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Circuit Breaker Sizing Calculator

Calculate standard circuit breaker ampere ratings applying the continuous load 80% / 125% rule.

1. Enter your numbers → 2. Check the answer → 3. Read the steps before you trust it

Continuous Load Current (runs ≥ 3 hours)

e.g. Water heaters, EV chargers, continuous lighting

Non-Continuous Load Current

Intermittent loads (e.g. microwave, garbage disposal)

Continuous Multiplier Rule

Circuit Breaker Recommendation

Recommended Standard Breaker

20 Amps

Minimum required circuit ampacity: 20 A

Continuous Load Component

16 A (Factored: 20.00 A)

Non-Continuous Component

0 A

Electrical Code Notice

Circuit breaker sizing must always protect the smallest connected conductor ampacity and comply with governing electrical codes (such as NEC Article 240 / IEC 60364). Motor circuits, HVAC compressors (MCA/MOP), and transformer inrush require specialized overcurrent calculations. Verify all installations with a licensed electrical contractor.

Step-by-Step Engineering Calculation

1Non-continuous load: $I_{\text{non-cont}} = 0\text{ A}$
2Continuous load (runs $\ge 3$ hrs): $I_{\text{cont}} = 16\text{ A} \times 125\% = 20.00\text{ A}$
3Minimum OCPD Ampacity: $I_{\text{min}} = 20.00\text{ A}$
4Standard commercial breaker rating: 20 A (NEC 240.6(A))

Formula & Theory

Min Breaker Rating = Non-Continuous Amps + (Continuous Amps × 125%)

Computes minimum required overcurrent protection device (OCPD) ampere ratings for branch and feeder circuits.

Variable Definitions

I_cont:: Continuous Load (Amperes (A)) – Load sustained for 3+ continuous hours
I_noncont:: Non-Continuous Load (Amperes (A)) – Intermittent branch load
Rating:: Standard Breaker (Amperes (A)) – Next standard OCPD commercial size

Worked Engineering Example

Commercial Continuous Lighting Circuit

A 16A continuous commercial LED lighting load.

Inputs:

Continuous Load: 16 A

Result: Min Ampacity = 20.00 A | Standard Breaker = 20 A (12 AWG Copper)

Key Assumptions

•Standard 80%-rated commercial molded-case circuit breaker (MCCB).

Limitations & Safety Scope

•Breakers must be matched to conductor thermal ampacity ratings. 100%-rated breakers require listed assemblies and special considerations.

Frequently Asked Questions

What is the 80% continuous breaker rule?

Under NEC Section 210.20, standard circuit breakers may only carry up to 80% of their nameplate rating continuously (for 3 hours or longer). Alternatively, continuous loads must be multiplied by 125% when sizing the breaker.

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Educational & Reference Notice: This calculator provides technical estimates based on standard physical formulas. Actual electrical installation, protective device sizing, and conductor selection must be verified against the applicable electrical code (NEC, IEC, CEC) by a licensed electrician or qualified professional engineer.