Voltage Drop Calculator
A 120 V circuit that arrives at the far end as 108 V is not a wiring "mistake" you can see — it's voltage drop, and it quietly cooks motors, dims lights, and prevents heaters from reaching temperature. The fix is almost always a thicker wire, not a bigger breaker. Enter your system voltage, load current, one-way run length, and wire gauge to see the exact drop and whether it clears the NEC 3% guideline — and get the minimum AWG that would.
Circuit & Wire
The Voltage Drop Formula
Drop depends on current, resistance, and — critically — the round-trip length of the circuit:
Single-phase: V_drop = 2 × I × L × R / 1,000
Three-phase: V_drop = √3 × I × L × R / 1,000
Here R is the conductor resistance in ohms per 1,000 ft (from the wire table), L is the one-way distance, and the factor of 2 accounts for current returning on a second conductor. Doubling the length or the current doubles the drop; halving the wire's resistance (going up ~3 AWG sizes) halves it.
Worked Example: A 120 V, 20 A Run, 100 ft on 12 AWG
12 AWG copper is 1.93 Ω/1,000 ft. The drop is 2 × 20 × 100 × 1.93 ÷ 1,000 = 7.72 V, or 6.4% — more than double the 3% NEC suggestion, arriving at only 112 V. Solving for the limit: the wire can have at most (0.03 × 120 × 1000) ÷ (2 × 20 × 100) = 0.9 Ω/1,000 ft. The smallest gauge meeting that is 8 AWG (0.764), which drops just 3.06 V (2.55%). So this run needs to jump two sizes — a classic case where ampacity says "12 AWG is fine for 20 A" but voltage drop says "use 8."
Why Ampacity and Drop Are Different Checks
The NEC ampacity tables answer "will the wire overheat?" (current vs. temperature). Voltage drop answers "will the load get enough voltage?" (resistance vs. length). A wire can be perfectly safe at 20 A yet still drop too much voltage over a long run. Always run both checks; the stricter one wins. For long feeder runs to sheds, shops, or EV chargers, the drop check is usually what forces the larger wire.
Frequently Asked Questions
What is voltage drop and why should I care?
It's voltage lost to wire resistance over a run. Excess drop overheats motors, dims lights, and stops heaters reaching temperature.
What does the NEC say about allowable drop?
The NEC recommends under 3% at the farthest outlet for branch circuits, under 5% combined. Many treat 3% as the hard limit.
How do I choose the right wire gauge?
Bigger wire = less resistance = less drop. Solve for max allowable resistance, then pick the smallest AWG at or below it. The tool does this lookup.
Why is the three-phase formula different?
Single-phase uses factor 2 (out and back); three-phase uses √3 because return current splits across three 120°-offset phases.
Copper or aluminum for the same drop?
Aluminum has ~1.6–1.7× copper's resistance, so it needs ~2 sizes larger and careful termination. The calculator applies the ratio automatically.