Electrical · overcurrent

Breaker size calculator, checked against the wire

Enter the continuous and non-continuous halves of a branch-circuit or feeder load and this page returns the breaker: the non-continuous amps plus 125% of the continuous amps, taken up to the next of the 37 standard ratings in NEC 240.6(A). It then runs the answer backwards and prints the smallest conductor that rating may protect, reduced for ambient, for bundling, for the 110.14(C) termination column and for the 240.4(D) small-conductor cap, so the page never hands back a breaker the wire underneath it cannot legally take.

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37 standard ratings
80% continuous rule
Conductor cross-check

The load, and the wire it runs on

Split the load at three hours. Article 100 calls anything expected to run three hours or more continuous, and that half alone is multiplied by 1.25.

Continuous load (A)

Non-continuous load (A)

Conductor metal

Insulation rating of the wire

Termination temperature marking

The lowest marking anywhere on the circuit wins — breaker, lug, splice or device.

Ambient around the raceway

Default 30 °C — NEC Table 310.16 heading: 'Based on Ambient Temperature of 30°C (86°F)'. An attic or a sunlit rooftop run is well above it.

Current-carrying conductors bundled

Grounds never count; a neutral usually does not, unless it is the neutral of a three-wire circuit off a four-wire wye or the load has real harmonic content.

Conductor you are installing

Set this when the wire went up a size for voltage drop — the grounding conductor has to follow it.

This circuit supplies more than one receptacle for cord-and-plug-connected portable loads — which removes the 240.4(B) round-up under 240.4(B)(1).

Breaker

40 A

Calculated 40 A, taken to the next rating in 240.6(A)

Minimum conductor

8 AWG

8.37 mm² of metal, copper

How the conductor’s allowable load was reduced, step by step
Table 310.16 at 90 °C	55 A
After 30 °C ambient and 3 conductors	55 A
110.14(C) ceiling, 75 °C column	50 A
240.4(D) small-conductor cap	none for this size
Largest breaker this conductor may take	50 A
Equipment grounding conductor	10 AWG

What decided it. The conductor stops at 50 A because of the 75 °C termination ceiling of 50 A under 110.14(C), and that figure is itself a 240.6(A) rating, so 50 A needs no rounding rule at all. The 40 A the load calls for sits at or below it.

NEC (NFPA 70) 2023, 210.20(A) for branch circuits, 215.3 for feeders and 230.90(A) for services: where a branch circuit supplies continuous loads, the rating of the overcurrent device shall not be less than the noncontinuous load plus 125 percent of the continuous load. 'Continuous load' is defined in Article 100 as a load where the maximum current is expected to continue for 3 hours or more. NEC 110.14(C): the temperature rating used for a conductor's ampacity may not exceed the lowest temperature rating of any connected termination, conductor or device. 110.14(C)(1)(a) puts circuits of 100 A or less, and equipment marked for 14 AWG–1 AWG, on the 60 °C column unless the equipment is listed for higher; 110.14(C)(1)(b) puts larger circuits on 75 °C. Equipment marked 90 °C at its terminals is rare outside of specific listings — do not assume it.

These are the code tables and the arithmetic over them. The page states what an article says and stops there; it does not inspect your installation, and it issues no finding that a circuit is compliant or safe. That judgment belongs to the electrician who installs it, the engineer who stamps it and the authority having jurisdiction who accepts it.

Standard breaker ratings to 400 A

The panel-door sheet: every 240.6(A) rating up to 400 A, the continuous load it may carry at 80%, and the smallest 75 °C conductor whose ampacity reaches the rating. Drawn at 30 °C with three current-carrying conductors on 75 °C terminations — change any of those above and the conductor column moves.

Breaker	Max continuous load	Copper	Aluminum	Cu ground
15 A	12 A	14 AWG	12 AWG	14 AWG
20 A	16 A	12 AWG	10 AWG	12 AWG
25 A	20 A	10 AWG	10 AWG	10 AWG
30 A	24 A	10 AWG	8 AWG	10 AWG
35 A	28 A	8 AWG	8 AWG	10 AWG
40 A	32 A	8 AWG	8 AWG	10 AWG
45 A	36 A	8 AWG	6 AWG	10 AWG
50 A	40 A	8 AWG	6 AWG	10 AWG
60 A	48 A	6 AWG	4 AWG	10 AWG
70 A	56 A	4 AWG	3 AWG	8 AWG
80 A	64 A	4 AWG	2 AWG	8 AWG
90 A	72 A	3 AWG	2 AWG	8 AWG
100 A	80 A	3 AWG	1 AWG	8 AWG
110 A	88 A	2 AWG	1/0 AWG	6 AWG
125 A	100 A	1 AWG	2/0 AWG	6 AWG
150 A	120 A	1/0 AWG	3/0 AWG	6 AWG
175 A	140 A	2/0 AWG	4/0 AWG	6 AWG
200 A	160 A	3/0 AWG	250 kcmil	6 AWG
225 A	180 A	4/0 AWG	300 kcmil	4 AWG
250 A	200 A	250 kcmil	350 kcmil	4 AWG
300 A	240 A	350 kcmil	500 kcmil	4 AWG
350 A	280 A	500 kcmil	750 kcmil	3 AWG
400 A	320 A	600 kcmil	1000 kcmil	3 AWG

