Calculator
Electrical Load & Breaker Sizing Calculator
Why Electrical Load Calculations Matter
Every panel upgrade, service call, and new circuit installation starts with the same question: Can the panel handle it? This electrical load calculator helps contractors answer that before rolling a truck. Add each circuit in the home, and the calculator returns individual breaker sizes, wire gauges per NEC 310.16, and a total demand load estimate using NEC Article 220 demand factors. Use it to verify panel capacity before quoting an EV charger installation, check whether a gas-to-electric water heater swap is feasible, or build accurate material lists for new construction. Pair the results with TackonFSM’s estimating and invoicing tools to turn load calculations into professional quotes your customers can approve on the spot.
Electrical Load & Breaker Sizing Calculator
Size breakers, estimate amperage & check panel capacity per NEC guidelines
Panel Information
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Results
| Circuit | Watts | Volts | Amps | Breaker | Wire (Cu) |
|---|
Total Connected Load—
Total Load (amps at 240V)—
Demand Factor Applied (NEC Article 220)—
Estimated Demand Load—
Panel Capacity—
Panel Load Status
0%
0%50%80%100%
NEC Wire Size Reference (Copper, 75°C — NEC 310.16)
| Breaker (A) | Wire Gauge (Cu) | Common Use |
|---|---|---|
| 15A | 14 AWG | Lighting, general outlets |
| 20A | 12 AWG | Kitchen, bathroom, garage outlets |
| 30A | 10 AWG | Dryer, small AC unit |
| 40A | 8 AWG | Range, cooktop, large AC |
| 50A | 6 AWG | Range, EV charger, hot tub |
| 60A | 6 AWG | Sub-panel, large equipment |
| 70A | 4 AWG | Sub-panel feeder |
| 100A | 3 AWG | Sub-panel, service entrance |
| 150A | 1/0 AWG | Service entrance |
| 200A | 2/0 AWG | Main service |
Calculation basis: Breaker sizing per NEC 210.20(A) — continuous loads at 125%, non-continuous at 100%. Wire sizing per NEC 310.16 (copper, 75°C). Demand factor uses simplified NEC Article 220 standard method. This is an estimate for planning purposes — always verify with the adopted NEC edition, local amendments, and a licensed electrician before installation.
Frequently Asked Questions
Divide the total wattage of all devices on the circuit by the voltage (120V or 240V) to get the amperage. For continuous loads — anything running 3 or more hours at a time — multiply the amperage by 1.25 per NEC 210.20(A). Then round up to the nearest standard breaker size (15A, 20A, 30A, 40A, 50A, etc.). For example: a 4,500W water heater on 240V draws 18.75A. As a continuous load: 18.75 × 1.25 = 23.4A, so you'd install a 25A or 30A breaker.
The 80% rule means a standard circuit breaker should not be loaded beyond 80% of its rated capacity for continuous loads. A 20A breaker should carry no more than 16A continuously. This is the same as the NEC 125% sizing rule — just expressed from the opposite direction. If a load runs for 3 hours or more, size the breaker so the load is at most 80% of the breaker's rating. Some breakers are rated for 100% continuous duty, but they're specialty products and must be listed for that use.
Most Level 2 home EV chargers draw 32A to 48A at 240V. Since EV charging is a continuous load, you need to apply the 125% rule. A 40A charger requires a 50A breaker (40 × 1.25 = 50). A 48A charger requires a 60A breaker. The wiring must match — 6 AWG copper for a 50A breaker, 6 AWG for 60A. Before installing, verify your panel has enough spare capacity. This calculator can help you check that.
It depends on two things: available breaker spaces and available capacity. Even if your panel has open slots, the total demand load must not exceed the panel's amp rating. Add up all your existing loads using this calculator, then check if the new circuit fits within the panel's capacity. If total demand exceeds 80% of the panel rating, you should consider a panel upgrade or a sub-panel before adding the new circuit. A licensed electrician can perform a formal load calculation per NEC Article 220.
Per NEC 310.16, a 30A breaker requires a minimum of 10 AWG copper wire (rated for 30A at 75°C). For longer runs over 50 feet, you may need to upsize to 8 AWG to compensate for voltage drop. The wire must always be rated equal to or greater than the breaker — never use 12 AWG or 14 AWG wire on a 30A breaker, as this creates a serious fire hazard. The breaker protects the wire, not the appliance.
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