Electrical Load Calculator Formula
Understand the math behind the electrical load calculator. Each variable explained with a worked example.
Formulas Used
Current Draw
amps = voltage > 0 ? total_watts / voltage : 0With Safety Factor
amps_with_safety = voltage > 0 ? total_watts / voltage * safety_factor / 100 : 0Recommended Breaker
breaker_size = voltage > 0 ? ceil(total_watts / voltage * safety_factor / 100 / 5) * 5 : 0Variables
| Variable | Description | Default |
|---|---|---|
voltage | Voltage(V) | 120 |
total_watts | Total Watts(W) | 1500 |
safety_factor | Safety Factor(%) | 125 |
How It Works
Electrical Load Formula
Amps = Watts / Volts
NEC Safety Factor = 125% (for continuous loads)
Breaker sizes are standard: 15A, 20A, 30A, 40A, 50A. Always round up to the next standard size.
Worked Example
1,500W load on a 120V circuit with 125% safety factor.
- 01Amps = 1,500W / 120V = 12.5A
- 02With safety = 12.5 × 1.25 = 15.6A
- 03Recommended breaker = 20A
Frequently Asked Questions
What is the NEC 80% rule?
The National Electrical Code requires that continuous loads not exceed 80% of the breaker rating (or equivalently, the breaker should be 125% of the load). A 20A breaker handles 16A continuous.
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 Electrical Load Calculator