How to read the ampacity chart correctly
The table gives you three numbers per conductor. Only one of them is the answer, and it is almost never the biggest one.
Usable ampacity = min( Table 310.16 × Cambient × Cfill , termination limit )Step 1 — find the table value
Look up the conductor in the chart above at its insulation temperature rating. THHN and XHHW-2 are 90 °C. THWN and XHHW are 75 °C. TW and UF are 60 °C.
Step 2 — correct for ambient (Table 310.15(B)(1))
The table assumes 30 °C (86 °F). A hot attic, a rooftop, or a boiler room will be higher, and the ampacity falls.
Step 3 — adjust for conduit fill (Table 310.15(C)(1))
More than three current-carrying conductors in the same raceway and the conductors cannot shed heat as easily. Four to six conductors: 80%. Seven to nine: 70%. Ten to twenty: 50%.
Step 4 — apply the termination limit (110.14(C)(1)) — this is the one people miss
A chain is as strong as its weakest link, and the weakest link in a conductor run is usually the lug it terminates on. Equipment rated 100 A or less is generally limited to the 60 °C column. Above 100 A, the 75 °C column. It does not matter that your THHN is rated 90 °C — if it lands on a 75 °C lug, you take the 75 °C ampacity.
The 90 °C column is not useless. It is what you derate from in steps 2 and 3, which often leaves you better off than starting at 75 °C. But whatever you derate to, you may never exceed the termination column.
The mistake this chart is designed to prevent
Take a 125 A load, 1 AWG copper THHN, 30 °C, three conductors in a conduit.
- The 90 °C column says 145 A. Plenty of room. Many charts stop here.
- But the breaker and its lugs are 75 °C-rated. NEC 110.14(C)(1) caps you at the 75 °C column: 130 A.
- 130 A still covers 125 A, so 1 AWG is correct — but only because we checked.
Now run the same logic in reverse. If you had sized purely from the 90 °C column, 3 AWG (115 A) would have looked adequate for a 110 A load. On a 75 °C terminal, 3 AWG is only good for 100 A. You would be 10 A over at the lug — which is exactly where conductors overheat, because that is where the heat has nowhere to go.
Wire ampacity — frequently asked
Ampacity is the maximum current a conductor can carry continuously without its temperature exceeding the rating of its insulation. In the United States it is set by NEC (NFPA 70) Table 310.16 for conductors in a raceway, cable or earth.
Because of NEC 110.14(C)(1). The conductor may be rated 90 °C, but the breaker lug or terminal it lands on is usually rated 60 °C (equipment 100 A or less) or 75 °C (above 100 A). The Code limits you to the ampacity column matching the lowest-rated part of the connection. The 90 °C column exists so you have headroom to derate from — it is not the ampacity you may use.
6 AWG copper: 55 A at 60 °C, 65 A at 75 °C, 75 A at 90 °C (NEC Table 310.16). In practice, on a 60 °C-rated terminal — which is typical for equipment rated 100 A or less — the usable ampacity is 55 A.
2/0 copper: 145 A at 60 °C, 175 A at 75 °C, 195 A at 90 °C. Because 2/0 is normally used above 100 A, the 75 °C termination column applies and the usable ampacity is 175 A.
4/0 aluminium: 150 A at 60 °C, 180 A at 75 °C, 205 A at 90 °C. On the 75 °C terminals typical above 100 A, the usable ampacity is 180 A. This is why 4/0 aluminium is the common choice for a 200 A residential service — 180 A after the 83% service-conductor allowance of 310.12 covers it.
Multiply the Table 310.16 value by the correction factor from Table 310.15(B)(1), which is based on a 30 °C (86 °F) ambient. For example, a 90 °C conductor at 45 °C ambient is multiplied by 0.87. Above roughly 55 °C ambient, 60 °C insulation can no longer be used at all.
NEC 310.15(C)(1) applies an adjustment factor: 80% for 4–6 current-carrying conductors, 70% for 7–9, 50% for 10–20, 45% for 21–30, 40% for 31–40 and 35% above that. Neutrals that carry only unbalanced current, and equipment grounding conductors, are generally not counted.
Generally no — NEC 240.4 requires the overcurrent device to protect the conductor. 240.4(B) permits the next standard size up in limited cases (600 A or less, not a receptacle branch circuit), and 240.4(D) caps 14, 12 and 10 AWG copper at 15, 20 and 30 A regardless of what the table says.