When sizing conductors for branch circuits, feeders, or subpanels, you need exact ampacity figures, not guesswork. Whether you are running NM-B (Romex) through wall cavities or pulling THHN through EMT conduit, the allowable current depends strictly on the conductor material, insulation temperature rating, and installation conditions. Often mistyped by apprentices searching for a wire guage table, this master chart provides the exact allowable ampacities for copper conductors based on the National Electrical Code (NEC).
Base Assumptions for this Chart: Copper conductors, not more than three current-carrying conductors in a raceway or cable, and an ambient temperature of 30°C (86°F). Always verify local AHJ requirements, as local code amendments can override baseline NEC guidance.
The Master Wire Gauge Table (NEC 310.16)
How to read this table: The ampacity columns represent the maximum continuous current the wire insulation can handle before degrading. The 60°C column applies primarily to non-metallic sheathed cables (NM-B/Romex) and older terminals. The 75°C and 90°C columns apply to individual conductors in conduit (like THHN/THWN-2). The final column lists the maximum standard overcurrent device (breaker) size permitted by NEC 240.4(D) for small conductors and standard rounding rules for larger sizes.
- 15A Lighting/Receptacle Circuits: Use 14 AWG (60°C column).
- 20A Kitchen/Bathroom Circuits: Use 12 AWG (60°C column).
- 30A Dryer/HVAC Circuits: Use 10 AWG (60°C column).
- 50A Range/Subpanel Feeder: Use 6 AWG NM-B or 8 AWG THHN (75°C column).
| AWG Size | 60°C (NM-B / TW) | 75°C (THHN Wet / XHHW) | 90°C (THHN Dry) | Max Standard Breaker |
|---|---|---|---|---|
| 14 AWG | 15A | 20A | 25A | 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 | 80A / 90A |
| 3 AWG | 85A | 100A | 110A | 100A |
| 2 AWG | 95A | 115A | 130A | 125A |
| 1/0 AWG | 125A | 150A | 170A | 150A |
* NEC 240.4(D) strictly limits 14 AWG to 15A, 12 AWG to 20A, and 10 AWG to 30A for standard branch circuits, regardless of the 75°C or 90°C column values.
Which Temperature Column Applies to Your Installation?
The most common mistake DIYers and junior electricians make is looking at the 90°C column for THHN wire and assuming they can use that higher ampacity to downsize the wire. According to NEC 110.14(C), the allowable ampacity of a circuit is dictated by the lowest temperature rating of any connected component.
In residential and light commercial work, standard breakers, receptacles, and switches are typically rated for 75°C terminations. However, if you are using NM-B cable (commonly known by the brand name Romex), the cable assembly itself is only rated for 60°C. Therefore, even if the individual THHN wires inside the NM-B jacket could theoretically handle 90°C, the entire assembly is legally bound to the 60°C column.
You can only use the 90°C column as your starting baseline when calculating derating factors (like bundling or high ambient temperatures). Once you apply the derating math, the final adjusted ampacity must still be equal to or less than the 60°C or 75°C terminal limit of your lugs and breakers.
If you are pulling individual THHN/THWN-2 conductors through EMT or PVC conduit and landing them on a 75°C rated breaker lug, you may use the 75°C column for your final ampacity. Always check the termination equipment datasheet; older equipment or specific motor controllers may still be restricted to 60°C.
How Derating Factors Modify Base Ampacity
The values in the master wire gauge table assume ideal conditions: an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors (CCCs) in a single raceway. When real-world conditions deviate, you must apply derating factors as outlined in NEC Article 310.15.
Bundling Derating (NEC 310.15(C)(1))
When you pull four to six current-carrying conductors in a single conduit, the heat generated by the wires cannot dissipate efficiently. You must multiply the base ampacity by 80%. For seven to nine conductors, the multiplier drops to 70%.
Worked Numeric Example:
You are pulling four 12 AWG THHN wires (two hots, one neutral, one ground) through a single 3/4" EMT conduit to feed a multi-wire branch circuit. The ground wire does not count as a CCC. You have 3 CCCs. Wait, if it's a standard MWBC, the neutral carries only the unbalanced load and may not count depending on the circuit type, but let's assume a scenario with 4 CCCs (e.g., two 240V circuits sharing a conduit, 4 hots, no neutral).
- Base 90°C Ampacity for 12 AWG: 30A (from the table above).
- Derating Factor (4-6 CCCs): 80% (0.80).
- Adjusted Ampacity: 30A × 0.80 = 24A.
- Terminal Limit Check: The breaker lugs are rated 75°C (25A limit for 12 AWG). Since 24A is less than 25A, the 24A adjusted value governs.
- Result: You can still protect this wire with a standard 20A breaker, but you cannot use it for a 25A or 30A load.
Ambient Temperature Derating
If your conduit runs through an attic in a southern climate where ambient temperatures regularly exceed 30°C, you must apply the temperature correction factors from the bottom of NEC Table 310.16. For example, in a 40°C (104°F) attic, the correction factor for 90°C insulation is 0.91. You multiply the 90°C base ampacity by 0.91 before checking against your terminal limits.
What the Wire Gauge Table Cannot Tell You
While this ampacity chart is the foundational reference for electrical sizing, it has strict limitations. Relying on it blindly without considering the following factors will result in failed inspections or poorly performing circuits.
1. Voltage Drop Over Distance
NEC Table 310.16 tells you what size wire will prevent the insulation from melting; it does not guarantee your equipment will receive adequate voltage. For long feeder runs (typically over 100 feet), you must calculate voltage drop using the resistance values found in NEC Chapter 9, Table 8. A 6 AWG wire might be legally rated for 65A at 75°C, but if you run it 200 feet to a 60A subpanel, the voltage drop will exceed the recommended 3% limit, causing motors to overheat and lights to dim. In that scenario, you must upsize to 4 AWG or 3 AWG purely for voltage drop mitigation.
2. Physical Lug Fit and Conduit Fill
The table assumes you can physically terminate the wire. A 1/0 AWG copper wire might be required for a 125A feeder, but many standard 125A residential breaker lugs are only rated to accept up to 2 AWG or 3 AWG. Always verify the breaker manufacturer's datasheet for maximum wire size. Similarly, upsizing wire for voltage drop changes your conduit fill calculations (NEC Chapter 9, Table 1), potentially requiring you to pull a larger diameter conduit.
3. Short-Circuit Thermal Withstand
Keep this page bookmarked on your phone for the jobsite. When in doubt, always defer to the specific manufacturer instructions for your terminations and the final authority of your local electrical inspector.
