Electrical Circuit Load & Breaker Calculator
Calculate total circuit amperage, wattage load, NEC 80% continuous safety limits, and recommended breaker size for home electrical circuits.
Connected Devices & Appliances
The National Electrical Code (NEC) 80% Rule
The National Electrical Code specifies that any circuit carrying a continuous load (defined as running for 3 hours or more continuously, such as space heaters, EV chargers, or servers) must not exceed 80% of the breaker's rated capacity. This 20% buffer prevents thermal circuit breaker fatigue and fire hazards.
About this tool
How to use it
- Select your circuit voltage (standard 120V in North America, 240V for heavy dryers/EV, or 230V in UK/EU).
- Choose your installed circuit breaker amperage rating (e.g., 15A for bedroom/lighting circuits, 20A for kitchen/bath).
- Add your connected appliances, their operating wattages, and toggle the '3h+ Continuous' indicator.
- Click Calculate Circuit Load to inspect total current draw, NEC 80% safety cap utilization, and trip risk.
Why it matters
Plugging multiple high-wattage heating or motor appliances into the same wall circuit causes circuit breakers to overheat and trip. Calculating total amperage ensures you stay within the National Electrical Code (NEC) 80% continuous threshold, avoiding annoying blackouts and protecting home copper wiring from thermal insulation breakdown.
When to use it
Use this tool before setting up a space heater in a bedroom, adding a portable air conditioner, plugging in a coffee maker next to a microwave on kitchen counters, or designing a garage workshop tool station.
Good to know
Amps = Watts / Volts. Motorized appliances like table saws, vacuums, and air conditioners draw a transient startup surge (inrush current) of 2x to 3x their rated wattage for the first second of operation.
Common questions
What is the NEC 80% rule for circuit breakers?
Under the National Electrical Code (NEC), any continuous electrical load (defined as operating for 3 hours or more continuously, such as space heaters or EV chargers) must not exceed 80% of the breaker's rated limit (e.g. 12A max on a 15A breaker, or 16A max on a 20A breaker).
Can I plug a 1,500W space heater and a hair dryer into the same outlet?
No. At 120V, a 1,500W heater draws 12.5 Amps and an 1,800W hair dryer draws 15.0 Amps. Operating both simultaneously requires 27.5 Amps, which will immediately trip both 15A and 20A circuit breakers.
What copper wire gauge corresponds to each breaker size?
15 Amp breakers require 14 AWG copper wire; 20 Amp breakers require 12 AWG; 30 Amp breakers require 10 AWG; and 50 Amp breakers require 6 AWG. Never install a larger breaker on undersized wiring.
What causes a circuit breaker to feel warm to the touch?
Circuit breakers utilize bimetallic strips that heat up as current passes through them. Operating near maximum capacity generates normal warmth, but if a breaker feels hot or buzzes, you are overloading the circuit and risking a thermal fire hazard.