Breaker Size Calculator — NEC Continuous-Load Rule and the 240.6(A) Standard Ratings

Background.

A breaker size calculator answers the question every renovation, EV charger install and shop fit-out runs into: the load is known, so what rating of circuit breaker does the code actually require? The answer is almost never the load itself. Under the National Electrical Code, an overcurrent device that will carry a load for three hours or more has to be rated for 125 percent of that load, which is why a 48-amp EV charger takes a 60-amp breaker rather than a 50-amp one, and why an office lighting feeder drawing 150 amps needs far more than a 150-amp device.

This page implements exactly two rules, and it is worth being able to name them. The first is the continuous-load multiplier. NEC 210.20(A) covers branch circuits and NEC 215.3 covers feeders, and the two sections are worded identically: the rating of the overcurrent device shall not be less than the noncontinuous load plus 125 percent of the continuous load. A continuous load is defined in NEC Article 100 as one whose maximum current is expected to continue for three hours or more — not one that runs often, and not one that runs all day intermittently. Both sections carry the same exception: where the whole assembly, including the overcurrent devices protecting the circuit, is listed for operation at 100 percent of its rating, the multiplier drops to 1. That listing belongs to the assembly, not to the breaker on its own, so the standard option here is the right one in almost every dwelling and most light commercial work.

The second rule is the ladder of standard sizes. NEC 240.6(A) fixes the standard ampere ratings for fuses and inverse-time circuit breakers at 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000 and 6000 amperes. The calculator takes the requirement produced by the first rule and returns the smallest entry in that list which is not below it. That comparison is made on the unrounded requirement, so a load that computes to exactly 60.0 amperes takes a 60-amp device while one that computes to 60.000001 amperes takes a 70-amp device. The result panel prints the requirement to four decimal places for exactly this reason.

What this page will not do matters as much as what it does, and it is stated beside the result rather than buried in an FAQ. It does not size motor branch circuits: NEC 430.52 sets the branch-circuit short-circuit and ground-fault protective device as a percentage of the motor full-load current, and routinely permits an inverse-time breaker far above the conductor ampacity, so applying the 125 percent rule to a motor gives an answer that is both wrong and nuisance-tripping. It does not size air-conditioning or refrigeration equipment, where NEC 440.22 makes the nameplate maximum overcurrent protective device the governing number — read the MOCP off the unit and use that. And it does not size the conductor. A breaker rating is only half of an overcurrent-protection design; the conductor has to satisfy NEC 240.4, including the small-conductor limits in 240.4(D), and no ampacity table is asserted anywhere on this page.

The sections quoted here are from NFPA 70, National Electrical Code, 2023 edition. Local amendments govern, and the edition adopted in your jurisdiction may not be the 2023 one — states, cities and counties adopt on their own cycles and amend the text freely, and some of those amendments change exactly this arithmetic. Treat the number this page returns as a design starting point that a licensed electrician or electrical engineer must confirm and sign off before any work proceeds. It is not a substitute for a permit, a plan review or an inspection, and installing an overcurrent device that is too large is one of the few electrical mistakes that shows no symptom at all until there is a fire.

What is breaker size calculator?

A circuit breaker is an overcurrent protective device: it protects the conductors of a circuit from being loaded beyond what they can carry without overheating. Sizing one is therefore not a matter of picking a rating that comfortably exceeds the load. It is a matter of satisfying a minimum set by the load and a maximum set by the conductor, and this page computes the minimum.

The vocabulary matters. A continuous load, per NEC Article 100, is one whose maximum current is expected to continue for three hours or more. A standard overcurrent device is what the trade calls 80 percent rated: it is tested to carry 100 percent of its rating, but the code will only let you plan a continuous load at 80 percent of it, which is the same statement as the 125 percent multiplier seen from the other side. An assembly listed for operation at 100 percent of its rating is a specific, labelled listing covering the enclosure, the bus and the device together; it is uncommon, it is expensive, and it is not something you can assume. The standard ampere ratings are the fixed ladder in NEC 240.6(A) — you cannot buy a 63-amp breaker to a North American panel schedule and call it standard.

The calculator is valid for non-motor branch circuits and feeders: receptacle circuits, lighting, electric heat, water heaters, ovens, and electric-vehicle supply equipment. It is not valid for motors, for air-conditioning and refrigeration equipment, for welders, for transformer primaries, or for service-entrance conductors, each of which has its own article and its own multiplier.

How to use this calculator.

1. 01Split the load into its continuous and noncontinuous parts first. The test is the NEC Article 100 one: is the maximum current expected to continue for three hours or more? EV charging, store and office lighting, electric heat and commercial refrigeration are continuous. A cooktop, a dryer and a receptacle circuit generally are not.
2. 02Choose whether you are entering amperes or watts. Amperes is the more direct route when you have a nameplate current; watts mode converts using the voltage, phase and power factor below it.
3. 03Leave the device type on standard unless you are physically reading a 100-percent-rated listing off an assembly label. The listing belongs to the assembly, not the breaker, and assuming it understates the breaker by 20 percent.
4. 04Set the circuit type and voltage. Three-phase mode wants the line-to-line voltage and applies the √3 factor when converting watts to amperes.
5. 05Set the power factor to 1 for resistive loads. It affects only the watts-to-amperes conversion, never the 125 percent rule.
6. 06Read the required minimum device rating before the breaker size. That number is the code requirement; the breaker size is just the next rung of the 240.6(A) ladder above it.
7. 07Check the continuous load the device may carry. If that figure is below your continuous load you have entered something inconsistent — on a standard device it is always exactly 80 percent of the rating.
8. 08Take the result to a licensed electrician along with the conductor size, which this page does not compute, and confirm both against the code edition and amendments adopted where the work is being done.

