size do I need for a 100-ft run at 20 amps?
On a 120 V circuit, 8 AWG copper: it drops 3.13 V (2.6%), just inside the NEC-recommended 3%. The 12 AWG you'd normally pair with a 20 A breaker drops 7.90 V (6.6%) at that distance — fine for ampacity, bad for voltage. On a 240 V circuit the same run passes with 10 AWG (4.97 V = 2.1%), because the drop is a smaller share of the higher voltage.
Is 6.6% voltage drop too much?
Yes — it's more than double the NEC-recommended 3% for branch circuits. At 6.6% a 120 V circuit delivers about 112 V under load: lights dim, motors run hot and start poorly, and the energy lost in the wire becomes heat in your walls. Upsize the conductor two gauges or shorten the run.
Is the NEC 3% voltage drop limit a legal requirement?
Usually not — it appears in informational notes (e.g., after NEC 210.19), which are recommendations rather than enforceable code in most jurisdictions. A few applications, like fire pumps and some sensitive-equipment circuits, do carry hard limits. Treat 3% branch / 5% total as the engineering standard for equipment that works properly.
Why is voltage drop so much worse on 12 V circuits?
Because the same volts lost are a far bigger share of the supply. Losing 1.5 V is a harmless 1.25% on a 120 V circuit but a crippling 12.5% on a 12 V one. That's why RV, boat, and solar wiring looks absurdly thick for the wattage it carries — at 12 V, even short runs need heavy gauge to stay under 3%.