# 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.

Phiên bản tương tác: https://www.calcopenly.com/vi/construction/electrical-load-calculator
Chủ đề: Máy tính xây dựng và nhà ở

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.

## Dữ liệu đầu vào

- **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** (lựa chọn: 120 V, 208 V, 230 V, 240 V, 400 V)
- **Supply** (lựa chọn: Single-phase, Three-phase)
- **Breaker sizes** (lựa chọn: 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.

## Kết quả

- Suggested breaker (A) — kết quả chính
- Total load (W)
- Current drawn (A)
- Minimum breaker rating (A)
- Continuous part (W)
- Breaker loading

## Công thức

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

## Ví dụ có lời giải

### 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**
- Nguồn đối chiếu: 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**
- Nguồn đối chiếu: 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%**
- Nguồn đối chiếu: 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**
- Nguồn đối chiếu: 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: có
- **Current drawn: 15.88 A**
- **Minimum breaker rating: 19.85 A**
- **Suggested breaker: 20 A**
- Nguồn đối chiếu: Python decimal: 11000/(√3 × 400) = 15.877 A × 1.25 = 19.846 A → IEC 20 A

## Câu hỏi

### 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.

### “Electrical load and breaker size calculator” chính xác đến mức nào?

Độ chính xác phụ thuộc vào dữ liệu nhập và giả định của phương pháp. Phép tính thập phân dùng 50 chữ số có nghĩa, nhưng ước lượng, phương pháp số và dữ liệu nguồn có thể kém chính xác hơn; làm tròn khi hiển thị không loại bỏ các giới hạn đó. Ví dụ có lời giải đã đối chiếu với nguồn độc lập: 5. Ví dụ, “Space heater (continuous), TV and two lamps at 120 V” được kiểm tra bằng Python decimal: 1500/120 × 1.25 + 270/120 = 15.625 + 2.25 = 17.875 A → next NEC 240.6(A) size 20 A.

### Phương pháp này lấy từ đâu?

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.

## Nguồn

- [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