Circuit Load Calculator
Calculate total load and available capacity on a circuit
Circuit breaker rating
120V or 240V
Total watts already on circuit
Watts of new device
Total Load
—
watts
Total Current
—
amps
Safe Capacity (80%)
—
watts
Can Add Load?
—
Disclaimer: Results are estimates based on standard formulas. Not a substitute for professional advice from a licensed contractor, engineer, or architect. Verify all calculations before making purchasing or construction decisions.
How to Use This Calculator
Our calculator helps determine if your circuit has capacity for additional loads.
- Check circuit breaker size
- Add up existing loads
- Enter wattage of new device
- If load > 80% limit, use separate circuit
Expert Insights
Smart analysis for your Electrical project
80% Load Rule Enforcement
NEC requires circuits loaded to no more than 80% of breaker rating for continuous loads. AI enforces this automatically in all calculations.
GFCI/AFCI Requirements
GFCI: kitchens, baths, garages, outdoors. AFCI: bedrooms, living areas (2023 NEC). AI flags required protection for your circuit type.
Voltage Drop Optimization
Over 3% drop causes motor overheating. AI recommends wire gauge upgrades when your run length would cause excessive voltage drop.
Calculation Formula
Total watts = Existing + New load
Total amps = Total watts / Voltage
Safe capacity = Breaker x Voltage x 0.80
Authority References
Our calculations are based on guidelines and standards from these authoritative sources:
- National Fire Protection Association (NFPA) — NFPA 70 — National Electrical Code (NEC). Learn more
- Underwriters Laboratories (UL) — Wire & Cable Safety Standards. Learn more
Frequently Asked Questions
What is the 80% rule for circuits?
NEC requires continuous loads not exceed 80% of circuit rating. A 20-amp at 120V has safe capacity of 1,920 watts.
How many outlets on a 20-amp circuit?
NEC does not specify a maximum. Practical limit is 10 outlets for a 20-amp circuit.