Two checks decide the wire size
A wire has to pass two tests. First, it must carry the current without overheating: its ampacity, set by the National Electrical Code (NEC) in Table 310.16, must be at least the load. Second, on long runs it must not lose too much voltage on the way, or motors run hot and lights dim. This calculator finds the smallest American Wire Gauge (AWG) size that passes both and tells you which one controls.
Required ampacity = load amps (× 1.25 if continuous)
Voltage drop = 2 × length × R × I ÷ 1,000 (single-phase) or √3 × length × R × I ÷ 1,000 (three-phase)
R = conductor resistance in ohms per 1,000 ft (NEC Chapter 9, Table 8)
Ampacity of common copper and aluminum wire
| AWG | Copper 60 °C | Copper 75 °C | Aluminum 60 °C | Aluminum 75 °C |
|---|---|---|---|---|
| 14 | 15 A* | 15 A* | – | – |
| 12 | 20 A* | 20 A* | 15 A* | 15 A* |
| 10 | 30 A* | 30 A* | 25 A* | 25 A* |
| 8 | 40 A | 50 A | 35 A | 40 A |
| 6 | 55 A | 65 A | 40 A | 50 A |
| 4 | 70 A | 85 A | 55 A | 65 A |
| 2 | 95 A | 115 A | 75 A | 90 A |
| 1/0 | 125 A | 150 A | 100 A | 120 A |
| 2/0 | 145 A | 175 A | 115 A | 135 A |
| 4/0 | 195 A | 230 A | 150 A | 180 A |
*NEC 240.4(D) limits the breaker on small conductors: 14 AWG copper to 15 A, 12 AWG to 20 A and 10 AWG to 30 A (12 AWG aluminum to 15 A, 10 AWG aluminum to 25 A), regardless of the higher table values. Values are for no more than three current-carrying conductors in a cable or raceway at an ambient temperature of 30 °C (86 °F).
Which temperature column applies?
Use the lowest temperature rating in the circuit. NM-B cable ("Romex") must be sized from the 60 °C column under NEC 334.80, even though its conductors are rated 90 °C, and NEC 110.14(C) treats terminals on equipment rated 100 A or less, or for 14–1 AWG wire, as 60 °C unless they are marked otherwise. The 75 °C column applies to wires such as THHN/THWN in conduit connected to terminals marked 75 °C, which is common on breakers and larger equipment. If you are unsure, choose 60 °C: it gives a larger, safer wire.
Worked example 1: an EV charger circuit
- A charger draws 32 A for hours at a time, so it is a continuous load: 32 × 1.25 = 40 A of required ampacity.
- Copper THHN at 75 °C: 10 AWG is only 35 A (and capped at 30 A), so 8 AWG (50 A) is the smallest that passes.
- Voltage drop over 60 ft at 240 V: 2 × 60 × 0.778 × 32 ÷ 1,000 = 2.99 V, or 1.24%, well under 3%. Answer: 8 AWG copper.
Worked example 2: a long run to a shed
- A 15 A, 120 V load 150 ft from the panel, in copper cable sized at 60 °C. Ampacity alone would allow 14 AWG.
- But 14 AWG would drop 2 × 150 × 3.14 × 15 ÷ 1,000 = 14.1 V, almost 12%. To stay within 3% (3.6 V) the wire must be 8 AWG, which drops 3.50 V (2.92%).
- Here voltage drop, not ampacity, sets the size. Running 240 V instead of 120 V halves the percentage drop, which is often the cheaper fix.
Why 3% voltage drop?
Informational notes in the NEC (210.19 and 215.2) suggest sizing conductors so that the voltage drop is no more than 3% on a branch circuit and no more than 5% in total from the service to the farthest outlet, for "reasonable efficiency of operation". They are recommendations, not mandatory rules in most places, but equipment makers often assume them. For the drop alone, use the voltage drop calculator.
Limits of this calculator
- Adjustment and correction factors are not applied. More than three current-carrying conductors together, or high ambient temperatures such as attics and rooftops, reduce ampacity (NEC 310.15).
- Motors, HVAC equipment and welders have their own sizing rules: use the minimum circuit ampacity (MCA) on the nameplate.
- Service and feeder conductors for dwellings may use special tables, and local codes can amend the NEC.
- Electrical work can be dangerous and usually needs a permit. Use this result for planning and have a licensed electrician confirm it.
Common mistakes
- Using the round-trip length as the one-way length. Enter the distance from the panel to the load; the formula doubles it.
- Forgetting the 125% rule for continuous loads such as EV chargers, water heaters and heaters.
- Reading the 90 °C column for NM cable or ordinary breakers.
- Swapping aluminum for copper at the same gauge. Aluminum carries less current; it usually needs to be two sizes larger.
Frequently asked questions
What size wire for a 30 amp circuit?
10 AWG copper is the usual minimum for a 30 A breaker under NEC 240.4(D). Long runs may need 8 AWG to keep voltage drop under 3%.
What gauge wire for 50 amps?
In the 75 °C column, 8 AWG copper is rated 50 A; at 60 °C (for example NM-B cable) you need 6 AWG copper, rated 55 A. Check voltage drop on long runs.
What size wire for 100 amps at 100 feet?
At 75 °C, 3 AWG copper (100 A) or 1 AWG aluminum (100 A) meets ampacity. At 240 V and 100 ft, 3 AWG copper drops about 4.9 V, or 2%, which is within 3%.
Why is my wire size bigger than the ampacity table says?
On long runs, voltage drop controls the size. The calculator picks the smallest wire that meets both the ampacity and the voltage drop limit.
Can I use 12 gauge wire on a 30 amp breaker?
No. NEC 240.4(D) limits 12 AWG copper to a 20 A overcurrent device, except for specific equipment such as motor circuits covered by other articles.
Sources
- NFPA 70, National Electrical Code (2023): Table 310.16 Ampacities of insulated conductors; 240.4(D) Small conductors; 110.14(C) Temperature limitations; 334.80 NM cable ampacity; 210.19(A) and 215.2(A) informational notes on voltage drop.
- NFPA 70, Chapter 9, Table 8: Conductor properties (DC resistance at 75 °C, stranded uncoated copper and aluminum).
Last reviewed 31 October 2026. Results are estimates; see our disclaimer.