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Wire size, with the rule that overrides the table

Table 310.16 gives 12 AWG copper 25 amps. NEC 240.4(D) will not let you protect it above 20. Most sizing mistakes live in that gap, so this tool leads with the override rather than the table.

20 A, not 25 what 12 AWG copper is actually good for Table 310.16 lists 12 AWG copper at 25 A in the 75 °C column. NEC 240.4(D) caps its overcurrent protection at 20 A regardless, and that cap overrides both the table value and the next-size-up allowance. It is the single most safety-critical number in this tool.

Enter the load in amps, the conductor metal and the termination temperature. The tool returns the smallest size that carries it, the table ampacity, the largest standard device — and whether NEC 240.4(D) has capped the answer below the table.

Two things it deliberately does not do. It applies no derating for ambient temperature or for more than three current-carrying conductors in a raceway, both of which reduce the answer. And it does not check run length: a long run can force a larger conductor than ampacity alone requires, which is what the voltage drop calculator is for.

Live electrical work is not a do-it-yourself subject here. Nothing on this site instructs a reader to open, probe or modify energised equipment.

Conductor screen

Ampacity before derating. Nothing is emailed.

— Smallest size that carries it
— NEC 240.4(D)
— Largest standard device
— Table 310.16 value
— Design load
— Before derating

What this assumed —

A screening tool. Not a design, not a permit document, and not instructions to perform electrical work.

The table is not usually what decides it

People look up an ampacity and stop. Two other rules routinely override it, and both make the answer more conservative, not less.

NEC 240.4(D) caps the small sizes below their own table value. 12 AWG copper is listed at 25 A in the 75 °C column and may not be protected above 20 A. 14 AWG is listed at 20 A and is capped at 15 A; 10 AWG is listed at 35 A and capped at 30 A. This override is the reason a 12 AWG circuit is a 20 A circuit no matter what the table says, and it exists because small conductors lack the thermal mass to survive a moderate overload for as long as a breaker takes to trip on it.

Terminations cap the rest. Under 110.14(C) the temperature rating of the equipment you land on generally governs, and much equipment rated 100 A or less is a 60 °C termination. A 90 °C conductor is common, and it seldom gets to use its 90 °C column — that column is mostly for derating arithmetic, not for picking a breaker. This tool defaults to 75 °C for that reason, and you should treat the 90 °C answer with suspicion.

Derating comes off afterwards, and this tool does not apply it. More than three current-carrying conductors in a raceway, or an ambient above 30 °C, both reduce the allowable ampacity — an attic in Phoenix is not a 30 °C ambient. Anything the tool returns is a ceiling before those adjustments, never after them.

Nothing on this site is a stamped load calculation, a design, or a permit document. A load calculation that an inspector accepts is produced by someone who has seen the building and signs for it. This page exists so you can follow a proposal, ask a better question, or understand why an electrician specified what they specified. Live electrical work is not a do-it-yourself subject here. Nothing on this site instructs a reader to open, probe or modify energised equipment.

Where the small-conductor rule bites

The three sizes where the table value and the maximum breaker disagree. These are the ones that matter in a house, because they are almost every branch circuit in it.

SizeTable 310.16 at 75 °CNEC 240.4(D) maximum deviceWhat that means
14 AWG copper20 A15 AA 14 AWG circuit is a 15 A circuit. A 20 A breaker on 14 AWG is a defect, and a common one in older work.
12 AWG copper25 A20 AThe standard 20 A branch circuit. The 25 A table value is never available to it.
10 AWG copper35 A30 ATypical for a dryer or a water heater at 30 A. The 35 A value is not usable as protection.
12 AWG aluminium20 A15 AAluminium branch conductors carry their own history and their own terminations question. Not a like-for-like swap for copper.
10 AWG aluminium30 A25 ANote the cap is lower than the copper equivalent at the same size.

What to ask when a size looks wrong

Five questions that do not require you to open anything. All five are ordinary questions a competent electrician answers without difficulty.

  • Which code edition applies here?

    Adoption and amendment of the National Electrical Code is local. A state or city may be on an older edition, may amend it, and may add rules of its own. Confirm the adopted edition and the amendments with the authority having jurisdiction before relying on any code figure here.

  • What termination temperature did you size to?

    If the answer is 90 °C on equipment rated 100 A or less, that is worth a follow-up. 110.14(C) generally holds the sizing to the equipment rating, not the conductor rating.

  • How many current-carrying conductors are in that raceway?

    Above three, the allowable ampacity comes down. It is a routine adjustment and a routine thing to omit.

  • What ambient did you assume?

    Table 310.16 is based on 30 °C. An attic, a south wall or an equipment room can run far above that, and the correction is downward.

  • How long is the run?

    Ampacity and voltage drop are different constraints and a long run can force a larger conductor than ampacity alone requires. Our voltage drop calculator covers that.

Questions this calculator answers

What size wire do I need for a 20 amp circuit?
12 AWG copper, in the ordinary case. The table lists it at 25 A at 75 °C, but NEC 240.4(D) caps its overcurrent protection at 20 A, which is exactly why the standard 20 A branch circuit is a 12 AWG circuit. That cap is not a rounding convention — it overrides both the table and the next-size-up allowance.
Why does the tool default to 75 °C and not 90 °C?
Because under NEC 110.14(C) the temperature rating of the equipment you terminate on generally governs, and much equipment rated 100 A or less carries a 60 °C termination. Conductors rated 90 °C are common, but that column is mostly used for derating arithmetic rather than for selecting a breaker. A 90 °C answer here should be treated with suspicion unless someone has confirmed the terminations support it.
Does this apply derating?
No, and that is a deliberate limitation stated on the output itself. More than three current-carrying conductors in a raceway reduces allowable ampacity, and so does an ambient above the 30 °C the table assumes — an attic in summer is not 30 °C. Treat every figure here as a ceiling before those adjustments.
Can I use this to wire a circuit myself?
No. Live electrical work is not a do-it-yourself subject here. Nothing on this site instructs a reader to open, probe or modify energised equipment. Nothing on this site is a stamped load calculation, a design, or a permit document. A load calculation that an inspector accepts is produced by someone who has seen the building and signs for it. This is a screening tool for understanding a proposal, checking why an electrician specified what they specified, or following a conversation — not a design, not a permit document, and not instructions.
Is a 20 A breaker on 14 AWG wire a problem?
Yes, and it is a common finding in older work. NEC 240.4(D) limits 14 AWG copper to a 15 A device. A 20 A breaker on 14 AWG conductors removes the protection the conductor depends on, and it is one of the defects worth having an electrician look at rather than living with.
Why is aluminium capped lower than copper at the same size?
Aluminium has lower conductivity, so the same physical size carries less current — 10 AWG aluminium is capped at 25 A against copper’s 30 A. Aluminium branch conductors also carry their own terminations and connection history, which is a larger subject than gauge and one to raise with an electrician rather than resolve from a table.
Which NEC edition are these values from?
Table 310.16 ampacities and the 240.4(D) caps used here were corroborated against two independent published references on 2026-09-05. Adoption and amendment of the National Electrical Code is local. A state or city may be on an older edition, may amend it, and may add rules of its own. Confirm the adopted edition and the amendments with the authority having jurisdiction before relying on any code figure here.
The tool says one size and my electrician specified another. Who is right?
Probably your electrician, and the useful move is to ask which factor drove it. Derating, ambient, termination temperature, run length, future load and the manufacturer’s installation instructions can each push the answer up, and this screen sees none of them. A specific answer to that question is easy for a competent electrician to give.

Sources and methodology

Figures dated 5 September 2026. Last reviewed .

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