How to Read NEC Table 310.16 (Conductor Ampacity, Explained)

If there's one table you must be able to open in your sleep, it's NEC Table 310.16 — the allowable ampacities of insulated conductors. It shows up on nearly every journeyman exam, and the rules around it cause more wrong answers than the table itself.

What the table gives you

Table 310.16 lists the ampacity (current-carrying capacity) of a conductor by its size and its insulation temperature rating — the 60°C, 75°C, and 90°C columns. For copper at 75°C, the values you should know cold:

• 14 AWG → 20 A
• 12 AWG → 25 A
• 10 AWG → 35 A
• 8 AWG → 50 A
• 6 AWG → 65 A
• 3 AWG → 100 A
• 1/0 AWG → 150 A
• 3/0 AWG → 200 A

The catch: which column do you use?

This is where people lose points. Even if you're running a 90°C-rated conductor like THHN, you usually can't use the 90°C ampacity for your final answer. Under 110.14(C), the conductor is limited to the temperature rating of the terminations — and most equipment is listed for 60°C or 75°C. So in practice you read the 75°C column for most installations.

The 90°C column isn't useless, though — you use it as the starting point for derating (temperature correction and the more-than-three-conductor adjustment), then check the result against the 75°C termination limit.

Don't forget the small-conductor rule

Table 310.16 says 12 AWG copper is good for 25 A. But 240.4(D) caps the overcurrent device for 12 AWG at 20 A anyway. So the ampacity and the breaker size are two different questions — exam writers love to test whether you know that.

Drill it until it's automatic

The fastest way to own Table 310.16 is reps: look up dozens of ampacities until your hand finds the right cell instantly, then practice applying 110.14(C) and 240.4(D). Our verified question bank drills exactly this — every ampacity question shows the worked answer and the code reference — so the table becomes free points instead of a stumbling block.