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How much current a wire may carry

Ampacity is the current a conductor can carry continuously without its insulation getting too hot, and it comes from a table by wire size and by the temperature rating of the insulation and the terminals.

Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.

What it is for

The wire does not trip. It just gets hot, quietly, inside a wall, for years. A wire loaded past its ampacity is the fire nobody sees starting, and the table is the only thing between the load you calculated and the wire you pull.

How to think about it

Look the size up in the column you are allowed to use. For ordinary house wiring the answer is the small-conductor rule: 14 AWG copper is rated 15 A, 12 AWG copper is rated 20 A, 10 AWG copper is rated 30 A, whatever the insulation says. From 8 AWG up, use the 60 °C column unless every terminal on the circuit is marked 75 °C, in which case the 75 °C column applies.

Worked example

14 AWG copper is rated 15 A
The lighting and general receptacle wire. Never on a breaker bigger than 15 A.
12 AWG copper is rated 20 A
Kitchen, bathroom, laundry and most receptacle circuits.
10 AWG copper is rated 30 A
Dryers, water heaters, small air conditioners.
8 AWG copper is rated 40 A at 60 °C and 50 A at 75 °C
From here the column matters. Ranges and subpanels.
6 AWG copper is rated 55 A at 60 °C and 65 A at 75 °C
A 50 A range circuit, or a 60 A feeder on 75 °C terminals.
4 AWG copper is rated 70 A at 60 °C and 85 A at 75 °C. 3 AWG copper is rated 85 A at 60 °C and 100 A at 75 °C
Feeder sizes. 2 AWG copper is rated 95 A at 60 °C and 115 A at 75 °C; 1 AWG copper is rated 110 A at 60 °C and 130 A at 75 °C.

Your turn

Name the rule that caps 14, 12 and 10 AWG at 15 A, 20 A and 30 A whatever the insulation says.

the  rule

The trap

Using the 90 °C column because the cable says THHN. The wire's insulation may be 90 °C, but the breaker terminal is 60 or 75 °C, and the circuit is rated by its weakest link. The 90 °C column is for derating maths, not for choosing a breaker.

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