How it works
Usable ampacity = ampacity at the terminal-rating column (110.14(C)); continuous load is limited to 80% of that (210.19/210.20).
Table 310.16 gives the base ampacity at 30 °C ambient for up to three current-carrying conductors. The conductor insulation may be rated 90 °C, but unless every termination is also rated 90 °C, NEC 110.14(C)(1) makes you size from the 60 °C column (terminations ≤ 100 A or marked 60 °C) or the 75 °C column (most equipment ≥ 100 A). The small conductors 14, 12 and 10 AWG carry an extra cap: NEC 240.4(D) limits their overcurrent protection to 15 A (14 Cu), 20 A (12 Cu), 30 A (10 Cu), and 15/25 A (12/10 Al) regardless of the 75/90 °C column — the chart and the reverse lookup apply this cap to the usable column. For a load that runs three hours or more, NEC 210.19(A) and 210.20(A) require the conductor and overcurrent device to be sized at 125% of the load — equivalently, the conductor is loaded to no more than 80% of its usable ampacity. More than three conductors in the raceway, or an ambient above 30 °C, requires additional derating.
Code references
- Allowable ampacities NEC 2023, Table 310.16 — 60/75/90 °C, Cu and Al
- Temperature limitation of terminations NEC 2023, 110.14(C)(1)
- Continuous-load 125% / 80% rule NEC 2023, 210.19(A) and 210.20(A)
- Small-conductor overcurrent limit NEC 2023, 240.4(D) — 14/12/10 AWG
FAQ
Why can’t I use the 90 °C column for my THHN wire?
THHN is rated 90 °C, but NEC 110.14(C) sizes the circuit to the lowest-rated component in the path — usually the breaker and lug terminals, which are listed for 60 °C or 75 °C. The 90 °C column is used only as the starting point for derating, not as the final usable ampacity.
Which column should I use?
For most circuits 100 A and below, terminations are rated 60 °C (use the 60 °C column) unless the equipment and conductors are both marked 75 °C. For circuits above 100 A, terminations are typically 75 °C. When in doubt, the 75 °C column is the common field default — verify the markings on your specific gear.
What does the continuous-load toggle do?
It applies the NEC 80% rule. A continuous load (3 hours or more, like store lighting or an EV charger) must not exceed 80% of the conductor’s usable ampacity, so the chart shows the reduced figure. A 50 A continuous load needs a conductor with at least 62.5 A usable ampacity.
Why does 12 AWG copper show only 20 A even at 90 °C?
NEC 240.4(D) caps the overcurrent protection — and therefore the usable load — for the small conductors regardless of their insulation column: 15 A for 14 AWG copper, 20 A for 12 AWG copper, 30 A for 10 AWG copper, and 15/25 A for 12/10 AWG aluminum. The 90 °C column for these sizes is only a derating starting point, so the chart and the reverse lookup apply the 240.4(D) cap to the usable column.
Does this chart account for conduit fill or ambient heat?
No. Table 310.16 assumes no more than three current-carrying conductors and a 30 °C (86 °F) ambient. Add NEC 310.15(C)(1) adjustment for four or more conductors and NEC 310.15(B)(1) correction for higher ambients — both reduce the usable ampacity further.
This chart is provided for estimation purposes. Always verify ampacity against the current NEC edition, the equipment terminal ratings, and local amendments with a licensed electrician or electrical engineer before sizing conductors.