Use this free electrical load calculator to add up the wattage of your appliances or circuits, flag continuous loads and phase type, and get the total amp draw, a recommended circuit breaker size, and a matching copper wire gauge — all in one step.
Each item has its own "continuous" checkbox — check it for loads that run 3+ hours straight (water heater, EV charger, space heater). Use nameplate wattage where possible: for example, most refrigerators list 400–800W on the nameplate even though average running draw is lower — using the nameplate figure keeps your estimate on the safe side. Calculations assume a power factor of 1; for motors, welders, or EV chargers use the equipment's nameplate Full Load Amps (FLA) if listed, instead of watts.
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Add a row for every appliance or circuit you want to power — name it, enter its wattage (from the nameplate), and set the quantity. Check the "continuous" box for any item that runs 3+ hours straight, choose your voltage and phase, then hit Calculate. You'll get total watts, base amps, required amps with the NEC continuous-load factor applied where flagged, a recommended breaker size, and a suggested copper wire gauge.
Undersized breakers trip constantly; oversized ones can let wires overheat before tripping — both are real problems for homeowners, electricians, and RV or workshop owners. If you're adding a subpanel, wiring a garage workshop with a compressor, or sizing shore power for a camper, you need to know your true amp draw before pulling permits or buying breakers. For example, a kitchen circuit with a 1,500W microwave, 1,200W toaster oven, and 700W coffee maker running together (none continuous) pulls 3,400W ÷ 120V = 28.3A, requiring a 30A breaker with 10 AWG wire — already too much for a standard 20A kitchen circuit and using most of a 30A breaker's capacity by itself. Getting this number right before you shop for parts saves a return trip to the hardware store and, more importantly, keeps your wiring safe and code-compliant.
For single-phase loads, the core formula is Amps = Watts ÷ Volts. For three-phase loads (assuming a purely resistive power factor of 1), the formula is Amps = Watts ÷ (√3 × Volts), since three-phase power splits current across three conductors. Each item is calculated separately and summed. If an item is flagged continuous, NEC 210.20 requires sizing that portion of the load at 125% of its current, since continuous loads generate sustained heat — non-continuous items are left at 100%. The calculator adds every item's required current together, rounds up to the next standard breaker size per NEC 240.6(A), and matches it to a minimum copper THHN wire gauge based on NEC Table 310.16 (75°C column) ampacity ratings, applying the NEC 240.4(D) small-conductor rule that caps 14/12/10 AWG conductors at 15/20/30A regardless of table ampacity. This tool assumes unity power factor; for motors, compressors, or chargers with a nameplate FLA rating, use that FLA figure directly (as an "amps" equivalent) instead of watts for a truer number.
Say you're wiring a laundry circuit at 120V, single-phase, with a 1,500W space heater (continuous) and a 900W iron (non-continuous):
For a three-phase example, a 10,000W 208V heater bank (continuous) would draw 10,000 × 1.25 = 12,500 required watts ÷ (1.732 × 208V) = 34.7A, rounding up to a 35A breaker with 8 AWG copper.
List every appliance that could run at once with its wattage and quantity, flag any that run 3+ hours continuously, then divide the total by voltage (or by √3×voltage for three-phase) to get amps. This calculator rounds that figure up to the nearest standard breaker size automatically.
At 120V single-phase, 1,500W draws 12.5A, which fits a 15A breaker if it's non-continuous. If that same load runs continuously for 3+ hours, the NEC 125% factor pushes it to 15.6A, requiring a 20A breaker instead.
Standard practice is 12 AWG copper wire for a 20A circuit, and 14 AWG copper for a 15A circuit, per the NEC 240.4(D) small-conductor rule. Always confirm with your local code since some jurisdictions add derating requirements.
NEC 210.20 requires continuous loads (running 3+ hours, like a water heater or EV charger) to be sized at 125% of the actual current draw so the breaker and wire don't overheat under sustained use. This tool lets you flag each appliance individually so mixed continuous and non-continuous loads calculate correctly.
Single-phase current is Watts ÷ Volts, while three-phase current is Watts ÷ (√3 × Volts) because the load is split across three conductors. Most homes are single-phase; three-phase is mostly found in commercial and industrial equipment, so pick the option that matches your actual electrical supply.
Yes for subpanels — list every circuit you plan to feed and enter your panel's amp rating to see utilization percentage. Whole-house service sizing also involves NEC demand factors that reduce the total, so have an electrician verify final numbers.
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