Circuit Breaker Sizing Calculator Formula
Understand the math behind the circuit breaker sizing calculator. Each variable explained with a worked example.
Formulas Used
Actual Current Draw
actual_amps = load_ampsDesign Current
design_amps = load_amps * safety_multiplierRecommended Breaker
breaker_amps = ceil(load_amps * safety_multiplier / 5) * 5Variables
| Variable | Description | Default |
|---|---|---|
total_watts | Total Connected Load(W) | 2400 |
voltage | Voltage(V) | 120 |
is_continuous | Continuous Load? | 1 |
load_amps | Derived value= voltage > 0 ? total_watts / voltage : 0 | calculated |
safety_multiplier | Derived value= is_continuous == 1 ? 1.25 : 1 | calculated |
How It Works
Circuit Breaker Sizing
Amps = Watts / Volts
Continuous load: multiply by 1.25 per NEC 210.20(A). Then round up to the next standard breaker size (15A, 20A, 30A, 40A, 50A, etc.).
Worked Example
2,400W continuous load on 120V.
- 01Amps = 2400 / 120 = 20A
- 02Design amps = 20 x 1.25 = 25A
- 03Next standard size = 25A, or use 30A breaker
Frequently Asked Questions
What makes a load continuous?
A continuous load operates for 3 hours or more without interruption. Examples include lighting, electric heaters, and HVAC equipment.
Can I use the full breaker rating for non-continuous loads?
Yes, non-continuous loads can use up to 100% of the breaker rating. However, mixed circuits with both continuous and non-continuous loads must account for each portion separately.
Learn More
Guide
Electrical Load Calculation Guide for Residential Buildings
Learn how to calculate electrical loads for a home. Covers general lighting loads, appliance circuits, demand factors, panel sizing, and NEC code requirements.
Ready to run the numbers?
Open Circuit Breaker Sizing Calculator