For electricians & homeowners
Circuit Breaker
Size Calculator
A breaker must be sized above a continuous load, not right at it — the NEC wants 125% headroom (the load can't exceed 80% of the breaker). Too small and it trips; too large and it won't protect the wire. This tool finds the right size in seconds.
A circuit breaker size calculator built for electricians — clear enough for anyone.
A 40 A continuous load needs 25% headroom. On a 40 A breaker it sits at 100% and trips; a 50 A breaker holds it at 80% — the NEC minimum.
NEC-based
210.20 · 240.6 standard sizes
Breaker + wire
Sized together, every time
Free & private
No signup, runs in your browser
Circuit Breaker Size Calculator
Size the breaker for your load — and the minimum wire to protect it.
Enter a load to size the breaker.
Load → breaker → wire
Compare standard sizes — tap to preview (loaded % · wire)
Method — how the breaker is sized
Standard sizes (NEC 240.6): 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400 A. Continuous loads sized at 125% (210.20(A)). Minimum wire from the 75 °C copper column with small-conductor limits (240.4(D)). Motor branch circuits: inverse-time breaker ≤ 250% FLA (430.52) with conductors at 125% FLA (430.22).
How to use it
How to use the circuit breaker calculator
It takes three quick inputs — your load, the voltage, and whether it runs continuously. The tool applies the NEC 125% rule, rounds up to a standard breaker size, and shows the wire gauge that breaker needs — so the two are always sized together.
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01
Pick your appliance
Choose a common load — water heater, dryer, AC, EV charger — or enter it yourself in watts or amps.
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02
Set voltage & continuous
Select the voltage (120 or 240 V) and flag a continuous load (running 3+ hours) — that triggers the 125% rule.
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03
Read the breaker size
Get the correct standard breaker size instantly, rounded up per NEC 240.6 — no guessing between sizes.
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04
Match the wire
See the wire gauge that breaker needs, plus any double-pole or GFCI/AFCI requirement. The breaker protects the wire — they're sized together.
The core idea
What determines breaker size
A short sequence — with one hard guardrail.
Breaker size follows a quick path: start with the load in amps, add 25% if it runs continuously (the NEC 125% rule), then round up to the next standard breaker. One guardrail overrides everything — the breaker can never be larger than the wire it protects.
Load in, breaker out — with a guardrail
The guardrail — the breaker protects the wire. Whatever the load math gives, the breaker can never exceed the wire's ampacity (NEC 240.4). Too large, and the wire can overheat before the breaker ever trips.
Breaker = round up( load × 1.25 ) — capped at the wire's ampacity
The method
How breaker size is calculated
The breaker formula is a short sequence: convert the load to amps (watts ÷ volts), multiply by 1.25 if it's continuous, then round up to the next standard breaker size. The result is capped by the wire's ampacity — a breaker can't be larger than the wire it protects.
There's no in-between — round up to a standard size (NEC 240.6)
Standard sizes (240.6): 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200 A… A calculated 47 A has no breaker, so it rounds up to 50 A.
Worked example
What size breaker for a 40 A EV charger?
A 40 A EV charger runs continuously, so it needs 25% headroom. On a 40 A breaker the load sits at 100% and trips; a 50 A breaker holds it at 80% — the NEC minimum — paired with 6 AWG copper.
40 A breaker
40 A ÷ 40 A = 100% loaded
continuous limit = 80%
100% > 80% → trips
50 A breaker
40 A × 1.25 = 50 A minimum
40 A ÷ 50 A = 80% loaded
80% ≤ 80% → holds
A 40 A continuous EV charger needs a 50 A breaker with 6 AWG copper — the smallest that carries the load with the required 25% headroom, on a double-pole 240 V circuit.
The code behind it
The NEC rules behind breaker sizing
Breaker sizing runs on a few NEC rules: size to 125% of a continuous load (210.20), round up to a standard size (240.6), and never exceed the wire's ampacity (240.4). Every breaker is matched to the wire it protects.
| Breaker | Copper wire (75 °C) | Typical use |
|---|---|---|
| 15 A | 14 AWG | Lighting, general outlets |
| 20 A | 12 AWG | Kitchen, bath, garage outlets |
| 30 A | 10 AWG | Dryer, water heater, small AC |
| 40 A | 8 AWG | Range, EV charger, large AC |
| 50 A | 6 AWG | Range, EV charger, hot tub |
| 60 A | 6 AWG | Subpanel, large hot tub |
| 100 A | 3 AWG | Subpanel, feeder |
Common 75 °C copper pairings. NM-B/Romex (60 °C) or long runs may need a size larger; aluminum runs about two sizes up. Size it exactly with the Wire Size calculator.
The 125% rule
Continuous loads (3+ hr) need a breaker rated 125% of the load — the load can't exceed 80% of the breaker.
Standard sizes
Breakers come in fixed sizes — 15, 20, 25, 30, 40, 50, 60… Round up to the next one; there's no in-between.
Small-wire caps
14 AWG → 15 A, 12 AWG → 20 A, 10 AWG → 30 A max — a hard ceiling, whatever the load math says.
Motors differ
Motor circuits size the breaker to about 250% of full-load amps for inrush — a separate calc from the 125% rule.
AFCI & GFCI
NEC 2023 requires AFCI on most 15/20 A circuits and GFCI in kitchens, baths, garages, and outdoors.
What affects it
What raises the size — and the mistakes to avoid
The breaker climbs with continuous loads (+25%), a bigger load, motor inrush, and any headroom you add for the future. The dangerous mistakes go the other way — oversizing past the wire, undersizing into nuisance trips, or splitting a 240 V circuit into two breakers.
