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Electrical load and breaker size calculator

Breaker size for one circuit: total watts and amps from a list of appliances, with the NEC 125% continuous-load rule and NEC or IEC breaker ratings.

Actualizado Ejemplos verificados: 5

Name, power in W, kW or A; add “continuous” for anything that runs 3 hours or more, and “x 2” for several of the same.
Más opciones
1 for heaters and lamps; motors and some electronics run 0.8–0.95.
Probar
Suggested breaker
A
Suggested breaker: 20 A
Número entero; Al más cercano; empates alejándose de cero
Total load
1,770W
Current drawn
14.75A
Minimum breaker rating
17.88A
Continuous part
1,500W
Breaker loading
73.8%
Size the wire to the breaker, not the load, and follow local code; this is an estimate, not a design.

The circuit draws 14.75 A (1,770 W). Counting continuous loads at 125%, the breaker must be at least 17.88 A, so the next standard size is 20 A.

Share of the load

Space heater (continuous)1,500 WTV150 WLamp × 2120 W

Current as a share of a 20 A breaker

Up to 80%80–100%Overloaded73.8%Up to 80%
Loads Filas: 3
LoadWattsAmpsType
Space heater1,50012.5Continuous
TV1501.25Non-continuous
Lamp × 21201Non-continuous
Cómo se calcula S

  1. Total load

    P=1,500+150+120=1,770 WP = 1{,}500 + 150 + 120 = 1{,}770\ \text{W}

  2. Corriente eléctrica

    I=1,770120×1=14.75 AI = \frac{1{,}770}{120 \times 1} = 14.75\ \text{A}

  3. Minimum breaker rating (NEC 2023 210.20(A))

    2.25+1.25×12.5=17.875 A2.25 + 1.25 \times 12.5 = 17.875\ \text{A}

    Continuous loads (3 hours or more) count at 125%, which keeps them at or under 80% of the breaker.

  4. Next standard size

    20 A≥17.875 A20\ \text{A} \ge 17.875\ \text{A}

    From the NEC 2023 240.6(A) standard ratings.

Acerca de Electrical load and breaker size calculator

Each load is converted to amps with I = P ÷ (V × power factor), or P ÷ (√3 × V × power factor) on a three-phase supply. Loads marked continuous, meaning they run for 3 hours or more, count at 125%, and the smallest standard breaker at or above the total is suggested (NEC 2023 210.20(A) and 240.6(A), or the IEC 60898-1 ratings).

The default circuit, a 1,500 W space heater running continuously plus a 150 W TV and two 60 W lamps on 120 V, draws 14.75 A. With the heater counted at 125% the breaker must be at least 17.875 A, so the suggestion is 20 A, loaded to 73.8%.

The wire must be sized to the breaker. Motors, feeders and whole-house service loads follow other NEC articles, so treat the result as a check on one branch circuit, not a design.

Ejemplos resueltos

Space heater (continuous), TV and two lamps at 120 V

Loads on the circuit (one per line)
Space heater, 1500 W, continuous TV, 150 W Lamp, 60 W x 2
Voltage
120 V
Supply
Single-phase
Breaker sizes
US (NEC)
Total load
1,770 W
Current drawn
14.75 A
Minimum breaker rating
17.88 A
Suggested breaker
20 A

Fuente de comprobación: Python decimal: 1500/120 × 1.25 + 270/120 = 15.625 + 2.25 = 17.875 A → next NEC 240.6(A) size 20 A

Kitchen appliances at 230 V, IEC breakers

Loads on the circuit (one per line)
Kettle, 2000 W Microwave, 1200 W Toaster, 800 W
Voltage
230 V
Supply
Single-phase
Breaker sizes
IEC (MCB)
Total load
4,000 W
Current drawn
17.39 A
Suggested breaker
20 A

Fuente de comprobación: Hand calculation: 4000/230 = 17.39 A, no continuous loads → IEC 60898-1 20 A

Edge: 1920 W continuous needs exactly 20 A

Loads on the circuit (one per line)
Heater, 1920 W, continuous
Voltage
120 V
Supply
Single-phase
Breaker sizes
US (NEC)
Current drawn
16 A
Minimum breaker rating
20 A
Suggested breaker
20 A
Breaker loading
80%

Fuente de comprobación: Hand calculation: 1920/120 = 16 A × 1.25 = 20 A, which a 20 A breaker satisfies (16 A is 80% of 20 A)

32 A EV charger on 240 V

Loads on the circuit (one per line)
EV charger, 32 A, continuous
Voltage
240 V
Supply
Single-phase
Breaker sizes
US (NEC)
Total load
7,680 W
Minimum breaker rating
40 A
Suggested breaker
40 A

Fuente de comprobación: NEC 625.41 / 210.20(A): 32 A continuous × 1.25 = 40 A breaker (the standard 40 A EVSE pairing)

Preguntas

What size breaker do I need?

