Standard breaker and fuse size calculator

How the rating is picked

Table 240.6(A) lists the standard ratings for fuses and inverse time circuit breakers. Which way to round depends on the job. A load rounds up: the device is no smaller than the load, with 125 percent of the continuous part. A wire is protected at its ampacity, so the rating rounds down, except that 240.4(B) lets it go up one size when the ampacity falls between standard ratings, the circuit is not a multi-receptacle branch circuit, and the device is 800 A or less.

Load

rating >= noncontinuous + 1.25 x continuous210.20(A) for a branch circuit and 215.3 for a feeder. A 40 A continuous load needs 50 A and a 50 A device. A 45 A continuous load needs 56.3 A, which rounds up to 60 A. 100 A continuous plus 100 A noncontinuous is 225 A, itself a standard rating.

100 percent rated assembly

rating >= noncontinuous + 1.00 x continuousThe exception in 210.20(A) and 215.3, only where the assembly, device included, is listed for continuous operation at 100 percent of its rating. The same 45 A continuous load then takes 45 A.

Wire, main rule

rating <= ampacity240.4: a conductor is protected at its ampacity after correction and adjustment. 500 kcmil copper at 75°C (167°F) is 380 A, and the largest standard rating at or below it is 350 A.

Next size up, 240.4(B)

next standard rating above the ampacity, if it is 800 A or lessAllowed when the ampacity is not itself a standard rating and the circuit does not supply more than one receptacle for cord-and-plug loads. 380 A takes 400 A; two sets, 760 A, take 800 A. 1/0 copper at 150 A is a standard rating already, so it stays at 150 A.

Over 800 A, 240.4(C)

rating <= ampacityAbove 800 A there is no rounding up. Three sets of 500 kcmil carry 1,140 A, under the 1,200 A device, so the largest rating is 1,000 A. Three sets of 600 kcmil, 1,260 A, carry 1,200 A.

Small conductors, 240.4(D)

rating <= 15, 20, 30 A on 14, 12, 10 AWG copperAnd 15 A on 12 AWG and 25 A on 10 AWG aluminum, unless 240.4(E) or (G) says otherwise. 12 AWG copper at 25 A of ampacity is still held to 20 A.

Eight wires through the same rules

The first four rows are Charles R. Miller's service and feeder examples in Electrical Contractor, and the 17.6 A row is his receptacle circuit: 12 AWG THWN at 38°C (100°F) with six current-carrying conductors. Each row shows the standard ratings either side of the ampacity and the rule that picks the breaker.

Table 240.6(A), standard ampere ratings

These ratings apply to fuses and inverse time circuit breakers. The 2023 edition added 10 A for breakers; before it, the smallest standard breaker was 15 A. Fuses also come in 1, 3, 6 and 601 A, the last one the start of the Class L range.

Standards and data sources

• NEC 2023, Table 240.6(A), 240.4(B), (C) and (D), 240.6(B) and (C), 210.20(A) and 215.3 - the standard fuse and breaker ratings, the next size up, the over 800 A rule, the small-conductor limits, adjustable-trip ratings and the 125 percent rule for continuous loads. The text was read as the sources below print it, not in NFPA's viewer.
• Mike Holt, EC&M Code Q&A, Fuse and Breaker Ratings (2019) and Rounding Up Rules (2019) - the three conditions of 240.4(B), nonstandard ratings being permitted, the fuse-only ratings and the adjustable-trip rules of 240.6(B) and (C), in the 2017 edition.
• Charles R. Miller, Electrical Contractor, Sizing Conductors, Part XIX - the worked examples this page is tested against: 500 kcmil at 380 A on 400 A, two sets at 760 A on 800 A, three sets at 1,140 A refused for 1,200 A under 240.4(C), and 600 kcmil and 350 kcmil sets that carry 1,200 A.
• Charles R. Miller, Electrical Contractor, Sizing Conductors, Part XVII - 380 A protected at 350 A by the main rule, and 12 AWG THWN at 17.6 A on a circuit of 11 receptacles that may not round up to 20 A.
• Open Exam Prep, Overcurrent Protective Devices (2023 NEC) - 10 A moved into Table 240.6(A) as a breaker rating in the 2023 edition, and the fuse-only ratings of 1, 3, 6 and 601 A.
• Mike Holt, EC&M, Code Requirement for Sizing the Overcurrent Protection of Conductors (2018) - the 240.4(D) limits by size, and the next size up not applying to tap conductors and transformer secondary conductors.

