What Size Wire for a 20 Amp Breaker?
Verified against NEC 2023, checked against the 2026 edition · Last checked August 18, 2026
A 20 amp breaker takes 12 AWG copper or 10 AWG aluminum with 75°C terminations, sized by NEC Table 310.16 with the small-conductor caps of 240.4(D). This assumes THHN-class conductors in conduit, no more than three current-carrying conductors, 86°F ambient and a non-continuous load. With 60°C terminations or NM-B cable, use 12 AWG copper · 110.14(C)(1)(a), 334.80.
Common on this rating: receptacle and small appliance circuits.
| Conductor | Size | Usable | Basis |
|---|---|---|---|
| Copper · 75°C terminals | 12 AWG | 20 A | 240.4(D) small-conductor rule |
| Aluminum · 75°C terminals | 10 AWG | 25 A | 240.4(D) small-conductor rule |
| Copper · 60°C terminals or NM-B | 12 AWG | 20 A | Table 310.16 · 60°C column · 110.14(C)(1)(a), 334.80 |
| Ground wire (EGC) | 12 AWG Cu or 10 AWG Al | Table 250.122 |
EGC from Table 250.122. If conductors are upsized for voltage drop, the EGC upsizes in proportion · 250.122(B).
What changes this answer
Continuous load · 210.19(A)
A load running 3 hours or more needs conductors sized at 125% of that current. Sizes above assume a non-continuous load.
More than 3 conductors · 310.15(C)(1)
Four or more current-carrying conductors in one raceway derate ampacity by 20% or more. Run the calculator with your count.
Hot locations · 310.15(B)(1)
Ambient above 86°F cuts ampacity. Attics and rooftop conduit routinely trigger this correction.
Long runs and voltage drop
Distance does not change the NEC minimum, but past roughly 100 feet voltage drop usually forces a larger conductor.
Round-up rule · 240.4(B)
Where a conductor ampacity is not a standard breaker rating, the next higher standard breaker is sometimes permitted. The table above never relies on it, so its sizes never depend on that permission.
Check your exact conditions
The calculator below starts at 20 A. Set your ambient, conductor count and continuous load, and it re-sizes the wire with every step cited.
| SIZE | 60°C | 75°C | 90°C |
|---|---|---|---|
| 14 AWG | 15 | 20 | 25 |
| 12 AWG | 20 | 25 | 30 |
| 10 AWG | 30 | 35 | 40 |
| 8 AWG | 40 | 50 | 55 |
| 6 AWG | 55 | 65 | 75 |
| 4 AWG | 70 | 85 | 95 |
| 3 AWG | 85 | 100 | 115 |
| 2 AWG | 95 | 115 | 130 |
| 1 AWG | 110 | 130 | 145 |
| 1/0 AWG | 125 | 150 | 170 |
| 2/0 AWG | 145 | 175 | 195 |
| 3/0 AWG | 165 | 200 | 225 |
| 4/0 AWG | 195 | 230 | 260 |
| 250 kcmil | 215 | 255 | 290 |
| 300 kcmil | 240 | 285 | 320 |
| 350 kcmil | 260 | 310 | 350 |
| 400 kcmil | 280 | 335 | 380 |
| 500 kcmil | 320 | 380 | 430 |
| 600 kcmil | 350 | 420 | 475 |
| 700 kcmil | 385 | 460 | 520 |
| 750 kcmil | 400 | 475 | 535 |
| 1000 kcmil | 455 | 545 | 615 |
Ampacities from NEC 2023 Table 310.16, 30°C ambient basis. Highlighted: your conductor size and insulation column.
Size it on the job site, offline.
The JobVolt app runs this same verified math with no signal, keeps every calculation in History, and turns it into an inspector-ready PDF submittal on your letterhead.
Common questions
- What size wire do I need for a 20 amp breaker?
- 12 AWG copper or 10 AWG aluminum with 75°C terminations, per NEC Table 310.16 and the small-conductor caps of 240.4(D). In NM-B cable or on 60°C terminals, 12 AWG copper.
- Does NM-B cable change the wire size for 20 amps?
- No. 12 AWG copper covers 20 A in NM-B cable too, since NM-B is sized from the 60°C column · 334.80.
- Does distance change the wire size for 20 amps?
- Not in Table 310.16, but long runs lose voltage. Size the run with JobVolt's free voltage drop calculator so the equipment still sees full voltage.