Wire Size & Voltage Drop Calculator
Wire that is too small for the current overheats, and wire that is too long for its size wastes voltage. This calculator checks both. It finds the smallest copper or aluminum conductor that is rated for your current and keeps voltage drop within your limit. It is handy for a generator inlet far from the panel, a detached garage or a long extension run.
Wire Size & Voltage Drop Calculator
Find the smallest conductor that carries your current and keeps voltage drop within your limit, for example between a generator inlet and the panel.
Uses NEC Chapter 9 Table 8 resistance and the 60 °C ampacity column of NEC Table 310.16 (with the small-conductor limits of 240.4(D)). Temperature, bundling, conduit fill and local amendments can require a larger size. Have an electrician confirm before installing.
How the wire size is chosen
- Ampacity: the conductor must be rated for at least your current. The calculator uses the 60 °C column of NEC Table 310.16, which applies to most residential terminations, with the small-conductor limits of NEC 240.4(D) (14 AWG copper 15 A, 12 AWG 20 A, 10 AWG 30 A).
- Voltage drop: the drop along the run must stay within your limit. Resistance values come from NEC Chapter 9, Table 8.
The result is the smallest size that passes both tests.
Voltage drop formula
- Single-phase: VD = 2 × L × I × R ÷ 1,000
- Three-phase: VD = √3 × L × I × R ÷ 1,000
L is the one-way length in feet, I is the current in amps and R is the conductor resistance in ohms per 1,000 feet. The factor 2 accounts for current flowing out and back.
Worked example: generator inlet 100 feet from the panel
A 30 A, 240 V generator inlet (L14-30) with a 100-foot copper run:
- 10 AWG is rated 30 A, so it passes on ampacity. But the drop is 2 × 100 × 30 × 1.24 ÷ 1,000 = 7.44 V, which is 3.1%, just over a 3% limit.
- 8 AWG drops 2 × 100 × 30 × 0.778 ÷ 1,000 = 4.67 V, which is 1.9%.
- Result: 8 AWG copper.
Reference: resistance and ampacity
| Size | Copper Ω/1,000 ft | Copper ampacity (60 °C) | Aluminum Ω/1,000 ft | Aluminum ampacity (60 °C) |
|---|---|---|---|---|
| 14 AWG | 3.14 | 15 A | — | — |
| 12 AWG | 1.98 | 20 A | 3.25 | 15 A |
| 10 AWG | 1.24 | 30 A | 2.04 | 25 A |
| 8 AWG | 0.778 | 40 A | 1.28 | 35 A |
| 6 AWG | 0.491 | 55 A | 0.808 | 40 A |
| 4 AWG | 0.308 | 70 A | 0.508 | 55 A |
| 2 AWG | 0.194 | 95 A | 0.319 | 75 A |
| 1/0 AWG | 0.122 | 125 A | 0.201 | 100 A |
| 2/0 AWG | 0.0967 | 145 A | 0.159 | 115 A |
| 4/0 AWG | 0.0608 | 195 A | 0.100 | 150 A |
Important limitations
- Ampacity must be reduced for high ambient temperatures, more than three current-carrying conductors in a raceway, and some cable types. Larger conductors may be allowed a higher temperature column when all terminations are rated for it.
- The NEC requires generator output conductors to be rated for at least 115% of the generator’s nameplate current in many installations (NEC 445.13).
- Local codes and inspectors have the final say. Always have permanent wiring installed by a licensed electrician.
Frequently asked questions
What size wire do I need for a 30 amp generator inlet?
10 AWG copper for short runs. Beyond about 97 feet at 240 V, voltage drop pushes it to 8 AWG if you keep to 3%. Run your exact distance in the calculator.
Does voltage drop matter for extension cords?
Yes. Motors run hotter and may fail to start on low voltage. Cord gauges for generators are covered in our guide to connecting a generator safely.