Raises the breaker
what makes it bigger
- Continuous loads (+25%)Anything running 3+ hours adds the NEC 125% factor.
- A bigger loadMore amps or watts pushes the required size up directly.
- Motor inrushMotors draw a startup surge, so 430.52 sizes them higher.
- Future headroomSizing with room for added equipment — common in kitchens and shops.
The dangerous mistakes
what not to do
- 01Don't oversize past the wireThe #1 danger — the breaker protects the wire; too big and the wire overheats first.
- 02Don't undersize into tripsToo small and it nuisance-trips during normal operation.
- 03Don't split a 240 V circuitTwo 120 V breakers won't fault-protect a 240 V load — use a double-pole.
- 04Don't ignore voltage dropA breaker never fixes drop on a long run — check it separately.
The one rule
The breaker protects the wire, not the load
Every sizing choice bows to this: the breaker can never exceed the wire's ampacity (240.4). Match it to the wire and both do their job — the breaker trips before the wire can overheat.
Why does a breaker keep tripping? Usually a real overload (too many devices on one circuit), a continuous load sized without the 125% headroom, a failing breaker tripping below its rating, or motor inrush. A bigger breaker is never the fix — find the cause.
Quick chart
What size breaker — by appliance or amps
Pick a common appliance or enter the load in amps to get the breaker size and the wire it needs. For the classic question — what size breaker for an electric dryer — the answer is a 30 A double-pole breaker with 10 AWG copper.
Breaker size
30 A
with 10 AWG copper
| Appliance | Breaker | Copper wire | Notes |
|---|---|---|---|
| General lighting (120 V) | 15 A | 14 AWG | AFCI |
| Kitchen / bath outlets (120 V) | 20 A | 12 AWG | GFCI |
| Microwave (120 V) | 20 A | 12 AWG | dedicated |
| Dishwasher (120 V) | 15 A | 14 AWG | — |
| Electric dryer (240 V) | 30 A | 10 AWG | double-pole |
| Water heater 4500 W (240 V) | 30 A | 10 AWG | 25 A NEC minimum |
| Electric range (240 V) | 50 A | 6 AWG | double-pole |
| Wall oven (240 V) | 30 A | 10 AWG | double-pole |
| Cooktop (240 V) | 40 A | 8 AWG | double-pole |
| Central AC 2-ton (240 V) | 30 A | 10 AWG | per nameplate MOCP |
| Heat pump (240 V) | 40 A | 8 AWG | per nameplate MOCP |
| EV charger Level 2, 40 A (240 V) | 50 A | 6 AWG | double-pole, continuous |
| Hot tub / spa (240 V) | 50 A | 6 AWG | double-pole, GFCI |
| Pool pump (240 V) | 20 A | 12 AWG | GFCI, continuous |
| Welder 240 V | 50 A | 6 AWG | per nameplate |
| Space heater (120 V) | 15 A | 14 AWG | — |
Typical sizes for planning. Confirm the exact breaker and wire from your appliance's nameplate with the calculator at the top of this page.
What size breaker do I need for a water heater?
A standard 4,500 W water heater at 240 V draws 18.75 A. As a continuous load it needs 125%: 18.75 × 1.25 = 23.4 A, which rounds up to a 25 A breaker (30 A is commonly used) on 10 AWG copper. Always size from your unit's nameplate.
What size breaker for an electric dryer?
Most electric dryers use a 30 A double-pole breaker with 10 AWG copper on a 240 V circuit. Check the nameplate — a few high-output models call for more.
What about an AC unit or an EV charger?
A central AC uses the breaker printed on its nameplate (the MOCP) — often 30–60 A by size — not a calculated value. A Level 2 EV charger at 40 A is continuous, so 40 × 1.25 = 50 A on 6 AWG copper.
What is the 125% rule, and why does it exist?
For loads running 3+ hours (continuous), the NEC requires the breaker to be rated at least 125% of the load — equivalently, the load can't exceed 80% of the breaker (210.20). The headroom keeps the breaker from slowly heating up and nuisance-tripping.
Can I put a bigger breaker on to stop it tripping?
No — this is the most dangerous mistake in the panel. The breaker protects the wire, so a larger one lets the wire carry more current than it's rated for and overheat before the breaker trips. Find the cause; don't upsize the breaker.
Why does my breaker keep tripping?
Usually a genuine overload (too many devices on one circuit), a continuous load sized without the 125% headroom, a failing breaker tripping below its rating, or a motor's startup inrush. A bigger breaker is never the fix.
Does the breaker size match the wire or the load?
Both. You size the breaker from the load (×1.25 if continuous, rounded up to a standard size), but it can never exceed the wire's ampacity. The breaker and wire are always chosen together — match the wire to the breaker, not to the load alone.
Do I need a double-pole, AFCI, or GFCI breaker?
A 240 V appliance needs a double-pole breaker (both legs). NEC 2023 requires AFCI on most 15/20 A living-area circuits and GFCI in kitchens, bathrooms, garages, and outdoors.
Keep going
Related electrical calculators
Sizing the breaker is one step in the circuit. These cover the rest — size the wire it protects, check the run, and fit it in the raceway — and run on the same NEC-based engine.
How we keep this accurate
Breaker sizing follows the National Electrical Code (NEC 2023): the 125% continuous-load rule (210.20), standard breaker sizes (240.6), the small-conductor caps (240.4(D)), and conductor protection (240.4). Results are for planning and estimating. Code adoption, local amendments, and field conditions vary — verify with your AHJ and a licensed electrician before installation.