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Wire size and ampacity calculator

Table 310.16 is the starting point, not the answer. Ambient temperature corrects it, conductor count adjusts it, and then 110.14(C) caps it at whatever your terminations are rated for, which is the step that gets skipped. This runs all four.

The conductor

Material

Conductor size

Insulation rating THHN and THWN-2 are 90°C

Termination rating 110.14(C). Check the breaker and the lug.

The conditions

Ambient temperature the table is built on 30°C / 86°F

Current-carrying conductors in the raceway leave out the EGC

Your load amperes, to size from scratch. 0 to skip.

Circuit type 240.4(B) rounding up is not allowed on some circuits

Usable ampacity

0 A

Table 310.16 base

0 A

After corrections

0 A

Termination ceiling

0 A

Max overcurrent device

0 A

The math

Built on Table 310.16 with the corrections in 310.15(B)(1) and 310.15(C)(1), the termination limit in 110.14(C) and the overcurrent rules in 240.4 and 240.6(A). This is a working tool, not a substitute for the codebook or your inspector. Local amendments override the national text, and the licensed person signing the permit owns the result. Check which edition your AHJ has adopted.

Four steps, and most people stop at two

Sizing a conductor for ampacity runs in order. Pull the base value out of Table 310.16 at your insulation's column. Correct it for ambient temperature using 310.15(B)(1). Adjust it for conductor count using 310.15(C)(1). Then cap the whole thing at the termination temperature under 110.14(C).

That last step is the one that gets left out, and it is the one that makes people think they have more conductor than they do.

The 90 degree column you cannot actually use

THHN is a 90 degree conductor, and the 90 degree column in Table 310.16 is generous. 10 AWG copper shows 40 amps there.

You will almost never get to use 40 amps of it. Breakers and lugs on ordinary equipment are rated 75 degrees, and 110.14(C) says the conductor ampacity has to be selected so it does not exceed the lowest-rated termination in the circuit. On a 75 degree termination that same 10 AWG is a 35 amp conductor, and the small conductor rule in 240.4(D) then limits the protection to 30 amps anyway.

So what is the 90 degree column for? Derating. You are allowed to start from the 90 degree value, apply your ambient and conductor-count factors to it, and then compare the result against the termination limit. Starting high gives you more room to lose, which is exactly why 90 degree wire is worth pulling even when you can never bill the full column value.

The small conductor rule

240.4(D) sets a hard ceiling on overcurrent protection for the three sizes that make up most of a house, regardless of what any column or correction says. Copper is 15 amps on 14 AWG, 20 on 12 AWG, and 30 on 10 AWG. Aluminum is 15 amps on 12 AWG and 25 on 10 AWG.

There are specific exceptions for motor circuits and a few other cases, but on general wiring this rule is why a 12 AWG conductor is a 20 amp circuit and arguing about columns does not change it.

Where the money is

Derating is a cost event. An attic at 50 degrees Celsius in August, or nine current-carrying conductors sharing a pipe, can take a conductor you priced as 12 AWG and turn it into 10 AWG. That is more copper, a bigger raceway, and in a retrofit it can be the difference between pulling into existing pipe and pulling new pipe.

Which is a quoting problem, not a code problem. The code just tells you what it has to be. Whether you found that out before or after you gave the customer a number is the part that costs you.

Common questions

How do I find the ampacity of a conductor?

Start with the base value in Table 310.16 for your conductor material and insulation temperature rating. Apply the ambient temperature correction factor from 310.15(B)(1), then the adjustment factor from 310.15(C)(1) if more than three current-carrying conductors share the raceway. Finally, limit the result to the Table 310.16 value at your termination temperature rating, as 110.14(C) requires.

Can I use the 90 degree column for THHN?

You can start the calculation there, but you usually cannot end there. 110.14(C) limits the final ampacity to the value at the lowest-rated termination in the circuit, and most breakers and lugs are rated 75 degrees. The 90 degree column is valuable because it gives you a higher number to apply correction and adjustment factors to.

What is the small conductor rule?

240.4(D) caps overcurrent protection on small conductors regardless of their calculated ampacity. For copper it is 15 amps on 14 AWG, 20 amps on 12 AWG, and 30 amps on 10 AWG. For aluminum it is 15 amps on 12 AWG and 25 amps on 10 AWG. There are exceptions for motor circuits and a few other specific cases.

When do I apply the ambient temperature correction?

Whenever the ambient temperature is anything other than 30 degrees Celsius, which is the temperature Table 310.16 is built on. Above 30 degrees the factor reduces ampacity, and below it the factor increases ampacity, which is a legitimate gain people often forget they can take.

Do I apply both the ambient correction and the conductor count adjustment?

Yes, both, multiplied together against the base ampacity. An attic run with several circuits in one raceway gets hit twice, and that combination is what most often forces a larger conductor than the load alone would suggest.

Can I round up to the next standard breaker size?

240.4(B) allows it when the conductor ampacity does not match a standard rating, the device is 800 amps or less, and the circuit does not supply receptacles for cord-and-plug-connected portable loads. On a branch circuit with receptacles you round down instead, and the 240.4(D) small conductor limits still apply either way.

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Instructors, JATC training coordinators, and apprenticeship programs: use this in class, put the link on a syllabus, or print the output for a handout. If you want an input added or a print-friendly version for your curriculum, tell me what you need and I will build it.

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Wire Size and Ampacity Calculator

Wire size and ampacity calculator

The math

Four steps, and most people stop at two

The 90 degree column you cannot actually use

The small conductor rule

Where the money is

Common questions

Link to this calculator

More free tools

Related reading

Wire Size and Ampacity Calculator

Wire size and ampacity calculator

The math

Four steps, and most people stop at two

The 90 degree column you cannot actually use

The small conductor rule

Where the money is

Common questions

Link to this calculator

More free tools

Related reading

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