The formula.

The arithmetic is two steps and the second one is where people go wrong.

Step one is the requirement. NEC 210.20(A) and 215.3 both say the rating of the overcurrent device shall not be less than the noncontinuous load plus 125 percent of the continuous load. So with a 48 A continuous load and no noncontinuous load, the requirement is 0 + 1.25 × 48 = 60.0 A. Nothing is rounded here. The multiplier is applied to the continuous portion only; the noncontinuous portion enters at its face value. When the assembly carries a listing for operation at 100 percent of its rating, the exception in both sections replaces 1.25 with 1.00, and the same 48 A load requires only 48 A.

Step two is the selection. NEC 240.6(A) fixes the standard ratings, and the device chosen is the smallest one that is not less than the requirement. This is a floor, not a rounding: 60.0 A selects 60 A because 60 is not less than 60. Push the requirement one part in ten million above a rung — 60.000001 A — and 60 A no longer satisfies it, so the answer becomes 70 A. That is why the result panel prints the requirement to four decimal places instead of the two it displays elsewhere, and why the boundary is tested at, just below and just above every one of the thirty-seven standard ratings. The comparison is made on the unrounded requirement, never on the displayed one.

The continuous-capacity figure is the same rule read backwards. If a continuous load of I requires a device of 1.25 × I, then a device of rating R may carry a continuous load of R ÷ 1.25, which is 0.8 × R. That is the origin of the trade phrase "80 percent rated", and it is why a 60 A device is the right home for a 48 A continuous load and the wrong home for a 50 A one.

Watts mode adds one conversion in front of step one. On a single-phase circuit, current is the real power divided by the product of the voltage and the power factor. On a balanced three-phase circuit the line-to-line voltage is used and the product gains a factor of √3, carried here at forty significant digits and never truncated to 1.732. The power factor plays no part whatsoever in the 125 percent rule — it only turns watts into amperes.

A worked example.

Frequently asked questions.

References& sources.

1. [1]NFPA 70, National Electrical Code, 240.6(A) 'Fuses and Fixed-Trip Circuit Breakers' — the standard ampere ratings for fuses and inverse-time circuit breakers: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000 and 6000 amperes, plus the fuse-only additional ratings of 1, 3, 6, 10 and 601 amperes. Authority: NFPA. Document revision: NEC 2023. Locator: 240.6(A). The code text itself is gated behind NFPA's free-access registration; the thirty-seven breaker ratings quoted here are reproduced identically by this host and by the independent deck cited below. This host abbreviates the fuse-only additional ratings to 1, 3, 6 and 10 A; the 601 A fuse-only rating is the code text's own addition, and all five are excluded from this page's ladder either way. Retrieved 2026-07-29.
2. [2]Minnesota Electrical Association, '2020 NEC Overcurrent Protection (2-hr), Part 1' instructor deck — an independent reproduction of the NEC 240.6(A) standard ampere ratings against a different code edition, used to confirm that the ladder is unchanged between the 2020 and 2023 editions. Authority: Minnesota Electrical Association (continuing-education provider). Document revision: 2020 NEC. Locator: 240.6(A). Access: open PDF. Independence: secondary-check. Retrieved 2026-07-29.
3. [3]UpCodes, 'Continuous and Noncontinuous Loads' — reproduces NEC 210.20(A) for branch circuits and NEC 215.3 for feeders, including the identical wording 'the rating of the overcurrent device shall not be less than the noncontinuous load plus 125 percent of the continuous load' and the exception for assemblies listed for operation at 100 percent of their rating. Authority: UpCodes (code-hosting service). Document revision: NEC as adopted. Locator: 210.20(A), 215.3. Access: open. Independence: primary text, secondary host. Retrieved 2026-07-29.
4. [4]Dakota Prep, 'Guide to Sizing Overcurrent Protection for Branch Circuits and Feeders' — the independent check for this page (BUILD-BRIEF §10.5). Two printed worked examples were compared against this calculator: 12 A continuous plus 5 A noncontinuous, printed as 'OCPD ≥ 20A' selecting a 20 A device; and 150 A continuous plus 45 A noncontinuous, printed as 'OCPD ≥ 232.5A' selecting 250 A. Both agree with the values this module returns, and both are asserted in breaker-size.test.ts. Authority: Dakota Prep (NEC exam preparation). Locator: branch-circuit and feeder OCPD examples. Access: open. Independence: primary check. Result: agree. Retrieved 2026-07-29.
5. [5]Electrical Contractor Magazine (NECA), 'Sizing Conductors' series — background on NEC 240.4 conductor protection, 240.4(B)'s next-higher-standard-device allowance and its three conditions, and 240.4(C) for devices rated over 800 amperes. Cited here to support the scope statement that this page sizes the overcurrent device only and does not size the conductor. Authority: National Electrical Contractors Association. Locator: 240.4(B), 240.4(C). Access: open. Independence: secondary-check. Retrieved 2026-07-29.
6. [6]IAEI Magazine, '100% vs 80%: Choosing the right OCPD solution' (May 2016) — explains why a standard overcurrent device may carry only 80 percent of its rating continuously, what an assembly listed for operation at 100 percent of its rating actually covers, and the conditions attached to that listing. Authority: International Association of Electrical Inspectors. Access: open. Independence: secondary-check. Retrieved 2026-07-29.