NEC (NFPA 70) 2023, 240.6(A) Fuses and Fixed-Trip Circuit Breakers — the standard ampere ratings for fuses and inverse time circuit breakers. Conductor columns are the 75 °C copper and aluminum entries of NEC Table 310.16 (21 sizes carried here); grounding column from Table 250.122 (19 rows). Ambient correction from Table 310.15(B)(1) (16 bands), bundling from Table 310.15(C)(1) (6 bands), small-conductor caps from 240.4(D) (7 rows). Round-up rule: NEC (NFPA 70) 2023, 240.4(B) Devices Rated 800 A or Less: the next higher standard overcurrent device rating above the ampacity of the conductors being protected is permitted, provided all three of the conditions listed in 240.4(B)(1) through (3) are met. NEC 240.4(C) Devices Rated over 800 A: where the overcurrent device exceeds 800 A, the ampacity of the conductors it protects shall be equal to or greater than its rating — there is no round-up above 800 A.

How to size a breaker for a 32 A continuous load

The worked example the fields open on: a 32 A continuous load, THHN copper on 75 °C lugs, three conductors in a raceway at 30 °C.

Split the load at three hours
Put the part of the load that runs three hours or more in the continuous field and the rest in the non-continuous one. A 32 A continuous load gives 32 × 1.25 = 40 A; the same 32 A drawn intermittently would give 32 A and a 35 A breaker. If your figure is in watts or volt-amperes rather than amps, convert it first — the amperage calculator does the nameplate arithmetic for a mixed panel.
Tell the page what the terminations are marked
Set the wire's own insulation rating and, separately, the temperature marking on the breaker and lugs. 90 °C wire on 75 °C lugs is the normal case and the two columns do different jobs: the 90 °C figure is the starting point for the ambient and bundling corrections, and the 75 °C figure is the ceiling those corrections may not lift the answer above. Raise the ambient if the raceway runs through an attic or across a roof.
Read the binding constraint, not just the number
The result panel shows the conductor's Table 310.16 value, what it drops to after correction, the termination ceiling and the 240.4(D) cap, then names which of the four decided. That sentence is what to write on the drawing: the 40 A this example calls for takes 8 AWG copper even though the 90 °C column gives 10 AWG exactly 40 A, because two limits sit under that figure — 110.14(C) holds 10 AWG to the 35 A of the 75 °C column, and 240.4(D)(7) then caps any device on 10 AWG copper at 30 A.

Technical specifications

Standard ratings	37 breaker ratings from 15 A to 6,000 A, NEC 240.6(A). There is no 55 A or 63 A breaker in that list, which is why a calculated 52 A becomes 60 A.
Continuous-load factor	125% of the continuous load, added to 100% of the rest — NEC 210.20(A) for branch circuits, 215.3 for feeders. The same rule read from the other end is the 80% every electrician quotes: 1 ÷ 1.25 = 0.80.
Conductor sizes checked	21 sizes, 14 AWG through 1000 kcmil, in copper and aluminum at 60, 75 and 90 °C. Aluminum has no 14 AWG row and the page returns nothing rather than substituting copper.
Small-conductor caps	7 rows of NEC 240.4(D): 15 A on 14 AWG copper, 20 A on 12 AWG copper, 30 A on 10 AWG copper, and 15 A / 25 A on 12 and 10 AWG aluminum. Applied after ambient and bundling, not before.
Round-up ceiling	800 A. NEC 240.4(B) permits the next standard rating above a conductor's ampacity only up to 800 A; above that 240.4(C) requires the conductor ampacity to equal or exceed the device rating, with no round-up at all.
Grounding conductor	Table 250.122, 19 rows, sized on the breaker rather than on the load. Where you install a conductor larger than the minimum, the page applies the 250.122(B) proportional increase to the ground as well.
Ambient band	10 °C or less through 85 °C, from Table 310.15(B)(1). Outside that band the code table has no factor and the page returns none — an interpolated correction is not an answer.
Where it runs	In your browser. No load figure, conductor choice or ambient you type leaves the tab, so the page works in a basement with no signal.

Frequently asked questions

Why does a 40 A continuous load need a 50 A breaker and not a 40 A one?

Because 210.20(A) makes you rate the device at 125% of a continuous load: 40 × 1.25 = 50 A. Article 100 defines continuous as a load whose maximum current is expected to continue for three hours or more, which covers most commercial lighting, an EV charger and a long-cycling heater, and does not cover a range or a dryer. The one way out is an assembly listed for operation at 100% of its rating, which 210.20(A) allows and which is rare below 400 A and always marked.

The 90 °C column says 12 AWG copper is good for 30 A. Can I put 25 A on it?

