Breaker Size Calculator
Size a circuit breaker for single-phase or three-phase loads under the NEC (125% continuous-load rule, 240.6(A) ratings) or IEC 60364 (Ib ≤ In ≤ Iz with IEC 60898-1 / 60947-2 ratings), and check it against the cable.
Breaker Size Calculator
NEC sizes continuous loads at 125%. IEC 60364 sizes In ≥ Ib and handles heat through cable and breaker derating instead.
How to Use the Breaker Size Calculator
- 1Pick the standard. IEC for installations to IEC 60364, BS 7671 and other IEC-based wiring rules; NEC for the US and NEC-based codes.
- 2Enter the load. Give power in W or kW with voltage and power factor, or enter the current directly in amps. For three-phase, use line-to-line voltage.
- 3NEC only: mark continuous loads. A load whose maximum current runs for 3 hours or more is continuous and is sized at 125%.
- 4Optional: enter the conductor ampacity Iz. Use the value after ambient, grouping and installation-method correction. The calculator then checks the breaker against the cable.
How to Calculate Breaker Size
Step 1: Load current
Step 2: Apply the sizing rule of your standard
| Item | NEC (NFPA 70) | IEC 60364 / BS 7671 |
|---|---|---|
| Basic rule | OCPD ≥ 125% of continuous load + 100% of non-continuous load (210.20(A) branch circuits, 215.3 feeders) | Ib ≤ In ≤ Iz and I2 ≤ 1.45 × Iz (IEC 60364-4-43, 433.1) |
| Continuous loads | 125% unless the breaker assembly is listed for 100% operation (exceptions to 210.20(A) and 215.3) | No 125% factor. Breakers are rated for continuous current at their reference temperature; heat is handled by derating |
| Rounding | Next standard rating from 240.6(A) | Next rating from the manufacturer's range (IEC 60898-1 preferred values for MCBs) |
| Conductor | Breaker ≤ conductor ampacity (240.4); next size up allowed up to 800 A (240.4(B)); small-conductor limits in 240.4(D) | In ≤ Iz. For IEC 60898-1 and 60947-2 breakers I2 ≤ 1.45 In, so this also meets I2 ≤ 1.45 Iz |
| Fault rating | Interrupting rating ≥ available fault current (110.9) | Icn (IEC 60898-1) or Icu (IEC 60947-2) ≥ prospective short-circuit current |
Worked example 1 (IEC): 12 kW three-phase load, 400 V
- Ib = 12,000 / (1.732 × 400 × 0.90) = 19.25 A
- Smallest rating with In ≥ Ib: 20 A (IEC 60898-1 MCB, curve B or C)
- Cable: Iz ≥ 20 A after correction factors.
- Ib is 96% of In. In a warm, full enclosure the manufacturer's derating may push you to 25 A, which then needs Iz ≥ 25 A.
Worked example 2 (NEC): 4.5 kW water heater, 240 V single-phase
- I = 4,500 / 240 = 18.75 A. A storage water heater of 120 gal or less is a continuous load (422.13).
- 18.75 × 1.25 = 23.44 A, so the next 240.6(A) rating is 25 A.
- Conductor: 12 AWG copper is limited to a 20 A OCPD by 240.4(D), so use 10 AWG copper (30 A maximum). Many installers fit a 30 A breaker on the same 10 AWG cable, which is also compliant.
The same 12 kW load from example 1 under NEC at 480 V and continuous duty gives 16.04 A × 1.25 = 20.05 A, which rounds up to 25 A.
Standard Breaker Ratings: NEC vs IEC
| Standard | Ratings (A) |
|---|---|
| NEC 240.6(A) | 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000, 6000 |
| IEC 60898-1 MCB (preferred values) | 6, 10, 13, 16, 20, 25, 32, 40, 50, 63, 80, 100, 125 (13 A is mainly used in the UK; the calculator skips it) |
| IEC 60947-2 MCCB / ACB (typical ranges) | 160, 200, 250, 315 or 320, 400, 500, 630, 800, 1000, 1250, 1600, then ACB 2000 to 6300 |
Breaker Types and Trip Curves
| Type | Product standard | Typical range | Short-circuit rating |
|---|---|---|---|
| MCB | IEC 60898-1 | Up to 125 A, fixed trip | Icn, standard values up to 25 kA (6 kA and 10 kA are common) |
| MCCB | IEC 60947-2 | About 16 to 1600 A, often adjustable | Icu (ultimate) and Ics (service, given as % of Icu) |
| ACB | IEC 60947-2 | About 630 to 6300 A | Icu, Ics and short-time withstand Icw for selectivity |
| US branch / molded-case breaker | UL 489 | Branch breakers from 15 A; molded-case frames to several thousand amps | Interrupting rating, commonly 10, 22, 65 kA or higher |
| Supplementary protector | UL 1077 | Small ratings | Not permitted as branch-circuit protection |
MCB trip curves (IEC 60898-1 instantaneous trip range)
| Curve | Instantaneous trip | Use for |
|---|---|---|
| B | 3 to 5 × In | Resistive loads, lighting, long cable runs with low fault current |
| C | 5 to 10 × In | General commercial loads, small motors, LED drivers with inrush |
| D | 10 to 20 × In | Transformers, welders and motors with high inrush |
UL 489 breakers are not marked B/C/D; their instantaneous trip is set by the manufacturer and shown on the time-current curve. A higher curve needs a lower earth-loop impedance to trip within the disconnection time required by IEC 60364-4-41.
