Circuit Breaker Size Calculator
This free breaker size calculator answers the second half of every circuit question: once you know the load, which standard breaker protects it? It converts watts to amps when that is what the nameplate gives you, applies the 125% rule for continuous loads like EV charging (NEC 210.20(A)), and rounds up to the next standard rating from Table 240.6(A) — never down.
It is equally honest about what a breaker calculator must NOT do: motor, HVAC, and welder circuits are sized from the equipment nameplate under their own code articles, where a breaker larger than the wire ampacity can be legal and correct. Pick the motor/HVAC load type and the calculator explains that path instead of guessing at it.
The breaker is only half the pairing — the conductor must be rated for it, including the small-conductor caps of 240.4(D) that limit 14, 12, and 10 AWG regardless of what the ampacity chart says. Every result links to the wire size calculator to complete the pair. Reference only: verify with a licensed electrician and your local AHJ. Free, no signup.
Breaker Size Calculator
What size breaker for your load? Enter amps or watts, mark continuous loads, and get the minimum standard breaker per NEC 240.6(A) with the 125% rule applied — plus the wire rating it must pair with. Free, no signup.
Load
Continuous = runs at max current 3+ hours (EV charging is the classic case) — the code requires 125% sizing. Motor/HVAC/welder circuits follow their nameplate, not the generic rules.
What you’re looking at inside the panel
The main breaker sets the service size. Below it, one slot means a 120 V circuit and two slots — a double-pole breaker — means 240 V, which is why every big-appliance breaker is a double.
The HVAC exception, straight from the nameplate
On air conditioners and heat pumps, two nameplate values replace the generic rules: the wire must meet the MCA, the breaker must not exceed the MOP — and the breaker legitimately may exceed the wire’s ampacity (Art. 440).
Calculation Formulas
Loads running 3+ hours at maximum current (EV charging, some heating) are sized at 125% per NEC 210.20(A).
Example:
16 A continuous → 16 × 1.25 = 20 A required → 20 A breaker.
Nameplates often give watts; the breaker is sized from the current.
Example:
A 5,500 W water heater element at 240 V draws 22.9 A → 25 A breaker (non-continuous).
Breakers come in standard ratings (15, 20, 25, 30, 35, 40, 45, 50, 60 A… in the residential range) — round up to the next one, never down.
Example:
20 A continuous → 25 A required → 25 A breaker (the next standard rating).
Permitted only when the circuit does not serve multiple receptacles for portable loads, the conductor ampacity does not match a standard rating, and the next size is ≤ 800 A. Above 800 A (240.4(C)), the conductor must meet or exceed the device rating.
Example:
A conductor with 55 A ampacity may be protected at 60 A when the conditions are met.
The wire pairing must respect the small-conductor limits regardless of table ampacity.
Example:
12 AWG copper may not be protected above 20 A even though its 75°C ampacity is 25 A.
Standard Constants
| Constant | Value | Description |
|---|---|---|
| Continuous-load factor | 125% | NEC 210.20(A) — OCPD sized at not less than 125% of continuous load. |
| Residential standard ratings | 15–200 A | From Table 240.6(A): 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200. |
| Fuse-only ratings | 1, 3, 6, 10, 601 A | Additional standard ratings that exist only for fuses. |
| Motor inverse-time allowance | up to 250% | NEC 430.52 — motor branch breakers may exceed conductor ampacity to ride through starting current (nameplate governs; not computed here). |
| 800 A boundary | 240.4(C) | Above 800 A, no next-size-up rounding — conductor ampacity must equal or exceed the device. |
Note: All calculations include appropriate waste factors based on project complexity and material type. Results are estimates and should be verified by professionals before purchasing materials.
NEC 210.20(A) — Continuous loads(NFPA 70 (2023) 210.20(A))
View StandardBranch-circuit overcurrent devices serve at least the non-continuous load plus 125% of the continuous load.
Key Requirements:
- •125% sizing for loads running 3+ hours
NEC Table 240.6(A) — Standard ratings(NFPA 70 (2023) 240.6(A))
View StandardThe standard ampere ratings for fuses and inverse-time circuit breakers.
Key Requirements:
- •Round up to the next standard rating
- •Nonstandard ratings permitted but uncommon
NEC 240.4(B)/(C)/(D) — Protection of conductors(NFPA 70 (2023) 240.4)
View StandardThe next-size-up allowance and its three conditions, the 800 A boundary, and the small-conductor caps.
Key Requirements:
- •14/12/10 AWG Cu capped at 15/20/30 A
- •Next-size-up conditions
NEC Articles 430 / 440 / 630 — Special circuits(NFPA 70 (2023) Art. 430, 440, 630)
View StandardMotors, HVAC, and welders size their overcurrent protection from the nameplate and their own articles — this calculator flags them instead of guessing.
Key Requirements:
- •HVAC: breaker = nameplate MOP
- •Motors: up to 250% inverse-time
- •Welders: duty-cycle allowances
Standards Disclaimer: Standards and codes are subject to periodic updates. Always verify current requirements with local building authorities and professional engineers before beginning construction. Links provided are for reference only.
Homeowner permits and panel work
The panel is the line many jurisdictions draw
Even states with homeowner electrical provisions often treat service equipment and panel interiors more strictly than branch wiring.
