Wire size calculator
A 40 A circuit at 240 V running 100 ft in copper needs 8 AWG — the ampacity table and the voltage drop agree on that size here, at 2.60% drop. They often do not, and the run length is what separates them: the same 40 A at 200 ft is decided by voltage drop alone and needs a larger conductor than the table asks for.
The smallest conductor that passes all four tests: the ampacity table, the temperature its terminations are rated to, the small-conductor rule, and the voltage drop over the run. It says which one decided the answer, because on a long run it is almost never the table.
NEC 310.16, 310.15 and 240.4(D) · How we check
Smallest conductor that satisfies every rule
8 AWG
8 AWG — decided by both the ampacity table and the voltage drop, which land on the same size. It carries 50 A here and drops 2.44%.
- Current the conductor is sized for
- 50.0 A
- Its ampacity here, after every derate
- 50 A
- Voltage lost in the run
- 5.86 V — 2.44%
- Ampacity alone would want
- 8 AWG
- Voltage drop alone would want
- 8 AWG
- Correction × adjustment
- 1 × 1 = 1.000
Every size, under these conditions
| Size | Table ampacity | Usable here | Drop over 75 ft |
|---|---|---|---|
| 14 AWG | 25 A | 20 A | 9.81% |
| 12 AWG | 30 A | 25 A | 6.17% |
| 10 AWG | 40 A | 35 A | 3.88% |
| 8 AWG ← | 55 A | 50 A | 2.44% |
| 6 AWG | 75 A | 65 A | 1.54% |
| 4 AWG | 95 A | 85 A | 0.97% |
Branch circuits, short runs
Copper and aluminium on 75°C terminations at 30°C, three conductors in the raceway, run short enough that voltage drop does not bind. These are the sizes to reach for before anything unusual applies.
| Breaker | Copper | Aluminium | Longest run at 240 V, 3% |
|---|---|---|---|
| 15 A | 14 AWG | 12 AWG | 76 ft |
| 20 A | 12 AWG | 10 AWG | 91 ft |
| 30 A | 10 AWG | 8 AWG | 96 ft |
| 40 A | 8 AWG | 8 AWG | 115 ft |
| 50 A | 8 AWG | 6 AWG | 92 ft |
| 60 A | 6 AWG | 4 AWG | 122 ft |
| 100 A | 3 AWG | 1 AWG | 146 ft |
| 125 A | 1 AWG | 2/0 AWG | 186 ft |
| 150 A | 1/0 AWG | 3/0 AWG | 196 ft |
| 200 A | 3/0 AWG | 250 kcmil | 234 ft |
The last column is where most of the surprises live. A 20 A circuit holds 3% for nearly 200 ft; a 100 A feeder on 3 AWG runs out at 87 ft, and a detached garage further away than that needs a larger conductor than any ampacity table will tell you.
Dwelling services, where the 83% rule applies
NEC 310.12 lets the service or feeder carrying the entire load of a one-family dwelling be sized at 83% of its rating. It is the reason every electrician says 2/0 for a 200 A service while table 310.16 says 3/0 — and the reason a calculator that only knows 310.16 looks wrong to the people who use it most.
| Service | Sized as | Copper | Aluminium | On 310.16 alone |
|---|---|---|---|---|
| 100 A | 83 A | 4 AWG | 2 AWG | 3 AWG |
| 125 A | 104 A | 2 AWG | 1/0 AWG | 1 AWG |
| 150 A | 125 A | 1 AWG | 2/0 AWG | 1/0 AWG |
| 200 A | 166 A | 2/0 AWG | 4/0 AWG | 3/0 AWG |
| 400 A | 332 A | 400 kcmil | 14 AWG | paralleled |
Questions people actually ask
- What size wire do I need for a 50 amp circuit?
- 8 AWG copper on ordinary 75°C terminations, up to about 90 ft at 240 V before voltage drop pushes it to 6 AWG. 8 AWG copper is 55 A in the 90°C column and 50 A at 75°C, and it is the 75°C figure that counts because that is what the breaker and the lugs are rated for.
- What size wire for a 200 amp service?
- 2/0 copper or 4/0 aluminium — and that is not table 310.16, which says 3/0 copper. NEC 310.12 lets the service or feeder carrying the entire load of a one-family dwelling be sized at 83% of the rating, so 200 A is sized as 166 A. Tick the dwelling box above and the calculator applies it; leave it clear and you get 3/0, which is the right answer for a 200 A load that is not a house service.
- Is 14 gauge wire good for 20 amps?
- No. 14 AWG copper is 20 A in the 90°C column of table 310.16, which is where the claim comes from, but NEC 240.4(D)(3) caps the overcurrent device protecting it at 15 A. The table ampacity is used for derating arithmetic; the breaker cannot exceed 15 A whatever the arithmetic produces.
- Why does my 90°C wire not get 90°C ampacity?
- NEC 110.14(C). A conductor may be derated starting from its own column, but the ampacity it is finally used at may not exceed the column matching its terminations — 75°C on ordinary breakers and lugs, and 60°C on equipment rated 100 A or less unless it is marked otherwise. THHN is a 90°C conductor landing on 75°C screws, so 6 AWG is 65 A rather than 75.
- How far can I run a circuit before voltage drop matters?
- It depends on the current and the voltage, and it bites much sooner at low voltage. 20 A at 240 V holds 3% for about 195 ft on 12 AWG copper; the same 20 A at 12 V needs 4 AWG to cross 20 ft. The NEC does not require 3% — it is an informational note under 210.19(A) — but equipment does not work well at the bottom of a long run.
- Does the number of wires in the conduit change the size?
- Yes, once there are more than three current-carrying conductors. NEC 310.15(C)(1) derates to 80% at four to six, 70% at seven to nine and 50% from ten to twenty. Neutrals on a balanced multiwire circuit and equipment grounds are not counted; a neutral carrying only harmonic current is.
- What about ambient temperature?
- Table 310.16 assumes 30°C. Above that, NEC 310.15(B)(1) applies a correction — 0.88 for a 75°C conductor at 36-40°C, 0.75 at 46-50°C. An attic or a rooftop conduit in summer is genuinely at 50°C, which is a fifth of the ampacity gone before the wire has carried anything.
Sources
- NFPA 70 — National Electrical Code, free online access
- NEC 310.15(B)(1) — correction factors for ambient temperature
- Copper Development Association — ampacity adjustments for wire and cable
An estimate from the published tables, not a design. The NEC has more rules than any calculator applies — derating for conduit fill, terminations marked otherwise, motor and welder circuits with their own articles — and the authority having jurisdiction decides. Work that needs a permit needs an electrician.