Electric Wire Chart Amps: NEC Table 310.16 Sizing Guide

When sizing conductors for a branch circuit or feeder, the electric wire chart amps you need are dictated by the National Electrical Code (NEC). The definitive reference is NEC Table 310.16 (formerly 310.15(B)(16)). This table provides the allowable ampacities for insulated conductors rated up to 2000 volts, based on conductor material (copper or aluminum), insulation type, and ambient temperature.

How to Read the NEC Ampacity Table

The most common mistake DIYers and junior apprentices make is blindly pulling numbers from the highest temperature column. To read the electric wire chart amps correctly, you must understand which column applies to your specific installation:

• 60°C Column (TW, UF, NM-B): Nonmetallic-sheathed cable (Romex) is legally limited to the 60°C ampacity column per NEC 339.10(4), even if the individual wires inside have 90°C insulation. This column also applies to older wiring and specific appliance cords.
• 75°C Column (RHW, THHW, THW, THWN, XHHW): This is the default column for most commercial and residential terminations. Standard circuit breakers and panel lugs are tested and rated for 75°C. Per NEC 110.14(C), your wire ampacity cannot exceed the temperature rating of the termination it connects to.
• 90°C Column (THHN, THHW, THW-2, THWN-2, XHHW-2): You cannot use this column to determine your final breaker size unless every single component in the circuit (breaker, lug, splice connector) is explicitly rated for 90°C—which is virtually never true in standard panels. Instead, the 90°C column is used as the base value before applying ambient temperature and bundling derating factors.

The Small Conductor Rule (NEC 240.4(D)): Regardless of what the 75°C or 90°C columns say, overcurrent protection for 14 AWG is capped at 15A, 12 AWG at 20A, and 10 AWG at 30A. You cannot put a 12 AWG wire on a 25A breaker just because the 90°C column lists it at 30A.

Copper Wire Ampacity Chart (NEC Table 310.16)

Below is the complete copper ampacity table based on an ambient temperature of 30°C (86°F). Bookmark this section for quick reference on the most queried circuit sizes.

Source: NEC Table 310.16 / Electrical Technology Reference. Assumes not more than three current-carrying conductors in a raceway or cable.

Derating and Edge Cases the Chart Hides

The electric wire chart amps listed above assume 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 that modify the base value.

How Derating Modifies the Base Value

Derating is calculated using the 90°C column as your starting baseline, then multiplying by the correction factor from NEC Table 310.15(B)(1) (ambient temperature) or Table 310.15(C)(1) (bundling). After derating, you compare the result to the 75°C column, and the lower of the two numbers becomes your final allowable ampacity.

What the Table Cannot Tell You

• Voltage Drop: NEC Table 310.16 only addresses thermal limits (preventing the insulation from melting). It does not account for voltage drop over long distances. For runs exceeding 100 feet, you must upsize the wire to maintain a maximum 3% voltage drop on branch circuits, as outlined in NEC informational note 210.19(A)(1).
• Physical Lug Fit: A 4/0 AWG wire might be rated for 230A at 75°C, but it physically will not fit into the lug of a standard 200A residential main breaker. Always verify the manufacturer's torque and wire-range specifications on the breaker label.
• Continuous Loads: If a load runs for 3 hours or more (like an EV charger or hardwired heater), NEC 210.20(A) requires the breaker and wire to be sized at 125% of the continuous load. A 40A continuous load requires a 50A breaker and wire sized for 50A.

Frequently Asked Questions