Wire Size Chart — NEC Ampacity Table

Copper Conductor Ampacity (NEC Table 310.16)

Copper is the most widely used conductor material in residential and commercial electrical installations. The table below shows the maximum continuous ampacity for copper conductors installed in raceways, cables, or directly buried, based on an ambient temperature of 30 degrees C (86 degrees F) with not more than three current-carrying conductors in the raceway or cable. These values come directly from NEC Table 310.16 and represent the maximum current each gauge can safely carry without exceeding its temperature rating.

Aluminum Conductor Ampacity (NEC Table 310.16)

Aluminum conductors have lower ampacity than copper for the same gauge size due to aluminum's higher resistivity. However, aluminum is significantly lighter and less expensive, making it the preferred choice for large service entrance conductors, feeder circuits, and utility distribution. The table below shows aluminum ampacity values from NEC Table 310.16 at all three temperature ratings. When using aluminum, always verify that terminations are rated for aluminum conductors (AL-CU marking) and apply anti-oxidant compound to all connections.

Understanding the Ampacity Table

The ampacity values in NEC Table 310.16 represent the maximum continuous current a conductor can carry without exceeding its insulation temperature rating. These values assume standard conditions: an ambient temperature of 30 degrees C (86 degrees F) and no more than three current-carrying conductors in a raceway or cable. When conditions deviate from these assumptions, adjustment and correction factors must be applied.

The three temperature columns correspond to different insulation types. The 60 degrees C column applies to TW and UF insulation types and is the baseline for most older residential installations. The 75 degrees C column covers THW, THWN, and XHHW insulation types and is the most commonly referenced column for modern installations. The 90 degrees C column applies to THHN, THWN-2, and XHHW-2 types and is primarily used as a starting point for ampacity derating calculations rather than as the actual conductor ampacity.

When selecting a wire gauge, remember that the conductor itself is often rated for 90 degrees C, but the circuit must be sized based on the lowest temperature rating of any component. Most residential circuit breakers and devices have 75 degrees C rated terminals, so even with 90 degrees C wire, you must use the 75 degrees C ampacity column. The 90 degrees C rating becomes valuable when you need to apply ampacity adjustment factors for bundled conductors or high ambient temperatures, as the higher starting ampacity provides additional margin.

For circuits with more than three current-carrying conductors in a raceway, NEC Table 310.15(C)(1) requires reducing the ampacity. For 4-6 conductors, the adjustment factor is 80%. For 7-9 conductors, it is 70%. These reductions account for the increased heat generated when multiple conductors share the same confined space. Similarly, if the ambient temperature exceeds 30 degrees C, correction factors from NEC Table 310.15(B)(1) must be applied to reduce the ampacity accordingly.

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