No — 240.4(D)(5) caps the overcurrent device on 12 AWG copper at 20 A regardless of what any ampacity column says. The 90 °C figure is not a permitted load; 110.14(C) already limits you to the termination column, and 240.4(D) then limits the device on top of that. This is the single most common wrong answer produced by breaker calculators that read one column and stop, and it is why this page prints all three reductions rather than the survivor alone.

My calculation came out at 63 A. Is there a 63 A breaker?

Not in NEC 240.6(A) — the North American list jumps 60, 70, 80, 90, 100, so 63 A rounds to 70 A. 63 A is an IEC preferred rating and you will find it on a DIN-rail breaker sold for a European installation; fitting one into a panel listed to UL 67 and UL 489 is a different problem than picking a number. 240.6 does recognize a non-standard rating where the device is specifically listed for it, which is how adjustable-trip and some equipment-mounted devices are handled.

When may the breaker be bigger than the wire's ampacity?

Under 240.4(B), and only when all three of its conditions hold: the circuit is not a multi-outlet branch circuit serving cord-and-plug-connected portable loads, the conductor's ampacity does not itself land on a standard rating, and the rating you round up to is 800 A or less. The page prints all three whenever it takes that round-up, and switches the round-up off when you tick the multi-outlet box. Above 800 A the round-up disappears entirely under 240.4(C).

If I up-size the wire for voltage drop, does the ground grow too?

Yes, proportionally, under 250.122(B) — and almost nothing on the internet does this step. Where the ungrounded conductors are increased above the size that had sufficient ampacity, the equipment grounding conductor is increased in the same ratio of circular mils. A 20 A circuit pulled in 8 AWG instead of 12 AWG for a long run is a ratio of 16,510 ÷ 6,530 = 2.53, which takes the 12 AWG ground's 6,530 cmil to 16,510 cmil and makes the ground 8 AWG as well. Set the installed conductor above and the page applies it.

Does the breaker itself lose rating in a hot panel?

The corrections on this page reduce the conductor, not the breaker, and the two are separate problems. A molded-case breaker's thermal element is calibrated in open air at a stated ambient, and manufacturers publish an ambient-derating curve for enclosure temperatures above it — a breaker in a full panel in a boiler room can trip below its marked rating. That curve is the manufacturer's and no code table replaces it, so read it off the catalog rather than off a calculator.

Does a two-pole breaker carry twice the rating?

No. A 30 A two-pole breaker is 30 A per pole, and the load current is the same 30 A flowing through both. What the second pole buys is the second ungrounded conductor of a 240 V or three-wire circuit and the common trip that opens them together; 240.15(B) is what governs when a multiwire branch circuit needs handle ties or a common trip. Sizing is done on the current in one conductor, which is the figure this page asks for.

About the 80% rule and the four limits under it

Sizing a breaker looks like one multiplication and is really two questions asked in opposite directions. Forward: what rating does the load require? That is 210.20(A) — the non-continuous load plus 125% of the continuous load, taken up to the next rating in 240.6(A). Backward: what rating may the conductor carry? That is 240.4, and the answer is the conductor’s ampacity after ambient correction and bundling adjustment, capped by the termination column of 110.14(C) and capped again for 14, 12 and 10 AWG by 240.4(D). A breaker is only an answer when both directions agree, and when they do not, the fix is a bigger conductor rather than a smaller breaker.

The termination rule is the one that catches people who know the ampacity table cold. 110.14(C) says the temperature rating used for a conductor may not exceed the lowest rating of any termination on the circuit, and almost every breaker, lug and receptacle you will touch is marked 75 °C. That does not make 90 °C insulation pointless: 110.14(C) explicitly lets you start the ambient and bundling corrections from the 90 °C column and only holds the corrected figure to the 75 °C value. Four conductors in a 40 °C attic on THHN is exactly that case, and it is where a page that only knows the 75 °C column over-sizes. If the run is long enough that the drop rather than the heat decides, the voltage drop calculator is the other half of the same decision, and the wire gauge chart carries the three ampacity columns for every size in one printable sheet.

Two circuits do not obey any of this, and it is worth knowing which so you do not spend an afternoon arguing with the wrong article. 240.4(G) sends motors, air conditioning, refrigeration, welders and fire alarm circuits to their own articles, where the device protects against short circuits only and may sit far above the conductor’s ampacity — that case, and the transformer primary, belong to the fuse size calculator. And nothing here says the breaker can open safely: a device only interrupts what its marked rating covers, which is a fault-current question answered on the short circuit current calculator. Sizing a whole panel of these at once is the electrical load calculator’s job. None of which is a verdict: this page reads published tables aloud and does the arithmetic between them, and whether a circuit as installed satisfies the code is settled by the person holding the meter and the inspector signing the card.

Where the circuit you type goes

Every number on this page is worked out by JavaScript running in the tab you are reading it in. Nothing you type — loads, lengths, nameplate ratings, the rates your utility charges you — is uploaded, logged or kept, which is also why the calculators carry on working in a mechanical room with no signal.

The panel-door chart at the foot of the tool is generated from the tables at build time, so printing it costs no request either — the page prints the same whether or not it can reach the network.