Copper Cable Ampacity Chart: NEC Table 310.16 Sizing & Derating Guide

The copper cable ampacity chart dictates the maximum continuous current a wire can carry before its insulation degrades or the conductor overheats. For standard residential branch circuits, you will almost always use the 60°C column for NM-B (Romex) and the 75°C column for THHN in conduit, per NEC Table 310.16. If you are sizing a 40-amp circuit using NM-B cable, you need 8 AWG copper. If you are pulling THHN in conduit for the same 40-amp circuit, 8 AWG copper is also your baseline pick before applying derating factors.

How to Read the NEC Copper Cable Ampacity Chart

Before pulling wire, you need to know how to read the chart correctly. The table below is sourced directly from NEC Table 310.16 (2023/2026 editions) for copper conductors with an ambient temperature of 30°C (86°F).

How to read this table: Locate your wire gauge (AWG or kcmil) in the left column. Then, move right to the temperature column that matches your cable's insulation type and termination rating. The number at the intersection is your baseline ampacity—the maximum current the wire can safely carry under ideal conditions (30°C ambient, no more than three current-carrying conductors bundled together).

Source: NFPA 70 (National Electrical Code), Table 310.16. Always verify with your local AHJ, as regional amendments may apply.

Which Temperature Column Actually Applies to Your Build?

The most common mistake DIYers make is looking at the 90°C column for THHN wire and assuming they can push that maximum current through the circuit. You cannot. The allowable ampacity is limited by the weakest link in the circuit, which is almost always the termination point (the breaker lug or receptacle screw).

• The 60°C Column: Use this for NM-B (Romex) cable. Even though modern NM-B insulation is rated for 90°C, NEC Article 334.80 mandates that the ampacity of NM-B cable must be determined using the 60°C column. Additionally, for 14, 12, and 10 AWG copper, NEC 240.4(D) strictly caps overcurrent protection at 15A, 20A, and 30A respectively, regardless of the chart.
• The 75°C Column: Use this for individual THHN/THWN-2 conductors pulled in conduit. Most modern breakers, lugs, and receptacles are rated for 75°C terminations. If your equipment is explicitly marked 75°C, you may use this column for your final ampacity.
• The 90°C Column: Use this only as the starting baseline for derating calculations (ambient temperature adjustments or bundling adjustments). The final calculated ampacity after derating must still be compared against the 60°C or 75°C termination limits, and the lower of the two numbers wins.

Derating: When the Chart's Base Numbers Shrink

The chart above assumes an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors in a raceway. When your installation deviates from these ideals, you must apply derating factors from NEC Table 310.15(B)(1) and Table 310.15(C)(1).

Worked Numeric Example: You are pulling four current-carrying 10 AWG THHN wires through a conduit in an attic where the ambient temperature reaches 40°C (104°F). You need to find the final ampacity.

1. Base Value: 10 AWG at 90°C = 40A.
2. Temperature Correction (40°C): The correction factor for 90°C insulation at 40°C ambient is 0.91. (40A × 0.91 = 36.4A).
3. Bundling Adjustment (4 conductors): The adjustment factor for 4-6 current-carrying conductors is 80%. (36.4A × 0.80 = 29.12A).
4. Termination Check: Your breaker lugs are rated 75°C. The 75°C column value for 10 AWG is 35A.
5. Final Verdict: Compare the derated 90°C value (29.12A) to the 75°C termination limit (35A). The lower number governs. Your 10 AWG wire is now legally limited to 29.12 amps. If your continuous load is 30A, 10 AWG has failed the calculation; you must step up to 8 AWG.

What This Chart Cannot Tell You

Ampacity charts only solve for thermal limits. They do not account for the physical or electrical realities of long runs and hardware constraints. Here is what the table leaves out:

• Voltage Drop: A 14 AWG wire can safely carry 15A thermally, but if you run it 150 feet to a 12A load, the voltage drop will exceed the NEC-recommended 3% limit, causing motors to overheat and lights to dim. Always run a voltage drop calculation for runs over 50 feet.
• Physical Lug Capacity: You might calculate that you need 4/0 AWG copper for a 200A service due to heavy derating in a hot environment. However, if the main breaker lugs on your specific panelboard are only physically rated to accept a maximum of 2/0 AWG, you cannot terminate the 4/0 wire. You must either use a lug reducer kit (if listed for the panel) or run parallel conductors.
• Neutral and Ground Sizing: The chart gives you the phase conductor size. The neutral may need to be the same size (or larger for high harmonic loads), while the equipment grounding conductor (EGC) is sized separately via NEC Table 250.122 based on the breaker rating, not the phase wire ampacity.

Wire Sizing Decision Tree: Pick Your Exact AWG

Stop guessing. Follow this decision path to lock in your exact wire size for a standard 240V, 50-amp continuous load (like a large EV charger or subpanel feeder) where the run is under 50 feet and ambient temps are normal.