If you need the direct answer for standard residential copper branch circuits: a 15-amp breaker requires 14 AWG, a 20-amp breaker requires 12 AWG, a 30-amp breaker requires 10 AWG, a 40-amp breaker requires 8 AWG, and a 50-amp breaker requires 6 AWG.
However, electrical wiring is not just about matching a single number. The National Electrical Code (NEC) dictates ampacity based on insulation temperature ratings, termination limits, and installation conditions. Below is the complete breaker size wire size chart based on NEC Table 310.16 and NEC 240.4, followed by the critical derating rules that modify these base values.
How to Read This Breaker Size Wire Size Chart
Before pulling wire, you must understand which temperature column applies to your specific installation. The chart below contains three ampacity columns: 60°C, 75°C, and 90°C. Here is how to determine which one governs your circuit:
- The 60°C Column: Per NEC 240.4(D), small conductors (14, 12, and 10 AWG copper) are strictly limited to the 60°C ampacity column for overcurrent protection, regardless of whether the wire insulation is rated for 90°C (like THHN). Additionally, Non-Metallic Sheathed Cable (NM-B, commonly known as Romex) is always evaluated using the 60°C column per NEC 310.15(B)(2)(3).
- The 75°C Column: This is the standard termination rating for most modern residential circuit breakers, receptacles, and switches (NEC 110.14(C)). For wire sizes 8 AWG and larger, you generally use the 75°C column to determine ampacity, provided your terminations are rated for 75°C.
- The 90°C Column: You cannot use the 90°C ampacity to size your breaker. This column is exclusively used as the starting baseline for calculating derating adjustments (like bundling wires in a conduit or high ambient temperatures). After applying derating math to the 90°C column, you compare the result to the 75°C column and use the lower of the two values.
Assumptions for this chart: Copper conductors, not more than three current-carrying conductors in a raceway, and an ambient temperature of 30°C (86°F).
Master NEC Copper Ampacity and Breaker Sizing Table
The following data is sourced directly from NFPA 70 (NEC 2023) Table 310.16 for copper wire ampacity, combined with NEC 240.4(B) and 240.6 for standard overcurrent device (breaker) sizing.
| Wire Size (AWG/kcmil) | 60°C Ampacity (NM-B / Small Wire Limit) | 75°C Ampacity (Standard Terminations) | 90°C Ampacity (THHN Derating Baseline) | Max Standard Breaker Size |
|---|---|---|---|---|
| 14 AWG | 15A | - | - | 15A |
| 12 AWG | 20A | 25A | 30A | 20A* |
| 10 AWG | 30A | 35A | 40A | 30A* |
| 8 AWG | 40A | 50A | 55A | 40A / 50A |
| 6 AWG | 55A | 65A | 75A | 60A |
| 4 AWG | 70A | 85A | 95A | 70A / 80A |
| 3 AWG | 85A | 100A | 115A | 100A |
| 2 AWG | 95A | 115A | 130A | 100A / 110A |
| 1 AWG | 110A | 130A | 145A | 125A |
| 1/0 AWG | 125A | 150A | 170A | 150A |
| 2/0 AWG | 145A | 175A | 195A | 175A |
| 3/0 AWG | 165A | 200A | 225A | 200A |
| 4/0 AWG | 195A | 230A | 260A | 225A / 250A |
*Note on 12 AWG and 10 AWG: While the 75°C column shows 25A and 35A, NEC 240.4(D) strictly mandates maximum overcurrent protection of 20A and 30A respectively for these specific sizes.
• 15A Circuit (Lighting/Bedrooms): 14 AWG Copper
• 20A Circuit (Kitchen/Bath Receptacles): 12 AWG Copper
• 30A Circuit (Dryer/Water Heater): 10 AWG Copper
• 50A Circuit (Range/Hot Tub): 6 AWG Copper
What This Chart Cannot Tell You: Derating and Voltage Drop
A common mistake among DIYers and junior apprentices is treating the ampacity table as an absolute law of physics. The table assumes ideal conditions. Here is what the chart omits and how you must adjust your wire size in the real world.
1. Conductor Bundling and Derating
If you pull more than three current-carrying conductors through a single conduit or bored hole, the wires heat each other up. Per NEC 310.15(C)(1), you must apply a derating factor. For example, if you run four 12 AWG THHN wires in a conduit (two circuits sharing a neutral, or a multi-wire branch circuit with specific harmonics), you multiply the 90°C baseline (30A) by 80%.
Math: 30A x 0.80 = 24A. Since 24A is greater than the 20A breaker requirement, 12 AWG is still acceptable. However, if you bundle 7 to 9 conductors, the multiplier drops to 70% (30A x 0.70 = 21A), pushing you dangerously close to the limit and requiring careful termination checks.
2. Ambient Temperature Corrections
If your conduit runs through a 115°F (46°C) attic in Texas or Arizona, the 30°C baseline is invalid. You must apply the temperature correction factors from NEC Table 310.15(B)(1). A 90°C THHN wire in a 46°C attic requires a 0.82 multiplier. Always do the math before sealing the walls.
3. Voltage Drop Over Distance
The NEC ampacity tables only address thermal limits (preventing the wire from melting). They do not account for voltage drop over long distances. If you are running a 240V circuit to a detached garage 150 feet away, a 6 AWG wire on a 50A breaker might not overheat, but the voltage at the far end could drop below 228V (a >5% drop), causing motors to overheat or electronics to brown out. For runs over 100 feet, always use a voltage drop calculator and expect to upsize your wire by one or two AWG steps.
Breaker and Wire Sizing FAQ
What size wire do I need for a 20-amp breaker?
You need 12 AWG copper wire. While 12 AWG THHN has a 90°C ampacity of 30A, NEC 240.4(D) explicitly caps the overcurrent protection for 12 AWG at 20A. If you are using NM-B (Romex) cable, it is strictly evaluated at the 60°C column (20A), making 12 AWG the exact and only match for a 20A breaker.
Can I use 10 AWG wire on a 20-amp breaker?
Yes. NEC 240.4(B) allows you to use the "next size up" breaker, which inherently means you can use a larger wire on a smaller breaker. Upsizing to 10 AWG on a 20A circuit is an excellent way to mitigate voltage drop on long runs. Warning: 10 AWG stranded wire may not physically fit into the backstab terminals or smaller screw terminals of standard 20A receptacles. You may need to use a 12 AWG pigtail or a receptacle specifically rated for #10 wire termination.
What size breaker for 6 AWG copper wire?
For standard residential applications using 6 AWG copper, the maximum breaker size is 60 amps. If you are using NM-B cable, the 60°C column rates 6 AWG at 55A. Because 55A is not a standard breaker size listed in NEC 240.6(A), NEC 240.4(B) permits you to round up to the next standard size, which is 60A. If using THHN in conduit (75°C column), the ampacity is 65A, but standard practice and termination limits usually keep this on a 60A breaker.
Does the ground wire count towards conduit derating?
No. Per NEC 310.15(B)(6), equipment grounding conductors (bare copper or green) are not counted as current-carrying conductors when applying the bundling adjustment factors in Table 310.15(C)(1). Only the hot and neutral (or grounded) conductors that carry normal operating current are counted. However, if you are running a multi-wire branch circuit (MWBC) with a shared neutral, the neutral is counted as a current-carrying conductor if it carries unbalanced current from non-linear loads.
