When sizing conductors for a branch circuit or feeder, the copper wire amp chart (officially NEC Table 310.16) is your single source of truth for baseline ampacity. However, reading the chart incorrectly—specifically by using the wrong temperature column or ignoring derating factors—is one of the most common causes of overheated terminals and failed inspections. This reference guide provides the complete master table, explains exactly which column applies to your specific installation, and breaks down the derating math you need to finalize your wire size.
How to Read the NEC Copper Wire Amp Chart
The copper wire amp chart is divided into three primary temperature columns: 60°C, 75°C, and 90°C. These columns represent the thermal rating of the wire's insulation and, more importantly, the temperature rating of the termination points (lugs, breakers, and receptacles) the wire connects to.
Which column applies to your installation? The National Electrical Code (NEC) mandates the "weakest link" rule under Article 110.14(C). You must use the lowest temperature rating of any component in the circuit. For almost all residential branch circuits up to 100 amps, standard breakers and receptacles are rated for 75°C, but NM-B (Romex) cable is strictly limited to the 60°C column, regardless of the fact that the individual conductors inside might have 90°C insulation. If you are pulling individual THHN/THWN-2 conductors in conduit and terminating them on 75°C rated lugs (common in subpanels and commercial gear), you use the 75°C column. The 90°C column is almost never used for final ampacity; it exists primarily as a baseline for calculating derating adjustments.
The Master Copper Wire Ampacity Table (NEC 310.16)
The table below contains the complete baseline ampacities for copper conductors. Source standard: NFPA 70 (NEC) Table 310.16, based on an ambient temperature of 30°C (86°F) and not more than three current-carrying conductors in a raceway. For deeper engineering reference, cross-check with the Copper Development Association Ampacity Tables.
Bookmark Quick-Jumps:
- 14 AWG (15A Max OCPD)
- 12 AWG (20A Max OCPD)
- 10 AWG (30A Max OCPD)
- 8 AWG (40A / 50A)
- 6 AWG (55A / 65A)
- 4 AWG (70A / 85A)
- 2 AWG (95A / 115A)
| AWG / kcmil | 60°C (NM-B / Romex) | 75°C (THHN in Conduit) | 90°C (Derating Base) |
|---|---|---|---|
| 14 | 15A* | 20A* | 25A* |
| 12 | 20A* | 25A* | 30A* |
| 10 | 30A* | 35A* | 40A* |
| 8 | 40A | 50A | 55A |
| 6 | 55A | 65A | 75A |
| 4 | 70A | 85A | 95A |
| 3 | 85A | 100A | 110A |
| 2 | 95A | 115A | 130A |
| 1 | 110A | 130A | 145A |
| 1/0 | 125A | 150A | 170A |
| 2/0 | 145A | 175A | 195A |
| 3/0 | 165A | 200A | 225A |
| 4/0 | 195A | 230A | 260A |
Derating and Edge Cases: What the Chart Cannot Tell You
The baseline copper wire amp chart assumes ideal conditions: an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors bundled together. When real-world conditions deviate, you must apply derating factors. How derating rows modify the base value: You always start your derating math using the 90°C column, multiply by the correction factor, and then compare the result to the termination temperature column (60°C or 75°C). The final allowable ampacity is the lower of the two numbers.
Example: Bundled Conductors
You are pulling four current-carrying conductors (two 120V circuits sharing a neutral) through a single conduit. According to NEC Table 310.15(C)(1), four to six conductors require an 80% derating factor. You want to use 10 AWG THHN on a 30A breaker.
1. Base 90°C ampacity for 10 AWG = 40A.
2. Derated ampacity = 40A × 0.80 = 32A.
3. Compare to 75°C termination limit (35A). The lower number is 32A.
4. Since 32A is greater than your 30A load/breaker, 10 AWG is legal and safe.
What the table cannot tell you: Voltage Drop
The amp chart only tells you what the wire can handle thermally before the insulation melts. It does not account for distance. If you run 10 AWG copper 150 feet to a 30A RV receptacle, the wire won't overheat, but the voltage drop will exceed the recommended 3% threshold (NEC Informational Note 310.14). For long runs, you must upsize the wire purely to maintain voltage, often jumping two AWG sizes larger than the chart strictly requires for ampacity. For comprehensive voltage drop math, refer to the NFPA National Electrical Code handbook annex notes.
Copper Wire Amp Chart FAQ
What size copper wire do I need for a 50 amp breaker?
For a standard 50A breaker (like a welder or EV charger circuit), you need 6 AWG copper wire if you are using NM-B (Romex), because 6 AWG in the 60°C column is rated for 55A. If you are pulling individual THHN conductors in conduit and terminating on 75°C lugs, 8 AWG copper is technically rated for 50A in the 75°C column. However, most electricians default to 6 AWG THHN for 50A circuits to provide a buffer for voltage drop and to accommodate standard 75°C/60°C mixed termination environments without inspector pushback.
Can I use the 90°C column for my residential THHN wire to get higher ampacity?
No. This is a frequent code violation. While the PVC insulation on THHN wire is indeed rated to withstand 90°C, the lugs inside your breakers, panels, and receptacles are almost universally rated for a maximum of 75°C (and sometimes 60°C). If you push 90°C-rated current through a 75°C lug, the lug will overheat, oxidize, and eventually fail, even if the wire insulation itself survives. The 90°C column is strictly a mathematical starting point for calculating derating adjustments, not a final ampacity limit.
Does the copper wire amp chart apply to aluminum wire?
No, aluminum has a higher electrical resistance and lower thermal conductivity than copper, meaning it requires a larger physical cross-section to carry the same current safely. NEC Table 310.16 contains separate, distinct columns for aluminum conductors. For example, while 6 AWG copper is sufficient for a 60A panel feeder (using the 75°C column), you must step up to 4 AWG aluminum to achieve the same 65A ampacity. Never mix the copper and aluminum columns when sizing wire.
How does voltage drop change the wire size on the amp chart?
Voltage drop does not change the ampacity ratings printed on the chart, but it overrides your final wire selection for long runs. The NEC recommends a maximum 3% voltage drop on branch circuits and 5% total from the service entrance to the farthest outlet. If your calculation shows a 4% drop using the minimum ampacity wire size from the chart, you must increase the wire gauge (e.g., moving from 10 AWG to 8 AWG) until the drop falls below 3%, even if the smaller wire is legally allowed to handle the thermal load of the breaker.
