The amp rating for 6 AWG copper wire is 55 amps in the 60°C column, 65 amps in the 75°C column, and 75 amps in the 90°C column, per NEC Table 310.16. However, for most residential branch circuits and subpanel feeders terminating on standard breakers, the practical maximum breaker size is 60 amps. If you are using 6 AWG NM-B (Romex) cable, the National Electrical Code strictly limits the ampacity to the 60°C column, capping the breaker size at 55 amps (requiring a 50-amp breaker, as 55-amp breakers are not standard for this application).
How to Read the NEC 310.16 Ampacity Table for 6 AWG
To accurately determine the ampacity of your wire, you must cross-reference the wire material (copper vs. aluminum), the insulation type (THHN, XHHW, NM-B), and the temperature rating of the terminals your wire connects to. The National Fire Protection Association (NFPA) publishes these limits in NEC Table 310.16.
Which column applies to your installation?
- 60°C Column: Applies to NM-B (Romex) cable, UF-B cable, and any circuit rated 100A or less where the equipment (breaker or receptacle) is not explicitly marked with a 75°C rating. Per NEC 110.14(C)(1)(a), this is your default assumption for older homes and standard residential receptacles.
- 75°C Column: Applies to THHN/THWN-2 or XHHW wires in conduit when terminating on modern, listed breakers and lugs explicitly rated for 75°C (most modern Square D, Eaton, and Siemens breakers are).
- 90°C Column: Used only for derating calculations. You cannot terminate a wire on a 90°C-rated lug in standard residential gear because 90°C lugs are exceptionally rare in panels under 600V.
| AWG Size | 60°C (140°F) Copper | 75°C (167°F) Copper | 90°C (194°F) Copper | 75°C (167°F) Aluminum |
|---|---|---|---|---|
| 8 | 40 | 50 | 55 | 40 |
| 6 (Target) | 55 | 65 | 75 | 50 |
| 4 | 70 | 85 | 95 | 65 |
| 3 | 85 | 100 | 110 | 75 |
| 2 | 95 | 115 | 130 | 90 |
Derating Factors: When 6 AWG Carries Less Current
The base ampacities in Table 310.16 assume 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 using the 90°C column as your starting baseline, even if your final termination is limited to 75°C.
How derating rows modify the base value:
Imagine you are pulling four current-carrying 6 AWG THHN copper conductors through a conduit in an attic where the ambient temperature reaches 40°C (104°F).
- Base Ampacity (90°C column): 75 amps.
- Ambient Temperature Correction (NEC Table 310.15(B)(1)): At 40°C, the correction factor for 90°C insulation is 0.91. (75A × 0.91 = 68.25A).
- Bundling Adjustment (NEC Table 310.15(C)(1)): Four to six current-carrying conductors require an 80% adjustment factor. (68.25A × 0.80 = 54.6A).
Your final derated ampacity is 54.6 amps. Because standard breakers do not come in 54-amp sizes, and NEC 240.4(B) does not allow rounding up to the next standard size if the calculated derated ampacity is below the standard breaker size (unless specific conditions apply), you must drop down to a 50-amp breaker for this specific run.
What the Ampacity Table Cannot Tell You
Looking up the amp rating for 6 AWG wire in a vacuum ignores three critical physical realities of electrical installations:
- Voltage Drop: NEC Table 310.16 only addresses thermal limits (preventing the insulation from melting). It does not account for voltage drop. If you are running a 60-amp, 240V circuit to a detached garage 150 feet away, 6 AWG copper will experience roughly a 4.5% voltage drop. To maintain the recommended 3% maximum drop for feeder circuits, you must upsize to 4 AWG or 3 AWG copper, regardless of the thermal ampacity.
- Continuous vs. Non-Continuous Loads: Under NEC 210.20(A), if a load is expected to run for three hours or more (like an EV charger or a workshop heater), the branch circuit must be sized at 125% of the continuous load. A 6 AWG wire on a 60-amp breaker can only safely supply a maximum continuous load of 48 amps (60A / 1.25 = 48A).
- Torque Specifications: The table assumes proper terminations. A 6 AWG wire loosely connected to a breaker lug will create a high-resistance joint, generating localized heat that can melt the breaker or start a fire long before the wire itself reaches its thermal ampacity limit. Always use a calibrated torque screwdriver set to the manufacturer's exact inch-pound specification (typically 35-45 in-lbs for 6 AWG in standard residential breakers).
Frequently Asked Questions
What size breaker do I use for 6 AWG copper wire?
For standard residential applications using THHN in conduit, a 60-amp breaker is the standard and safest choice, aligning with the 60°C termination rule for general equipment. If your wire is 6 AWG NM-B (Romex), the NEC limits it to 55 amps, meaning you must use a 50-amp breaker because 55-amp breakers are not standard for this wire class.
Can I use 6 AWG aluminum wire for a 60-amp subpanel?
No. According to the 75°C aluminum column in NEC Table 310.16, 6 AWG aluminum is only rated for 50 amps. To safely feed a 60-amp subpanel using aluminum conductors, you must step up to 4 AWG aluminum, which is rated for 65 amps at 75°C, allowing you to use the standard 60-amp breaker.
How many watts can 6 AWG wire handle at 240 volts?
Assuming a 60-amp breaker and a standard 240V residential circuit, the maximum theoretical wattage is 14,400 watts (60A × 240V). However, if the load is continuous (running for 3+ hours), you must apply the 80% continuous load rule, limiting the practical wattage to 11,520 watts (48A × 240V).
Does the amp rating for 6 AWG change if it is buried underground?
Yes, depending on the cable type. If you are burying 6 AWG UF-B (Underground Feeder) cable directly in the dirt, it is strictly limited to the 60°C column, capping its ampacity at 55 amps. If you are pulling 6 AWG THWN-2 individual conductors through a buried PVC conduit, you can utilize the 75°C column, giving you an ampacity of 65 amps (typically protected by a 60-amp breaker). Always check local OSHA and local AHJ guidelines for minimum burial depths.