Selection Checklist and Common Mistakes
- Rated current: from this calculator, then check it against the conductor (In ≤ Iz, or NEC 240.4).
- Breaking capacity: Icn / Icu (IEC) or interrupting rating (NEC 110.9) at least equal to the prospective fault current at the panel. Series (cascade) ratings are valid only for combinations the manufacturer has tested.
- Ambient temperature: IEC 60898-1 MCBs are calibrated at 30 °C; UL 489 breakers and most IEC 60947-2 MCCBs at 40 °C. Above that, use the manufacturer's derating table.
- Altitude: the normal service condition in IEC 60947-1 is up to 2000 m. Above that, derate current and voltage per the manufacturer.
- Poles and voltage: 1P, 1P+N, 2P, 3P or 4P to suit the earthing system; DC circuits need a DC-rated breaker.
- Motors: use NEC 430.52 / Table 430.52 or IEC 60947-4-1 coordination tables, not this general-load rule. See the motor breaker calculator.
Common mistakes: applying the NEC 125% factor and then rounding to IEC ratings (or the reverse); sizing the breaker without checking the cable; ignoring kA rating; using a D-curve MCB on a long circuit where the fault current cannot reach 20 × In.
Breaker Size Chart (NEC vs IEC)
| Load current Ib | NEC continuous (×1.25) | NEC non-continuous | IEC 60364 (In ≥ Ib) |
|---|---|---|---|
| 10 A | 12.5 A → 15 A | 15 A | 10 A |
| 16 A | 20 A → 20 A | 20 A | 16 A |
| 24 A | 30 A → 30 A | 25 A | 25 A |
| 32 A | 40 A → 40 A | 35 A | 32 A |
| 40 A | 50 A → 50 A | 40 A | 40 A |
| 48 A | 60 A → 60 A | 50 A | 50 A |
| 64 A | 80 A → 80 A | 70 A | 80 A |
| 100 A | 125 A → 125 A | 100 A | 100 A |
| 160 A | 200 A → 200 A | 175 A | 160 A |
| 320 A | 400 A → 400 A | 350 A | 400 A |
The breaker is only correct if the conductor is rated for it. Check Iz (IEC) or the ampacity table (NEC 310.16) after derating.
Breaker Size Calculator FAQ
Find the load current (I = P / (V × PF), or P / (√3 × V × PF) for three-phase). Under NEC, multiply continuous loads by 1.25 and round up to a 240.6(A) rating. Under IEC 60364, pick the smallest rating with In ≥ Ib and confirm In ≤ Iz of the cable.
It is the NEC 125% rule seen from the other side: a continuous load may use at most 80% of a standard breaker's rating (1 / 1.25 = 0.8). So a 20 A breaker can carry 16 A continuously. Breakers and assemblies listed for 100% operation are exempt (exceptions to 210.20(A) and 215.3).
No. IEC 60364-4-43 only requires Ib ≤ In ≤ Iz and I2 ≤ 1.45 × Iz. Continuous operation is covered by derating the cable (ambient, grouping, installation method) and, where the manufacturer requires it, the breaker for enclosure temperature.
It depends on the cable's Iz for the installation method and correction factors; the breaker must not exceed it. In the UK, 2.5 mm² copper is commonly used on 20 A radial circuits and on 32 A ring final circuits. The nearest US sizes are 12 AWG (20 A maximum per NEC 240.4(D)) and 10 AWG (30 A).
At unity power factor: 230 V × 32 A = 7.36 kW single-phase, and √3 × 400 V × 32 A = 22.17 kW three-phase. Multiply by the power factor for inductive loads. Under NEC, a continuous load is limited to 80% of the rating.
At least the prospective short-circuit current at the point of installation. Calculate it from the transformer or supply data, or get it from the utility. Under NEC this is the interrupting rating (110.9); under IEC it is Icn for MCBs (IEC 60898-1) or Icu for MCCBs (IEC 60947-2).
Standards and References
The formulas and reference values on this page follow these standards. For design work, check results against the current edition that applies in your country and the equipment manufacturer's data.
- NFPA 70 (NEC) – 110.9 interrupting rating; 210.19(A), 210.20(A), 215.2(A) and 215.3 continuous loads; 240.4, 240.4(B), 240.4(D) conductor protection; 240.6(A) standard ratings; 240.87 arc energy reduction; 422.13 water heaters. Published by NFPA.
- UL 489 – Molded-case circuit breakers (US). UL 1077 – Supplementary protectors.
- IEC 60364-4-43 – Protection against overcurrent (Ib ≤ In ≤ Iz, I2 ≤ 1.45 Iz). IEC 60364-4-41 – Automatic disconnection times. Published by IEC.
- IEC 60898-1 – Circuit-breakers for household and similar installations (MCB ratings, B/C/D curves, Icn).
- IEC 60947-1 and 60947-2 – Low-voltage switchgear general rules and circuit-breakers (MCCB/ACB, Icu, Ics, Icw).
- BS 7671 – IET Wiring Regulations (UK application of IEC 60364).
Related Calculators
Motor Breaker Size Calculator
Calculate the correct circuit breaker size for electric motors using motor power, voltage, phase type, efficiency, and…
Open calculator GeneratorGenerator Breaker Size Calculator
Use our generator-breaker-size-calculator to find the perfect circuit breaker.
Open calculator Panel DesignElectrical Fuse Size Calculator
Calculate the correct electrical fuse size for single-phase & three-phase loads.
Open calculator