# 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.

Versione interattiva: https://www.calcopenly.com/it/construction/electrical-load-calculator
Argomento: Calcolatori per edilizia e casa

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.

## Dati

- **Loads on the circuit (one per line)**: 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.
- **Voltage** (opzioni: 120 V, 208 V, 230 V, 240 V, 400 V)
- **Supply** (opzioni: Single-phase, Three-phase)
- **Breaker sizes** (opzioni: US (NEC), IEC (MCB))
- **Treat every load as continuous**
- **Power factor**: 1 for heaters and lamps; motors and some electronics run 0.8–0.95.

## Risultati

- Suggested breaker (A) — risultato principale
- Total load (W)
- Current drawn (A)
- Minimum breaker rating (A)
- Continuous part (W)
- Breaker loading

## Formula

$$
I = \frac{P}{V \cdot \text{pf}}\ \big(\text{or } \tfrac{P}{\sqrt{3}\,V \cdot \text{pf}}\big),\qquad I_{\text{breaker}} \ge I_{\text{non-cont}} + 1.25\, I_{\text{cont}}
$$

## Esempi svolti

### 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**
- Fonte di verifica: 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**
- Fonte di verifica: 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%**
- Fonte di verifica: 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**
- Fonte di verifica: NEC 625.41 / 210.20(A): 32 A continuous × 1.25 = 40 A breaker (the standard 40 A EVSE pairing)

### 11 kW continuous on 400 V three-phase

- Loads on the circuit (one per line): Heat pump, 11 kW
- Voltage: 400 V
- Supply: Three-phase
- Breaker sizes: IEC (MCB)
- Treat every load as continuous: sì
- **Current drawn: 15.88 A**
- **Minimum breaker rating: 19.85 A**
- **Suggested breaker: 20 A**
- Fonte di verifica: Python decimal: 11000/(√3 × 400) = 15.877 A × 1.25 = 19.846 A → IEC 20 A

## Domande

### 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.

### Quanto è preciso «Electrical load and breaker size calculator»?

La precisione dipende dai dati inseriti e dalle ipotesi del metodo. Il calcolo decimale usa 50 cifre significative, ma stime, metodi numerici e dati di origine possono essere meno precisi; l’arrotondamento visualizzato non elimina questi limiti. Esempi svolti verificati con fonti indipendenti: 5. Per esempio, «Space heater (continuous), TV and two lamps at 120 V» viene verificato con Python decimal: 1500/120 × 1.25 + 270/120 = 15.625 + 2.25 = 17.875 A → next NEC 240.6(A) size 20 A.

### Da dove proviene il metodo?

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.

## Fonti

- [NFPA 70 (NEC) 2023, 210.20(A) — overcurrent device ≥ noncontinuous load + 125% of continuous load](https://up.codes/s/continuous-and-noncontinuous-loads)
- [NFPA 70 (NEC) 2023, 240.6(A) — standard ampere ratings](https://up.codes/s/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)](https://up.codes/s/small-conductors)
- IEC 60898-1 — preferred rated currents for miniature circuit breakers