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Electrical

Circuit Breaker Size Calculator

Find the next standard breaker size from continuous and non-continuous load current using the 125% continuous-load rule.

Formula Ireq = Inc + 1.25 × Ic, then choose the next standard breaker size ≥ IreqReviewed Aug 29, 2026

A circuit breaker is sized from the load current it must carry without nuisance tripping, while still protecting the conductors installed on that circuit. This circuit breaker size calculator handles the common branch-circuit sizing step: add non-continuous load at 100%, add continuous load at 125%, then round up to the next standard breaker rating. It can work directly from amps, or convert watts to amps from voltage and power factor.

Calculation Bench
Load Input Mode
01

I_nc · Enter nameplate or measured current.

02

I_c · Runs 3+ hours; enter A or 0 if none

Solution

Enter continuous and/or non-continuous load to size the breaker.

Ireq = Inc + 1.25 × Ic, then choose the next standard breaker size ≥ Ireq

Formula Sheet

Ireq=Inc+1.25IcI_{req}=I_{nc}+1.25I_c
I=PV×PFI=\dfrac{P}{V\times PF}
  • BBreaker Size
  • I_reqRequired Current
  • I_ncNon-Continuous Load
  • I_cContinuous Load
  • PLoad Power
  • VVoltage
  • PFPower Factor

Variables & Units

SymbolVariableDescriptionCommon Units
BBreaker SizeThe selected standard breaker rating.A
I_reqRequired CurrentThe current after applying the 125% continuous-load factor.A
I_ncNon-Continuous LoadLoad current that is not expected to run continuously for 3 hours or more.A
I_cContinuous LoadLoad current expected to operate for 3 hours or more.A
PLoad PowerReal power entered in watts when using watts mode.W, kW
VVoltageCircuit voltage used to convert watts to amps.V
PFPower FactorRatio of real power to apparent power for AC loads; defaults to 1.

How to Use This Calculator

  • 01Choose whether you want to enter the load in amps or watts.
  • 02Enter the non-continuous load - equipment that is not expected to run at its maximum current for 3 hours or more.
  • 03Enter the continuous load - equipment expected to run for 3 hours or more, such as EV charging, lighting, heating, or process loads.
  • 04If you selected watts, enter the circuit voltage and, if needed, a power factor below 1.
  • 05Read the required current and the next standard breaker size. Use the result as a planning estimate, then check wire ampacity, equipment nameplates, and local code before installation.

How the Formula Works

For general branch-circuit overcurrent protection, the familiar 80% breaker rule and the 125% continuous-load rule are the same idea written from opposite directions. A 32 A continuous load needs a 40 A standard breaker because 32 × 1.25 = 40, and 32 A is 80% of 40 A.

Non-continuous loads are added at 100% because they are not assumed to sit at maximum current long enough to heat the breaker in the same sustained way. Continuous loads are multiplied by 1.25 before the breaker is selected.

Real breakers come in standard ratings, so the calculated required current is rounded up, never down. If the requirement is 34.5 A, a 30 A breaker is too small and the calculator selects 35 A.

When loads are entered in watts, the calculator first converts to current using I = P / (V × PF). For purely resistive loads PF is usually close to 1; for motors and other inductive loads, nameplate full-load current is usually better than estimated wattage.

Worked Example 01

32 A continuous EV-style load

Known

  • Non-continuous load: 0 A
  • Continuous load: 32 A

Formula

Ireq = Inc + 1.25 × Ic, then choose the next standard breaker size ≥ Ireq

Substitution

Ireq = 0 + 1.25 × 32 = 40 A

Result

40 A breaker

A 32 A load running for 3 hours or more uses the 125% continuous-load rule. The required current is exactly 40 A, so a 40 A standard breaker is the first size that qualifies.

Worked Example 02

Mixed workshop load

Known

  • Non-continuous load: 12 A
  • Continuous load: 18 A

Formula

Ireq = Inc + 1.25 × Ic, then choose the next standard breaker size ≥ Ireq

Substitution

Ireq = 12 + 1.25 × 18 = 34.5 A

Result

35 A breaker

The non-continuous portion is counted once, while the continuous portion is weighted at 125%. Since 34.5 A is greater than 30 A, the calculator rounds up to the next standard size: 35 A.

