Total amps, breaker size, and service capacity check
List each appliance or device on the circuit along with its wattage (found on the nameplate or in the manual), and this calculator sums the load, converts to amps, and recommends the next standard breaker size. It also checks the total against your service panel so you can see whether a new high-draw appliance — a tankless water heater, EV charger, or hot tub — fits your service.
A 120V circuit running a 1500W space heater, 300W of LED lights, and a 600W TV: total = 2400W → 2400 ÷ 120 = 20A. A 20A breaker at 80% continuous capacity (16A) is overloaded if the heater runs continuously — put the heater on a dedicated circuit, leaving 900W (7.5A) for the rest, which fits a 15A breaker.
Why only 80% of a breaker rating? NEC requires that continuous loads (3+ hours) not exceed 80% of a breaker's rating, so breakers don't trip under sustained heat. Non-continuous loads can use the full rating.
Can I upsize a breaker? Never without verifying the wire gauge — a 20A breaker needs 12 AWG copper, 30A needs 10 AWG. An oversized breaker on undersized wire is a fire hazard.
Do I need a service upgrade? If your calculated load exceeds ~80% of your service capacity, or you're adding an EV charger or tankless water heater to a 100A panel, get a load calculation from a licensed electrician (NEC Article 220 allows demand factors this simple tool doesn't).
Is this calculator code-compliant? No — it's a planning aid. Permit-level load calculations follow NEC Article 220 with demand factors, diversity, and area-based allowances. Always have a licensed electrician verify.