Quick answer: Divide total watts by voltage to get amps, then multiply by 1.25 (NEC safety factor) to find the minimum breaker size. A 2,400W appliance on 240V draws 10A and needs a 15A breaker minimum.
Related Calculators
What size breaker do I need for a dryer?
A standard electric dryer requires a dedicated 240V circuit with a 30A double-pole breaker and 10 AWG (6mm²) wire. Some larger dryers may need 40A — check the nameplate for rated amps and multiply by 1.25 per NEC code.
Can I add a circuit to my panel myself?
In most jurisdictions, homeowners can do their own electrical work in their own home, but a permit and inspection are usually required. The work must meet NEC code. In the UK, any work in kitchens, bathrooms or new circuits must be done by a Part P certified electrician or notified to building control.
What is the 80% rule for breakers?
The NEC 125% rule (inverse of 80% rule) states that a circuit should not carry more than 80% of its breaker rating continuously. A 20A breaker should carry no more than 16A continuously. This calculator applies the 1.25× factor automatically when sizing breakers.
How many outlets can be on a 20 amp circuit?
The NEC allows up to 13 outlets on a 20A circuit (based on 1.5A per outlet rule), but best practice is 8–10 for living areas. Kitchens require dedicated 20A circuits for countertop appliances. Never put heavy appliances like refrigerators on shared circuits.
What causes a breaker to trip repeatedly?
Repeated tripping means the circuit is overloaded, there's a short circuit, or the breaker is faulty. First calculate the actual load using this tool — if you're exceeding 80% of breaker capacity, you need a larger circuit or load redistribution. If load is within limits, call an electrician.
What is a GFCI breaker and when do I need one?
A GFCI (Ground Fault Circuit Interrupter) trips within milliseconds when it detects current leaking to ground — preventing electrocution. Required by NEC in bathrooms, kitchens, garages, outdoors, basements and near pools. In the UK the equivalent is an RCD (Residual Current Device).
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.