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Circuit Load Calculator

Adds up the appliance wattages on one branch circuit, converts them to amperes at the given voltage and power factor, compares the load with the breaker rating (applying the NEC 80 % continuous-load limit when requested) and recommends the smallest standard breaker size that fits.

When to use

You want to know whether a set of appliances will trip a breaker, how much capacity a circuit has left, or which standard breaker size a load needs.

Do not use when: You need conductor sizing or voltage drop on a long run (use wire-voltage-drop), motor inrush or generator sizing (use generator-sizing), or a whole-service load calculation per NEC Article 220; local code and a licensed electrician decide the installed breaker.

Formula

total_watts = Σ loads_w; total_amps = total_watts / (voltage_v × power_factor); breaker_capacity_percent = total_amps / breaker_amps × 100; max_continuous_amps = 0.8 × breaker_amps; limit = continuous_load ? max_continuous_amps : breaker_amps; within_limit = total_amps ≤ limit; headroom_w = (limit − total_amps) × voltage_v × power_factor; recommended_breaker_amps = min standard size S with total_amps ≤ S × (continuous_load ? 0.8 : 1)

Single-phase circuit. NEC 210.20(A) requires the overcurrent device to be rated at least 125 % of a continuous load (equivalently the load ≤ 80 % of the rating); non-continuous loads may use the full rating. The standard-size list mixes IEC 60898 (10, 13, 16, 20, 25, 32, 40, 50, 63 A) and North American (15, 20, 30, 40, 50 A) ratings; choose the ones sold in your region.

Inputs

Parameter	Type	Unit	Required	Description
`voltage_v`	number	V	default 230	Nominal circuit voltage: 230 (Europe, Australia), 120 or 240 (North America). Range: ≥ 12, ≤ 1000
`breaker_amps`	number	A	default 16	Rating of the circuit breaker or fuse protecting the circuit. Range: > 0, ≤ 1000
`loads_w`	number_list	W	yes	Rated power of each appliance on the circuit in watts, comma-separated (use the nameplate value).
`continuous_load`	boolean		default false	true when the load runs 3 hours or more at a time (heaters, lighting, EV chargers): NEC 210.20(A) then limits it to 80 % of the breaker rating.
`power_factor`	number		default 1	Power factor of the loads: 1 for resistive heaters and kettles, 0.8–0.95 for motors and electronics. Range: > 0, ≤ 1

Outputs

Output	Type	Unit	Description
`total_watts`	number	W	Sum of loads_w.
`total_amps`	number	A	total_watts / (voltage_v × power_factor).
`breaker_capacity_percent`	number	%	total_amps / breaker_amps × 100.
`max_continuous_amps`	number	A	0.8 × breaker_amps, the NEC limit for continuous loads.
`within_limit`	boolean		true when total_amps ≤ breaker_amps (non-continuous) or ≤ 80 % of it (continuous).
`headroom_w`	number	W	Watts that can still be added before the applicable limit; negative when over the limit.
`recommended_breaker_amps`	integer	A	Smallest standard size (10, 13, 15, 16, 20, 25, 30, 32, 40, 50, 63 A) whose limit covers the load; absent when the load exceeds 63 A.

Example

230 V, 16 A breaker, 2000 + 800 + 500 W (non-continuous): {"voltage_v":230,"breaker_amps":16,"loads_w":[2000,800,500]} → {"total_watts":3300,"total_amps":14.35,"breaker_capacity_percent":89.67,"max_continuous_amps":12.8,"within_limit":true,"headroom_w":380,"recommended_breaker_amps":15}

Same loads running continuously: {"voltage_v":230,"breaker_amps":16,"loads_w":[2000,800,500],"continuous_load":true} → {"total_amps":14.35,"within_limit":false,"headroom_w":-356,"recommended_breaker_amps":20}

GET https://tttkmbb.com/api/v1/calculate/circuit-load?voltage_v=230&breaker_amps=16&loads_w=2000%2C800%2C500

Machine access

API: GET https://tttkmbb.com/api/v1/calculate/circuit-load (query parameters) or POST with a JSON body {"inputs": {...}}
Schema: https://tttkmbb.com/api/v1/calculators/circuit-load · Markdown: https://tttkmbb.com/energy/circuit-load.md · JSON definition: https://tttkmbb.com/energy/circuit-load.json
MCP: server https://tttkmbb.com/mcp, tool run_calculator with calculator_id="circuit-load"
OpenAPI operationId: calculate_circuit_load
Freshness: static. Authentication: none. Rate limit: fair use (see rate limits).

Sources

NFPA 70 – National Electrical Code (Article 210, Branch Circuits) (standard)
Wikipedia – National Electrical Code (reference)
Wikipedia – Circuit breaker (reference)

FAQ

What is the 80 % rule?

NEC 210.20(A) sizes the breaker at 125 % of any load expected to run for 3 hours or more, so continuous loads may use only 80 % of a breaker's rating: 12 A on a 15 A circuit, 16 A on a 20 A circuit.

Can I install the recommended breaker myself?

The breaker protects the wiring, so it may only be upsized if the conductors are rated for it (e.g. 2.5 mm² / 12 AWG for 20 A). Have an electrician verify conductor size and local code.

Should I use rated or measured watts?

Use nameplate ratings for a conservative check; measured draw is usually lower (a 2,000 W kettle draws its full rating, a 800 W microwave draws about 1,200 W input).

Related calculators

Wire Voltage Drop Calculator — Voltage drop and conductor size for the same current.
Electric Power Calculator — Watts, volts and amps of a single appliance.
Generator Sizing Calculator — Generator capacity for the same loads including motor starting.