Limitations of this calculator

• This page gives preliminary numbers for estimating and for checking a quote. A licensed electrician sizes and installs the circuit under the permit and the inspection of the local authority having jurisdiction (AHJ). Nothing here approves an installation.
• The NEC text was read as Mike Holt, Charles R. Miller and an exam-prep summary of the 2023 edition print it, not in NFPA's own viewer. Most of the worked examples use 2017 numbering. Check the edition and the local amendments your AHJ enforces; 10 A became a standard breaker rating only in 2023.
• Equipment with its own rules is not worked: motors (Article 430), air conditioners and heat pumps (Article 440, the nameplate MOCP), and the other circuits 240.4(G) lists. Those can call for a breaker larger than the wire ampacity. Use the motor, AC breaker and AC wire calculators for them.
• Taps (240.21), transformer secondary conductors (240.4(F)) and flexible cords and fixture wires (240.5) are not worked, and the next size up does not apply to taps or secondaries.
• Adjustable-trip breakers are noted, not worked. A breaker is rated at its highest long-time pickup setting unless the adjusting means has restricted access (240.6(B) and (C)).
• The ampacity is your entry. Which column applies (60, 75 or 90°C, that is 140, 167 or 194°F), the ambient correction and the conductor-count adjustment come from Table 310.16 and 310.15; the wire ampacity derating calculator works them.
• Which loads are continuous, the interrupting rating (110.9), the series rating of a panel and selective coordination are outside this page.

Common mistakes with breaker and fuse sizes

1. Rounding up on a receptacle circuit. 12 AWG THWN copper at 38°C (100°F) with six current-carrying conductors carries 17.6 A. On a branch circuit with more than one receptacle the next size up is not allowed, so the largest breaker is 15 A, too small for a 20 A circuit. The same wire on a single-outlet circuit could take 20 A. Miller's answer for the receptacle circuit is 10 AWG.
2. Using the next size up above 800 A. Two sets of 500 kcmil go on 800 A, so three sets look right for 1,200 A. They carry 1,140 A, and above 800 A the ampacity must be at least the rating (240.4(C)), so the largest device is 1,000 A. Three sets of 600 kcmil (1,260 A) or four sets of 350 kcmil (1,240 A) carry 1,200 A.
3. Rounding up from an ampacity that is already standard. 1/0 copper in the 75°C (167°F) column is 150 A, a standard rating, so it gets a 150 A breaker, not 175 A. Rounding up works only from an ampacity that falls between two standard ratings (240.4(B)(2)).
4. Skipping 240.4(D) on small wire. 12 AWG copper is 25 A in the 75°C (167°F) column, and 25 A is a standard rating, but 240.4(D) holds it to 20 A. 10 AWG at 24 A after derating is not a 30 A wire either; it takes 25 A. Air conditioners and motors are the exceptions, through 240.4(G).
5. Taking a load straight to the table. A 45 A continuous load is not a 45 A breaker. The breaker is at least 125 percent of it, 56.3 A, so 60 A. Only an assembly listed for 100 percent continuous operation takes 45 A, and most panel breakers are not listed that way.
6. Looking for a 601 A breaker. 601 A is a fuse-only rating, the start of the Class L range. A wire of 600.5 A ampacity can take a 601 A fuse under the next size up, but a breaker on the same wire goes to 700 A. The fuse-only ratings are 1, 3, 6 and 601 A.

Common questions about standard breaker and fuse sizes

Related calculators

• Wire size calculator The conductor for a load, with the 125 percent rule and the breaker that goes with it.
• Wire ampacity derating calculator The ampacity after the ambient correction and the conductor count, the number this page starts from.
• AC breaker size calculator Air conditioner and heat pump breakers from the nameplate MCA and MOCP, under Article 440.
• Equipment grounding conductor size calculator The ground wire for the breaker or fuse rating this page gives.
• kW to amps calculator The load current from kW, kVA or watts before it is matched to a standard rating.

Used on the same job

• Wire ampacity derating calculator The ampacity to enter here.
• Wire size calculator The wire for the load and the breaker together.
• Equipment grounding conductor size calculator The ground wire for the rating.