Electrical Load Calculator — Breaker Size & Wire Gauge
Free tool · Works offline · 120V–400V · Single & three phase
How to calculate electrical load
Electrical load calculation has three steps. First, add up the watts of every device on the circuit. Second, divide by voltage to get amps (Amps = Watts ÷ Volts). Third, multiply by 1.25 — the NEC/IEE safety factor that prevents continuous loads from exceeding 80% of breaker capacity. Round up to the next standard breaker size: 15A, 20A, 30A, 40A, 50A, 60A, 100A. This calculator does all three steps automatically.
What size breaker do I need?
The breaker size depends on the total continuous load on the circuit. As a rule of thumb: 15A breakers for lighting circuits and general outlets · 20A for kitchen appliances, bathrooms, garage outlets · 30A for dryers, water heaters · 40–50A for electric ranges and ovens · 60–100A for subpanels and EV chargers. Never upsize a breaker to fix a tripping problem — the breaker trips to protect the wire. Upsize the wire instead.
What wire size goes with each breaker?
Wire size must match the breaker — this is a safety requirement, not a guideline. The wire must be rated for at least as much current as the breaker. Standard pairings: 15A → 14 AWG (1.5mm²) · 20A → 12 AWG (2.5mm²) · 30A → 10 AWG (4mm²) · 40A → 8 AWG (6mm²) · 50A → 6 AWG (10mm²) · 60A → 4 AWG (16mm²). For runs over 30m (100ft), go up one wire size to compensate for voltage drop.
Single phase vs three phase — which do you have?
Most homes and small businesses have single phase supply — one live wire at 120V (US) or 230V (UK/EU). Larger commercial and industrial premises have three phase supply — three live wires at 208V (US) or 400V (EU) line-to-line. Three phase is more efficient for heavy motor loads. If you're not sure, check your main panel — three phase will have three large incoming cables, single phase has two (live + neutral).
The 80% rule — why your breaker must not run at full load
A breaker rated at 20A should never carry more than 16A continuously (80%). This isn't a guideline — it's code (NEC 210.20, IEE 18th Edition). Continuous overloading causes the breaker to run hot, degrades the contacts, and can cause nuisance tripping. The 1.25 multiplier in this calculator accounts for this automatically. If your calculated load is 18A, you need a 30A breaker (18 × 1.25 = 22.5A, round up to 30A).
How to calculate three-phase load
For three-phase circuits the formula is: Amps = Watts ÷ (Volts × √3 × Power Factor). √3 ≈ 1.732. A 10kW motor at 400V three-phase with power factor 0.85 draws: 10,000 ÷ (400 × 1.732 × 0.85) = 17.0A per phase. The three-phase option in this calculator handles this automatically — just select the phase and voltage.
Frequently asked questions
Can I add more outlets to a 15A circuit?
Yes, as long as the total connected load stays under 12A (80% of 15A) and you don't exceed the maximum number of outlets per circuit in your local code — typically 8–10 outlets on a 15A residential circuit.
Why does my breaker keep tripping?
Either the circuit is overloaded (total load exceeds 80% of breaker rating), there's a short circuit, or the breaker itself is failing. Calculate the actual load using this tool — if it's under 80%, the breaker likely needs replacing.
What is power factor and do I need to worry about it?
Power factor (PF) measures how efficiently a device uses electricity. Resistive loads like heaters have PF=1.0. Motors and compressors typically have PF=0.8–0.9. For residential circuits with mainly resistive loads, use PF=1.0. For commercial motor loads, use 0.85.
How many watts can a 20A circuit handle?
At 120V: 20A × 120V × 0.8 = 1,920W continuous. At 230V: 20A × 230V × 0.8 = 3,680W continuous. Never load a circuit beyond 80% of its rated capacity for continuous use.