Add the non-continuous amps and 125% of the continuous amps, then take the next standard rating up (NEC 2023 210.20(A) and 240.6(A); standard ratings include 15, 20, 25, 30, 35 and 40 A). A 1,500 W heater running 3 hours or more on 120 V is 12.5 A × 1.25 = 15.625 A; with a 150 W TV and two 60 W lamps (2.25 A) the circuit needs 17.875 A, so a 20 A breaker.

What is the 80% rule for circuit breakers?

It is the 125% continuous-load rule read the other way: a load that runs 3 hours or more may use at most 80% of a standard breaker's rating, because 1 ÷ 1.25 = 0.8 (NEC 2023 210.20(A)). A 20 A breaker can carry 16 A continuously, which is 1,920 W at 120 V. Breakers in assemblies listed for continuous operation at 100% of their rating are the exception.

How do you convert watts to amps?

Divide watts by volts, and by the power factor for motors and electronics: I = P ÷ (V × pf). On a three-phase supply divide by √3 × V × pf instead. A 2,000 W kettle on 230 V draws 8.7 A; an 11 kW load on a 400 V three-phase supply draws 11,000 ÷ (1.732 × 400) = 15.88 A per phase.

What wire size goes with a 15, 20 or 30 amp breaker?

Under NEC 2023 240.4(D), copper conductors may be protected at no more than 15 A for 14 AWG, 20 A for 12 AWG and 30 A for 10 AWG. A 20 A circuit therefore needs at least 12 AWG copper. Hot locations and bundled cables lower a conductor's ampacity, and long runs add voltage drop, either of which can call for a larger size.

What are the standard MCB sizes in the UK and Europe?

IEC 60898-1 lists rated currents of 6, 8, 10, 13, 16, 20, 25, 32, 40, 50, 63, 80, 100 and 125 A. A kettle, microwave and toaster totalling 4,000 W on 230 V draw 17.4 A, so the next MCB rating is 20 A. The 125% continuous-load factor is an NEC rule; leave loads unmarked to size to the plain current.

¿Qué precisión tiene «Electrical load and breaker size calculator»?

La precisión depende de tus datos y de los supuestos del método. El cálculo decimal usa 50 cifras significativas, pero las estimaciones, los métodos numéricos y los datos de origen pueden ser menos precisos; el redondeo mostrado no elimina esos límites. Ejemplos resueltos comprobados con fuentes independientes: 5. Por ejemplo, «Space heater (continuous), TV and two lamps at 120 V» se comprueba con Python decimal: 1500/120 × 1.25 + 270/120 = 15.625 + 2.25 = 17.875 A → next NEC 240.6(A) size 20 A.

¿De dónde procede el método?

NFPA 70 (NEC) 2023, 210.20(A) — overcurrent device ≥ noncontinuous load + 125% of continuous load; NFPA 70 (NEC) 2023, 240.6(A) — standard ampere ratings; NFPA 70 (NEC) 2023, 240.4(D) — small conductors: 14 AWG 15 A, 12 AWG 20 A, 10 AWG 30 A (copper); IEC 60898-1 — preferred rated currents for miniature circuit breakers.

Acerca de esta calculadora

I=PV⋅pf (or P3 V⋅pf),Ibreaker≥Inon-cont+1.25 IcontI = \frac{P}{V \cdot \text{pf}}\ \big(\text{or } \tfrac{P}{\sqrt{3}\,V \cdot \text{pf}}\big),\qquad I_{\text{breaker}} \ge I_{\text{non-cont}} + 1.25\, I_{\text{cont}}

Fuentes

  1. NFPA 70 (NEC) 2023, 210.20(A) — overcurrent device ≥ noncontinuous load + 125% of continuous load
  2. NFPA 70 (NEC) 2023, 240.6(A) — standard ampere ratings
  3. NFPA 70 (NEC) 2023, 240.4(D) — small conductors: 14 AWG 15 A, 12 AWG 20 A, 10 AWG 30 A (copper)
  4. IEC 60898-1 — preferred rated currents for miniature circuit breakers

Verificado con las referencias

Esta calculadora incluye 5 ejemplos resueltos con respuestas de fuentes independientes. Forman parte del conjunto de pruebas y también puedes ejecutarlos aquí.

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