Worked Example 03

4800 W continuous load at 240 V

Known

  • Continuous load: 4800 W
  • Voltage: 240 V
  • Power factor: 1.0

Formula

I = P / (V × PF)

Substitution

I = 4800 / (240 × 1.0) = 20 A; Ireq = 1.25 × 20 = 25 A

Result

25 A breaker

The wattage is first converted to current. Because the load is continuous, the 20 A operating current becomes a 25 A breaker-sizing current.

Applications

  • 01Estimating breaker size for simple branch circuits before checking conductor ampacity
  • 02Understanding why a 32 A continuous EV charger commonly needs a 40 A breaker
  • 03Converting a wattage load at 120 V or 240 V into an approximate breaker size
  • 04Teaching the relationship between the 80% continuous-load rule and 125% sizing

Standard Breaker Sizes Used by This Calculator

RangeStandard sizes
15-100 A15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100 A
110-250 A110, 125, 150, 175, 200, 225, 250 A
300-1200 A300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200 A
1600-6000 A1600, 2000, 2500, 3000, 4000, 5000, 6000 A

Assumptions

  • 01Uses the common branch-circuit sizing relationship: 100% of non-continuous load plus 125% of continuous load.
  • 02Rounds up to a standard breaker size list from 15 A through 6000 A.
  • 03Watts mode assumes single-phase style current conversion using I = P / (V × PF); use nameplate amperes when available.
  • 04Continuous means the load is expected to operate at maximum current for 3 hours or more.

Where This Model Stops

  • 01Does not size or verify the conductor. The breaker must not exceed the wire ampacity after all applicable derating and code rules.
  • 02Does not handle motor branch-circuit short-circuit/ground-fault sizing, HVAC MCA/MOCP rules, transformer primary protection, welders, taps, or other special NEC article rules.
  • 03Does not replace local electrical-code review, permits, inspection, equipment listing instructions, or a licensed electrician.
  • 04Does not apply demand factors, panel/service load calculations, ambient-temperature derating, conductor bundling, terminal temperature limits, or voltage-drop checks.

References

  1. [1]

    NEC 210.20(A) continuous and noncontinuous loads

    National Electrical Code (NFPA 70)

    General branch-circuit overcurrent device sizing basis: noncontinuous load plus 125% of continuous load.

  2. [2]

    NEC 240.6(A) standard ampere ratings

    National Electrical Code (NFPA 70)

    Standard overcurrent device ratings used for rounding the required current up to an actual breaker size.

  3. [3]

    NEC 240.4 conductor protection

    National Electrical Code (NFPA 70)

    Breaker selection must be coordinated with conductor ampacity and code limits; this calculator does not replace that conductor check.

Frequently Asked Questions

What is the 80% rule for circuit breakers?

For a standard breaker serving continuous load, the load is commonly limited to 80% of the breaker rating. The same rule can be written as breaker size = continuous load × 1.25. For example, 16 A continuous load fits a 20 A breaker because 16 is 80% of 20.

Do I always multiply breaker size by 125%?

No. The 125% multiplier is for continuous loads. Non-continuous loads are added at 100%. If a circuit has both, use Ireq = non-continuous current + 1.25 × continuous current.

Can I install the breaker size this calculator recommends?

Not by itself. A breaker protects wiring, so the conductor ampacity, insulation rating, terminal temperature rating, derating factors, equipment nameplate, and local code rules must also allow that breaker size.

Why is motor or air-conditioner breaker sizing different?

Motors and HVAC equipment often have special code rules and nameplate values such as MCA and MOCP. Their breakers may be selected for starting current and short-circuit protection, not just continuous ampacity, so this general calculator should not be used for those final selections.

Should I enter watts or amps?

Use nameplate amps when you have them. Watts mode is useful for rough planning, but AC loads with power factor below 1 draw more current than watts divided by volts would suggest.

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