AmpSizerBreaker and wire size chart
At a 75 °C terminal on a short run: 15 A takes 14 AWG copper, 20 A takes 12, 30 A takes 10, 40 A takes 8, 50 A takes 8, 60 A takes 6, 100 A takes 3, and 200 A takes 3/0. Aluminium is two sizes larger.
Notice that 40 A and 50 A share a conductor. 8 AWG copper carries 50 A, so it covers both — the pairing is a floor, not a one-to-one mapping.
Three things move any row of this chart: 240.4(D) caps 14, 12 and 10 AWG regardless of the table, a continuous load raises the current by 25% before you start, and distance can push the conductor up without changing the breaker at all.
The chart
| Breaker | Copper | Aluminium | Copper ground | What it usually feeds |
|---|---|---|---|---|
| 15 A | 14 AWG | 12 AWG | 14 AWG | Lighting and general receptacles |
| 20 A | 12 AWG | 12 AWG | 12 AWG | Kitchen, bathroom and garage receptacles |
| 30 A | 10 AWG | 10 AWG | 10 AWG | Dryer, small water heater, RV outlet |
| 40 A | 8 AWG | 8 AWG | 10 AWG | Range, 40 A EV charger |
| 50 A | 8 AWG | 6 AWG | 10 AWG | Range, welder, 50 A EV charger, hot tub |
| 60 A | 6 AWG | 4 AWG | 10 AWG | Sub panel, large hot tub, tankless heater |
| 70 A | 4 AWG | 3 AWG | 8 AWG | Sub panel, larger tankless heater |
| 100 A | 3 AWG | 1 AWG | 8 AWG | Sub panel, or a small service |
| 125 A | 1 AWG | 2/0 AWG | 6 AWG | Sub panel |
| 150 A | 1/0 AWG | 3/0 AWG | 6 AWG | Service, or a large sub panel |
| 200 A | 3/0 AWG | 250 kcmil | 6 AWG | Typical whole-house service |
The three rules that move a row
240.4(D), the small-conductor rule. 14 AWG copper is held to 15 A, 12 AWG to 20 A and 10 AWG to 30 A — whatever Table 310.16 says they can carry. It is why 12 AWG reads 25 A in the table and is still a 20 amp circuit, and it is the difference between the ampacity chart and this one.
Continuous loads. Anything on for three hours or more is taken at 125%. A 40 A EV charger is a 50 A circuit; a 48 A charger is a 60 A circuit. The device moves and the conductor moves with it.
Distance. Voltage drop can force the conductor up two sizes on a long run while the breaker stays exactly where it was. When that happens, 250.122(B) pulls the ground up in the same proportion, and that is the step most people miss.
In cable rather than conduit
Non-metallic sheathed cable is used at the 60 °C ampacity under 334.80, so the pairing changes:
| Circuit | In conduit (THHN) | In NM cable |
|---|---|---|
| 15 A | 14 AWG | 14/2 |
| 20 A | 12 AWG | 12/2 |
| 30 A | 10 AWG | 10/3 |
| 40 A | 8 AWG | 8/3 |
| 50 A | 8 AWG | 6/3 |
| 60 A | 6 AWG | 4/3 |
The 50 and 60 A rows are where the two columns part company, and it is exactly the range where hot tubs, tankless heaters and sub panels live.
What each circuit usually feeds
The right-hand column of the main chart is the practical version of this whole page. Two rows are worth calling out because they are the ones people get wrong:
30 A at 240 V is a dryer or a small water heater — and it is also the largest circuit that 10 AWG can serve, so it is the natural ceiling of a Romex-only installation.
60 A at 240 V is where sub panels start, and where the cable-versus-conduit distinction stops being academic. A 60 A feeder in NM cable is 4/3, which is expensive and stiff; the same feeder in conduit is 6 AWG. That difference decides the wiring method on a lot of garage projects.
Work out your own pairing, including distance and derating, on the breaker size calculator or the wire size calculator.
Frequently asked questions
What size wire for a 60 amp breaker?
6 AWG copper or 4 AWG aluminium at 75 °C, with a 10 AWG copper ground. In NM cable it is 6/3, which is rated 55 A on the 60 °C column — so a full 60 A load in Romex needs 4/3. That gap between cable and conduit conductors catches people on sub panel feeds and tankless heaters.
Can I use a 30 amp breaker on 12 gauge wire?
No. 240.4(D) holds 12 AWG copper to a 20 amp device whatever the ampacity table says, so a 30 A breaker on 12 AWG leaves the conductor unprotected. This is the most common unsafe modification in domestic wiring, and it usually starts with a breaker that kept tripping.
Why do 40 and 50 amp circuits use the same wire?
Because 8 AWG copper carries 50 A at 75 °C, which covers a 40 A circuit with room to spare. The chart gives the smallest conductor each device may protect; using the same size for both is normal and means an existing 40 A circuit can often be uprated to 50 by changing the breaker — after checking the terminals and the load.
Does the ground wire scale with the breaker?
It scales in steps rather than continuously: everything from 25 to 60 A uses a 10 AWG copper ground, 70 to 100 A uses 8 AWG, and 110 to 200 A uses 6 AWG. Table 250.122 is indexed on the device rating, not on the conductor size, so upsizing the hots does not by itself change the ground — unless you upsized them for voltage drop, and then 250.122(B) does.
Is this chart the same for a feeder as for a branch circuit?
The conductor arithmetic is the same, but two things differ. A feeder normally carries a neutral, which changes the conduit and can change the derating count. And a dwelling service or main feeder may use the 83% allowance of 310.12, which lets a 100 A feeder be 4 AWG copper instead of 3.
How we calculate this
Sources: