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Electrical

How to Calculate Circuit Breaker Size

Circuit breaker sizing starts with the load current, but the most important distinction is whether the load is continuous. Non-continuous load is counted at 100%; continuous load is counted at 125%; the final current is then rounded up to a real standard breaker rating.

Final answer from the example

Required current

34.5 A

12 A + 125% of 18 A

Breaker size

35 A

Next standard rating above 34.5 A

Wire check

Still required

Breaker must protect the conductor

Key formulas

Continuous and non-continuous load rule

Ireq = Inc + 1.25 × Ic

Use this for the common branch-circuit sizing step before selecting a standard breaker.

Watts to amps

I = P / (V × PF)

Use nameplate amperes when available; watts mode is a planning estimate for AC loads.

80 percent relationship

Continuous load ≤ 0.80 × breaker rating

This is the same relationship as multiplying continuous load by 1.25 from the load side.

Standard breaker selection

Breaker size = next standard rating ≥ Ireq

Round up, never down, then separately verify wire ampacity and equipment rules.

Variables and units

SymbolMeaningTypical units
IreqRequired breaker sizing currentA
IncNon-continuous load currentA
IcContinuous load currentA
PLoad power when starting from wattsW
VCircuit voltageV
PFPower factor for AC loadsdimensionless

Quick reference conversions

Continuous load

3 hours or more

Common code definition used for the 125% factor.

Continuous multiplier

1.25

Equivalent to using 80% of the breaker rating.

Common branch sizes

15, 20, 30, 40, 50, 60 A

Real products use standard ratings.

Watts to amps

I = P/(V × PF)

Use nameplate amps for motors when possible.

32 A continuous

40 A breaker

Classic EV-style example: 32 × 1.25 = 40.

20 A continuous

25 A breaker

20 × 1.25 = 25.

Step-by-step solved example

Example problem

A workshop circuit has 12 A of non-continuous tool load and 18 A of continuous lighting or process load. Find the minimum standard breaker size using the 125% continuous-load rule.

1. Separate the load types

The non-continuous portion is 12 A. The continuous portion is 18 A because it may operate at maximum current for 3 hours or more.

2. Apply the 125% continuous-load factor

Continuous contribution = 1.25 × 18 A = 22.5 A.

3. Add non-continuous load at 100%

Ireq = 12 A + 22.5 A = 34.5 A.

4. Round up to a standard breaker rating

A 30 A breaker is too small because 34.5 A is greater than 30 A. The next standard breaker size is 35 A.

5. Check the conductor separately

The breaker result is not enough by itself. The installed conductor must have sufficient ampacity after terminal temperature limits, ambient temperature, bundling, and other derating rules are applied.

6. Compare with a watts example

For a 4,800 W continuous load at 240 V and PF = 1, current is 4,800 / 240 = 20 A. The breaker sizing current is 1.25 × 20 = 25 A, so the first standard size is 25 A.

Practical field notes

Breaker size and wire size are linked

A breaker protects the conductor. You cannot choose a larger breaker just because the load asks for it unless the conductor, terminals, equipment, and code rules allow that breaker.

Round up the breaker, not down

If the required current is 34.5 A, a 30 A breaker is below the calculation. The next standard size, 35 A, is the first candidate before conductor and equipment checks.

Motor and HVAC circuits have special rules

Motor starting current and HVAC nameplate values such as MCA and MOCP can change the final breaker selection. Do not use this general example for those final decisions.

The 80 percent rule is not a universal load cap

The 80% language is mainly the continuous-load relationship for standard breakers. Non-continuous loads and equipment listed for 100% operation are handled differently.

Common mistakes to avoid

  • Multiplying every load by 125% instead of only the continuous portion.
  • Rounding 34.5 A down to 30 A instead of up to 35 A.
  • Using watts divided by volts for an AC motor without considering power factor or nameplate current.
  • Choosing a breaker without checking wire ampacity, terminal temperature ratings, and local code.
  • Using this general method for motors, HVAC equipment, welders, transformers, or other special-code cases.

When to use the calculator instead

Use the Circuit Breaker Size Calculator when you need to switch between amps and watts, combine continuous and non-continuous loads, or quickly see the next standard breaker rating.

Calculation FAQs

How do you calculate circuit breaker size?

Add non-continuous current at 100%, add continuous current at 125%, then choose the next standard breaker rating at or above the result. The formula is Ireq = Inc + 1.25 × Ic.

What is the 80 percent rule?

For a standard breaker serving continuous load, the continuous load is commonly limited to 80% of breaker rating. From the load side, that is the same as multiplying continuous current by 1.25.

Do I size the breaker or wire first?

In real design they are checked together. The load determines the minimum circuit requirement, but the breaker must be coordinated with conductor ampacity and equipment rules.

Can I use watts to choose a breaker?

For simple resistive loads, yes as an estimate: I = P/V. For AC loads with power factor or motors, use I = P/(V × PF) or, better, the nameplate current.

Is a 35 A breaker common?

It is a standard NEC 240.6(A) ampere rating, but availability depends on panel type and manufacturer. If the exact size is not available, the design may need conductor, load, or equipment review rather than arbitrary upsizing.

References

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