Electrical Load Calculator
Sizing breakers and panels correctly keeps your wiring safe and your inspection passing. This calculator takes the total wattage on a circuit, applies NEC continuous-load rules (125% factor), and tells you exactly what breaker size and wire gauge you need. Works for both single-phase residential and three-phase commercial circuits.
How Electrical Load Calculation Works
The core formula is simple: Amps = Watts / Voltage for single-phase, or Amps = Watts / (Voltage x 1.732) for three-phase.
But the NEC (National Electrical Code) adds an important safety layer. Section 210.20 requires that any load running continuously for 3 hours or more must be multiplied by 125% when sizing the breaker. This is the "80% rule" contractors talk about -- a 20A breaker should carry no more than 16A of continuous load.
Standard Breaker Sizes
Residential panels typically use these breaker sizes:
- 15A -- Lighting circuits
- 20A -- General receptacles, kitchen, bathroom
- 30A -- Dryers, water heaters
- 40A -- Ranges, cooktops
- 50A -- Ovens, large appliances
- 100-200A -- Main panel breakers
Wire Gauge Quick Reference (Copper, 75C)
| Breaker | Wire Gauge |
|---|---|
| 15A | 14 AWG |
| 20A | 12 AWG |
| 30A | 10 AWG |
| 40A | 8 AWG |
| 50A | 6 AWG |
Always verify wire sizing with NEC Table 310.16 and check local code amendments.