The ampacity of 10 AWG copper wire is 30 amps in the 60°C column, 35 amps in the 75°C column, and 40 amps in the 90°C column, according to NEC Table 310.16. However, for standard residential and commercial overcurrent protection, NEC Article 240.4(D) strictly limits the breaker size for 10 AWG copper to a maximum of 30 amps, regardless of the insulation's higher temperature rating.
Knowing the raw number from the ampacity chart is only the first step. To safely size your conductors and breakers without failing inspection or creating a fire hazard, you must understand which temperature column applies to your specific installation, how bundling derates your wire, and where the National Electrical Code (NEC) overrides the base table values.
The Core AWG 10 Amp Rating Chart (NEC Table 310.16)
Before pulling wire, you need to know how to read the official ampacity tables. The data below is extracted from NEC Table 310.16 (formerly 310.15(B)(16) in older code cycles), which dictates the allowable ampacities of insulated conductors rated up to 2000 volts.
| AWG Size | 60°C Column (140°F) | 75°C Column (167°F) | 90°C Column (194°F) | Common Insulation Types |
|---|---|---|---|---|
| 14 AWG | 15A | 20A | 25A | TW, UF-B |
| 12 AWG | 20A | 25A | 30A | NM-B (Romex), THW |
| 10 AWG | 30A | 35A | 40A | NM-B, THHN, THWN-2 |
| 8 AWG | 40A | 50A | 55A | NM-B, THHN |
| 6 AWG | 55A | 65A | 75A | NM-B, THHN, XHHW |
Bookmark Quick-Jump Notes for 10 AWG:
- NM-B Cable (Romex): Use the 60°C column (30A). Even if the individual wires inside the sheath are rated 90°C, the NEC mandates NM-B be treated as 60°C for ampacity.
- THHN in Conduit: Use the 90°C column (40A) strictly as your starting point for derating calculations, but terminate based on the 75°C column.
- Breaker Sizing: Capped at 30A by NEC 240.4(D).
Selecting the Right Temperature Column for Your Wire
A common mistake on the jobsite is looking at a spool of THHN wire, seeing the '90°C' printed on the jacket, and immediately jumping to the 40-amp column. This is a violation of NEC 110.14(C), which governs termination provisions.
The rule is simple: the ampacity of your circuit is limited by the lowest temperature rating of any connected device, terminal, or conductor in the entire run. You must evaluate the entire chain to find the weakest link.
Scenario A: Running NM-B (Romex) to a Receptacle
If you are wiring a 240V baseboard heater or a heavy-duty 30A RV outlet using 10/2 NM-B cable, your wire is legally bound to the 60°C column. NM-B cable assemblies are explicitly restricted to the 60°C ampacity column by NEC 334.80. Your maximum continuous load capacity is 30 amps, and you must use a 30-amp breaker.
Scenario B: Pulling THHN through EMT Conduit
If you are pulling individual 10 AWG THHN/THWN-2 conductors through metal EMT conduit to a subpanel or a hardwired appliance, the wire insulation itself is rated for 90°C. However, the lugs on your standard residential breakers and disconnects are almost universally rated for 75°C.
Therefore, your final allowable ampacity for termination is the 75°C column (35 amps). You use the 90°C column (40 amps) only for math—specifically, to calculate derating adjustments before the wire hits the terminal. For a deeper look at termination temperature rules, refer to the NFPA National Electrical Code guidelines.
Derating Factors and the 30-Amp Breaker Hard Cap
Ampacity tables assume ideal conditions: a 30°C (86°F) ambient environment and no more than three current-carrying conductors bundled together. When real-world conditions deviate, you must apply adjustment factors. This is where the 90°C column becomes a vital tool, even if your terminations are only rated for 75°C.
How Derating Modifies the Base Value
According to NEC 310.15(C)(1), when you bundle more than three current-carrying conductors in a single conduit, the heat generated by the wires cannot dissipate efficiently. You must multiply the base ampacity by a derating percentage.
Worked Numeric Example:
You are pulling four 10 AWG THHN current-carrying conductors (two 240V circuits sharing a neutral, or two multi-wire branch circuits) through a single conduit in a 30°C attic.
- Start with the 90°C column for THHN: 40 amps.
- Check NEC Table 310.15(C)(1) for 4-6 conductors: The adjustment factor is 80%.
- Multiply: 40A × 0.80 = 32 amps.
- Compare to termination limits: 32A is less than the 75°C column limit (35A), so the derated wire is perfectly safe to terminate on 75°C lugs.
If you had used six conductors (derating to 50%), your math would be 40A × 0.50 = 20 amps. You would be forced to upsize to 8 AWG wire to maintain a 30-amp circuit.
What the Table Cannot Tell You: NEC 240.4(D)
The most critical piece of information missing from Table 310.16 is the overcurrent protection limit. If you only read the table, you might assume that because 10 AWG THHN can handle 40 amps (after derating) and your lugs are rated for 75°C (35 amps), you could protect the circuit with a 35A or 40A breaker.
This is a code violation. NEC Article 240.4(D) is a specific override for small conductors. It states that the overcurrent protection for 10 AWG copper conductors shall not exceed 30 amps, regardless of the insulation rating or the calculated derated ampacity.
This rule exists because standard residential terminals and cheap aftermarket receptacles may not reliably handle the thermal stress of 35A or 40A continuous faults on a wire this thin. There are exceptions to 240.4(D) for specific equipment like motor circuits (Article 430), air conditioning equipment (Article 440), and welders (Article 630), where the equipment listing dictates the breaker size. But for general branch circuits, lighting, and standard appliance feeds, 30 amps is your absolute ceiling.
For comprehensive manufacturer specifications on wire insulation thermal limits and bundling capacities, consult the Cerrowire official ampacity and derating tables. Always verify your final design against your local Authority Having Jurisdiction (AHJ), as local inspectors may have amendments that further restrict conductor sizing in your specific municipality.
