Wiring
Voltage drop calculator
Voltage drop = 2 × length × resistance per foot × current for single-phase circuits. The NEC suggests no more than 3% on a branch circuit and 5% total; 12 AWG copper carrying 16 A over 100 ft drops 6.2 V, or 5.1% at 120 V.
5.15%voltage drop (6.18 V)
- Voltage at the load
- 113.8 V of 120 V
- Power lost in the wire
- 99 W
- Smallest size for ≤ 3%
- 8 AWG
- Smallest size for ≤ 5%
- 10 AWG
Check: Over 5%: motors run hot and lights dim. Go up a size or shorten the run.
Ampacity still applies: a size that passes on voltage drop may still be too small for the breaker. Check it in the wire size calculator.
Voltage drop formula
single phase / DC: Vdrop = 2 × L × R × I ÷ 1,000 three phase: Vdrop = 1.732 × L × R × I ÷ 1,000 percent = Vdrop ÷ source voltage × 100 L = one-way length (ft), R = ohms per 1,000 ft (NEC Ch. 9 Table 8, 75 °C), I = amps
The factor 2 counts the trip out and back: current flows through both the hot and the neutral (or both hots at 240 V). Resistance values are DC resistance at 75 °C conductor temperature from NEC Chapter 9, Table 8, which is what most electricians use. For AC circuits in conduit with large conductors (above about 1/0), reactance adds a little; Table 9 has those figures.
How far can you run each wire size?
Maximum one-way distance for 3% drop at full load, copper. "Too small" means the size can't carry that current on ampacity alone (60 °C column), whatever the length.
| Copper size | 15 A @ 120 V | 20 A @ 120 V | 30 A @ 240 V | 40 A @ 240 V | 50 A @ 240 V |
|---|---|---|---|---|---|
| 14 AWG | 39 ft | too small | too small | too small | too small |
| 12 AWG | 62 ft | 46 ft | too small | too small | too small |
| 10 AWG | 99 ft | 74 ft | 99 ft | too small | too small |
| 8 AWG | 157 ft | 117 ft | 157 ft | 117 ft | too small |
| 6 AWG | 244 ft | 183 ft | 244 ft | 183 ft | 146 ft |
| 4 AWG | 389 ft | 292 ft | 389 ft | 292 ft | 233 ft |
| 2 AWG | 618 ft | 463 ft | 618 ft | 463 ft | 371 ft |
Doubling the voltage halves the percentage drop for the same power, which is why long runs to a well pump or a shop are usually 240 V.
Low-voltage DC is a different world
At 12 V, every volt is 8%. A 12 V, 30 A run of 10 AWG copper only 15 ft long drops 1.09 V — 9.1%. Inverters will shut down on low voltage long before the battery is empty. For RV, van and solar battery wiring, keep runs short, use 24 or 48 V for anything big, and aim for 2–3% at most.
Is the 3% rule required?
For most circuits, no. The 3% branch / 5% total figures sit in Informational Notes to NEC 210.19 and 215.2, which are advice rather than requirements. They become mandatory for some equipment (fire pumps, sensitive electronics under 647.4(D)) and when the equipment manufacturer requires a minimum voltage. They are still good engineering: motors run hotter and fail sooner on low voltage.
Safety. Voltage drop does not replace ampacity. A size that passes on drop may still be too small for the breaker; check with the wire size calculator.
Common questions
What is an acceptable voltage drop?
3% on a branch circuit and 5% total from service to outlet, per the NEC informational notes. At 120 V that is 3.6 V and 6 V.
Does voltage drop waste electricity?
Yes: the lost volts times the current is power turned into heat in the wire. A 5% drop on a 2,000 W load wastes about 100 W.
Do I measure length one way or round trip?
One way, from the panel to the load. The formula already doubles it for the return path.