Regional Examples:
AFCI/GFCI breaker requirements
Where the plain breaker is not enough
Modern code cycles require AFCI protection on most living-area circuits and GFCI in wet/outdoor locations — usually implemented as the breaker type.
Regional Examples:
Adopted NEC edition
2020 / 2023 / 2026
Standard ratings and the 125% rule are unchanged across editions; AFCI/GFCI reach expands each cycle.
Regional Examples:
Before You Build
- •Contact your local building department for specific requirements
- •Verify frost line depths, wind zones, and seismic requirements for your area
- •Check if permits are required and schedule required inspections
- •Consult with a local contractor familiar with local codes
Want to Learn More?
How to tell if your house has 100, 150, or 200 amp service — main breaker rating, service wire size, and meter class, plus the cases where each one lies.
Read the What Amp Service Do I Have? 3 Ways to Tell (100 vs 200)
Plan disposal before you start
Smaller jobs still produce more debris than a few trash bags can hold. Check what's allowed in a dumpster and which disposal option fits the scope.
See disposal options →
Related Calculators
Wire Size Calculator
What gauge wire do you need? Free NEC wire size calculator by amps, wiring method, and run length — with derating, 240.4(D), and appliance presets.
Voltage Drop Calculator
How much voltage do you lose over a wire run? Free NEC K-factor calculator for volts and % drop by wire size, length, and load — with the 3% check.
EV Charger Circuit Calculator
What wire and breaker does your EV charger need? Free NEC Article 625 calculator by EVSE amps — 125% rule, hardwire vs plug, GFCI, and run length.
How to Use This Calculator
- Enter the load — amps directly, or watts plus voltage if that is what the nameplate shows.
- Choose the load type: non-continuous for typical appliances, continuous for anything running 3+ hours at full current (EV charging is the classic case), or motor/HVAC/welder to see why the nameplate governs those.
- Click Calculate for the minimum standard breaker (NEC 240.6(A)), the required capacity behind it, and the conductor rating it must pair with.
- Size the wire to match with the wire size calculator — breaker and conductor are one decision, not two.
Why Breakers Round Up — and When They May Not
Breakers come in standard ratings (15, 20, 25, 30, 35, 40, 45, 50, 60 and up per NEC Table 240.6(A)), so a 22.9 A water-heater load lands on a 25 A breaker — the next standard size. But rounding up past the CONDUCTOR's ampacity is only permitted under the three conditions of 240.4(B): the circuit can't serve multiple receptacles for portable loads, the conductor ampacity can't already match a standard rating, and the next size can't exceed 800 A. And small conductors have hard caps regardless (240.4(D)): 14 AWG copper stops at 15 A, 12 at 20 A, 10 at 30 A. The one place bigger-than-expected breakers are routinely correct: motors and HVAC, where starting current is allowed to dictate protection (up to 250% for inverse-time breakers under 430.52, or whatever the HVAC nameplate MOP says) while the wire is sized separately. That is why this calculator hands those loads to the nameplate instead of computing them.
Frequently Asked Questions
What size breaker do I need for a 4500 watt water heater?
A 4,500 W element at 240 V draws 18.75 A; a 5,500 W element draws 22.9 A. Both land on a 25 A or 30 A breaker depending on how your jurisdiction treats water heaters — many electricians size storage water heaters at 125% (which makes 4,500 W → 23.4 A → 25 A, and the common install is 30 A with 10 AWG wire). Enter your element wattage and voltage in the calculator, and confirm the load type treatment with your inspector — then pair the wire with the wire size calculator.
What are the standard breaker sizes?
Per NEC Table 240.6(A), the standard ampere ratings in the residential range are 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, and 200 A (the table continues to 6,000 A, and 1/3/6/10/601 A exist for fuses only). Your required capacity rounds UP to the next standard rating — a 22.9 A load takes a 25 A breaker, a 52 A load takes a 60 A breaker. Never round down.
When do I multiply by 125%?
When the load is continuous — expected to run at maximum current for 3 hours or more (NEC 210.20(A)). EV charging is the classic residential continuous load: a 48 A charger needs 48 × 1.25 = 60 A of protection. Most ordinary appliances — dryers, ranges, dishwashers — cycle on and off and are sized as non-continuous. When in doubt, the equipment installation manual states the required circuit, and it always governs.
Can my breaker be bigger than my wire's rating?
Almost never — and the exceptions are specific. The general rule is the conductor must be protected at or below its ampacity, with small conductors hard-capped by 240.4(D) (14 AWG Cu at 15 A, 12 at 20 A, 10 at 30 A). The 240.4(B) allowance lets you round UP to the next standard breaker only when the ampacity doesn't match a standard rating, the circuit isn't a multi-receptacle portable-load circuit, and you stay at or under 800 A. The legitimate big exception: motors and HVAC, where starting current allows oversized protection under Articles 430/440 — sized from the nameplate, not this calculator.
Why won't this size my AC or well pump breaker?
Because the code sizes those from the equipment nameplate, not from the running amps. HVAC equipment lists an MOP (Maximum Overcurrent Protection) on its nameplate — that IS the breaker answer, and under Article 440 it may exceed the wire's ampacity to ride through compressor starting current. Motor circuits similarly allow inverse-time breakers up to 250% of full-load current (430.52). Feeding those loads through the generic 125% math produces wrong (usually too-small) breakers that nuisance-trip — so the calculator routes you to the nameplate instead.