How voltage drop is calculated
Every wire has resistance, so some voltage is lost between the panel and the load. The loss grows with current and length and shrinks with larger wire. This calculator uses the DC resistance at 75°C published in NEC Chapter 9, Table 8:
- Single-phase and DC: drop = 2 × amps × ohms per 1,000 ft × one-way feet ÷ 1,000
- Three-phase: drop = 1.732 × amps × ohms per 1,000 ft × one-way feet ÷ 1,000
Resistance of common copper wire
| Size | Solid (Ω per 1,000 ft) | Stranded (Ω per 1,000 ft) |
|---|---|---|
| 14 AWG | 3.07 | 3.14 |
| 12 AWG | 1.93 | 1.98 |
| 10 AWG | 1.21 | 1.24 |
| 8 AWG | 0.764 | 0.778 |
| 6 AWG | — | 0.491 |
| 4 AWG | — | 0.308 |
Questions people ask
What is an acceptable voltage drop?
Informational notes in the National Electrical Code recommend no more than 3% on a branch circuit and 5% for the feeder and branch circuit combined. They are recommendations, not requirements, but following them keeps lights and motors working properly.
How do you calculate voltage drop?
For single-phase circuits, voltage drop = 2 × current × resistance per foot × one-way length. For three-phase, replace 2 with 1.732 (√3). Resistance comes from NEC Chapter 9, Table 8.
Should I enter the one-way or round-trip length?
Enter the one-way length from the panel to the load. The formula already accounts for the current going out and back.
What size wire do I need for 100 feet?
It depends on the current and voltage. At 120 V and 16 A, 100 ft of stranded copper needs 8 AWG to stay under 3%: 12 AWG drops about 5.3% and 10 AWG about 3.3%. Enter your circuit to see the smallest size that works.
Does this tell me what size wire I need?
It shows the smallest wire that both keeps voltage drop within your target and is rated for the current under NEC Table 310.16, including the 15, 20 and 30 A breaker limits for 14, 12 and 10 AWG copper (240.4(D)). It does not apply derating for hot locations or bundled cables, so check those with an electrician.