AmpSizer

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Ampacity, with the derating shown.

The number in the table is a starting point, not an answer. Pick a conductor and your actual conditions, and this shows the published value, each correction applied to it, and the ampacity you can actually design around.

Lookup

Conductor and conditions

86°F — table basis is 30°C.

Result

Ampacity after derating

65 A
  • Published table value (75°C)65 A
  • Ambient correction at 30°C×1
  • Adjustment for 3 conductors×1
  • Derated ampacity65 A
  • Usable ampacity65 A

Terminations on most equipment are listed for 60°C or 75°C. The 90°C column is normally used only as the starting point for derating, not as the final ampacity.

Full 6 AWG copper reference
6 AWG65 A usable

Source: NFPA 70, National Electrical Code, Table 310.16; NFPA 70, National Electrical Code, Table 310.15(B)(1)(1); NFPA 70, National Electrical Code, Table 310.15(C)(1); NFPA 70, National Electrical Code, 240.4(D). Published by NFPA.NEC 2023

Why the table value is rarely the final number

Published ampacity assumes specific conditions: an ambient of 30°C (86°F), and no more than three current-carrying conductors in a raceway or cable. A conductor is rated for those conditions because heat is the actual limit — insulation degrades and eventually fails at temperature, and current is just the thing that produces the heat.

Change the conditions and you change how much heat the conductor can shed. An attic at 50°C leaves less room between ambient and the insulation's limit, so the conductor carries less. Bundle nine conductors in one pipe and each one is heated by its neighbours, so all of them carry less. Both corrections multiply together, and both are applied here.

The small-conductor ceiling

Separately from ampacity, the smallest conductors carry a fixed overcurrent limit: 15 A for 14 AWG copper, 20 A for 12 AWG copper, 30 A for 10 AWG copper, with lower values for aluminum. This holds even where the 90°C column reads higher, which is why you cannot put 14 AWG on a 20 A breaker by pointing at a table. The rule carries specific exceptions this calculator does not model.

Terminations usually govern in practice

The 90°C column is normally used as the starting point for derating, not as the final ampacity, because the breakers and equipment the conductor lands on are typically listed for 60°C or 75°C. Your conductor may be rated 90°C, but the weakest link in the path decides. Check what the equipment is actually listed for.

Ready to size a run? Try the wire size calculator, or browse the full copper ampacity table.