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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.

Updated Checked against 5 worked examples

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.
More options
1 for heaters and lamps; motors and some electronics run 0.8–0.95.
Try
Suggested breaker
A
Suggested breaker: 20 A
Shown to whole number, half-up
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 (3 rows)
LoadWattsAmpsType
Space heater1,50012.5Continuous
TV1501.25Non-continuous
Lamp × 21201Non-continuous
How it's calculated S

  1. Total load

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

  2. Current

    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.

About the 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.

Worked examples

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

Checked against: 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

Checked against: 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%

Checked against: 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

Checked against: NEC 625.41 / 210.20(A): 32 A continuous × 1.25 = 40 A breaker (the standard 40 A EVSE pairing)

Questions

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.

How accurate is the electrical load and breaker size calculator?

Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 5 worked examples whose answers come from independent sources; for example, “Space heater (continuous), TV and two lamps at 120 V” is checked against Python decimal: 1500/120 × 1.25 + 270/120 = 15.625 + 2.25 = 17.875 A → next NEC 240.6(A) size 20 A.

Where does the method come from?

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.

About this calculator

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}}

Sources

  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

Checked against references

5 worked examples with independently sourced answers ship with this calculator. They run in the test suite; you can run them here too.

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