The definitive wire gauge and amp chart for US residential and commercial copper wiring is based on NEC Table 310.16. For standard 120V/240V branch circuits using NM-B (Romex) cable or THHN in conduit with standard breakers, you will almost always use the 60°C column for circuits 100A or less, and the 75°C column for circuits over 100A or specific THHN conduit runs with 75°C-rated terminals. Never size your breaker based on the 90°C column unless you are using it strictly as a starting point for derating calculations.
The Master Wire Gauge and Amp Chart (NEC Table 310.16)
The following table reproduces the allowable ampacities for insulated copper conductors rated up to 2000 volts, based on the 2023/2026 National Electrical Code (NEC). Source standard: NFPA 70 (NEC) Table 310.16. Ambient temperature is assumed to be 30°C (86°F).
| AWG / kcmil | 60°C (140°F) NM-B, TW, UF |
75°C (167°F) THWN, XHHW, RHW |
90°C (194°F) THHN, XHHW-2 |
Max Standard Breaker (NEC 240.4) |
|---|---|---|---|---|
| 14 | 15A | 20A | 25A | 15A |
| 12 | 20A | 25A | 30A | 20A |
| 10 | 30A | 35A | 40A | 30A |
| 8 | 40A | 50A | 55A | 40A |
| 6 | 55A | 65A | 75A | 60A* |
| 4 | 70A | 85A | 95A | 70A |
| 3 | 85A | 100A | 115A | 100A* |
| 2 | 95A | 115A | 130A | 110A |
| 1 | 110A | 130A | 145A | 125A |
| 1/0 | 125A | 150A | 170A | 150A |
| 2/0 | 145A | 175A | 195A | 175A |
| 3/0 | 165A | 200A | 225A | 200A |
| 4/0 | 195A | 230A | 260A | 225A |
*Note on NEC 240.4(B): If the calculated ampacity does not match a standard breaker size, you are permitted to round up to the next standard size (e.g., 6 AWG at 55A allows a 60A breaker; 3 AWG at 100A allows a 100A breaker).
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For the most common residential branch circuits, memorize these baseline pairings (assuming standard 60°C terminations and NM-B cable):
- 15 Amp Breaker: 14 AWG (Lighting, general receptacles)
- 20 Amp Breaker: 12 AWG (Kitchen/bathroom receptacles, window ACs)
- 30 Amp Breaker: 10 AWG (Dryers, water heaters, RV 30A outlets)
- 40 Amp Breaker: 8 AWG (Electric ranges, EV Level 2 chargers)
- 50 Amp Breaker: 6 AWG (Hot tubs, subpanels, RV 50A outlets)
How to Read the Chart: Temperature Columns and Insulation
The most common mistake DIYers and junior electricians make is looking at a spool of THHN wire, seeing the 90°C rating printed on the jacket, and sizing the breaker using the 90°C column. This is a code violation and a fire hazard.
Which column applies to your installation? The NEC mandates that the ampacity of a circuit is limited by the lowest temperature rating of any connected component in the loop (NEC 110.14(C)).
• Use the 60°C column for circuits rated 100A or less, or for cables like NM-B (Romex) and UF, regardless of the fact that the individual wires inside NM-B might technically have 90°C insulation. The cable assembly as a whole is limited to 60°C.
• Use the 75°C column for circuits over 100A, or when using individual THWN/XHHW conductors in conduit terminated at equipment explicitly marked for 75°C (like most modern main lugs and large breakers).
The 90°C column is almost exclusively used as a mathematical starting point for derating calculations, not for final breaker sizing. For a deeper dive into termination temperature rules, refer to Electrical Contractor Magazine's code interpretations on NEC 110.14.
Derating: When the Chart's Base Values Drop
The ampacities in the master chart assume two things: an ambient temperature of 30°C (86°F), and no more than three current-carrying conductors bundled together. When you exceed these parameters, the wire cannot dissipate heat as efficiently, and you must apply derating factors (NEC 310.15).
Adjustment Factors for Bundled Conductors
| Number of Current-Carrying Conductors | Adjustment Factor (Multiplier) |
|---|---|
| 1 - 3 | 100% (No derating) |
| 4 - 6 | 80% |
| 7 - 9 | 70% |
| 10 - 20 | 50% |
Worked Numeric Example: You are pulling four current-carrying 12 AWG THHN wires through a single conduit to feed two separate 20A circuits.
- Base ampacity for 12 AWG at 90°C is 30A.
- Apply the 80% derating factor for 4 conductors: 30A × 0.80 = 24A.
- The derated wire is now good for 24A.
- However, your breaker terminals are rated 60°C (or 75°C). The 60°C limit for 12 AWG is 20A.
- Final Result: You must use a 20A breaker. The derating didn't force you to upsize the wire in this specific case, but if you had 7 wires in the pipe (70% derating: 30A × 0.7 = 21A), you would still be capped at the 20A terminal limit.
Decision Tree: Picking Your Exact AWG and Breaker
Use this if-then decision path to terminate your wire sizing process with a concrete pick. This assumes standard residential copper wiring, 60°C/75°C terminals, and runs under 75 feet.
| If your load requires... | And you are using... | Then buy and install... |
|---|---|---|
| Up to 12A continuous / 15A max | NM-B (Romex) | 14 AWG wire + 15A breaker |
| Up to 16A continuous / 20A max | NM-B (Romex) | 12 AWG wire + 20A breaker |
| Up to 24A continuous / 30A max | NM-B or THHN | 10 AWG wire + 30A breaker |
| Up to 32A continuous / 40A max | NM-B or THHN | 8 AWG wire + 40A breaker |
| Up to 40A continuous / 50A max | NM-B or THHN | 6 AWG wire + 50A breaker |
| Up to 48A continuous / 60A max | NM-B (60°C limit) | 6 AWG wire + 60A breaker (per 240.4(B)) |
| Up to 48A continuous / 60A max | THHN in conduit (75°C) | 6 AWG wire + 60A breaker |
| Up to 80A continuous / 100A max | THHN in conduit (75°C) | 3 AWG wire + 100A breaker |
What This Chart Cannot Tell You (Voltage Drop & Length)
The ampacity chart dictates the maximum current a wire can carry before the insulation melts or degrades. It does not account for voltage drop over distance. If you push 50 amps through 150 feet of 6 AWG copper wire, the wire won't catch fire, but the voltage at the far end will sag below the acceptable 5% threshold, causing motors to overheat and electronics to brown out.
NEC 310.15(B) Informational Note recommends a maximum 3% voltage drop on branch circuits and 5% total drop from the service entrance to the furthest outlet.
Concrete Default Recommendation: If your one-way wire run exceeds 75 feet, ignore the baseline chart and upsize your wire by exactly one AWG step to compensate for voltage drop.
- Example: A 50A EV charger located 120 feet from the panel. The ampacity chart says 6 AWG. The voltage drop calculator shows a 4.2% drop on 6 AWG at 240V. Decision: Upsize to 4 AWG copper to bring the drop down to a safe 2.6%.
Always pair your ampacity lookup with a voltage drop calculation for long runs. When in doubt, larger copper is always safer, provided it physically fits into the breaker and device lugs without excessive force or improper stranding.
