Breaker Size Calculator

Determine the correct circuit breaker size for your electrical loads.

Breaker Size Calculator

Compliant with NEC & IEC Standards

Results:

Design Current (Ib) (A): 0.00

Min. Breaker Rating (A): 0.00

Recommended Breaker (In) (A): 0

Bus Bar Size Calculator
Contactor Size Calculator

How to Use the Calculator

Calculate from Amps

  1. Select "Amps (Current)" as the calculation method.
  2. Enter the Load Current in Amperes.
  3. Select the Load Type (e.g., Continuous, Non-Continuous).
  4. Click "Calculate" to get the recommended breaker size.

Calculate from Power (kW)

  1. Select "Power (kW)" as the calculation method.
  2. Enter the Power in Kilowatts (kW).
  3. Select System Type (AC/DC), Voltage, and Phase (for AC).
  4. Select the Load Type.
  5. Click "Calculate" to first derive current, then breaker size.

IEC/NEC Verification

  1. Check the box for the desired standard (IEC or NEC).
  2. Enter your conductor's ampacity (Iz), the system fault level, and select relevant derating factors.
  3. The calculator will verify the selected breaker against the chosen standard's rules for safety and compliance.

How to Calculate Circuit Breaker Size (IEC-Compliant Guide)

Selecting the correct circuit breaker is critical for safety and reliability. An undersized breaker causes nuisance tripping, while an oversized one won't protect your cables from overheating. This guide follows IEC 60947-2 and IEC 60364-5-52 standards for proper sizing.

Step 1: Identify Load Type

The breaker's trip curve must match the load's electrical characteristics.

  • B Curve: For resistive loads (heaters, lamps).
  • C Curve: For mixed or inductive loads (motors, lighting, HVAC). This is the most common type.
  • D Curve: For high-inrush loads (transformers, welders).

Step 2: Calculate Load Current (Ib)

Use the power formula to find the design current (Ib). For a 3-phase system:

I = P / (1.732 x V x PF)

Example: A 500 kW motor at 400V with a 0.9 PF has a design current (Ib) of 801 A.

Step 3: Select Breaker Rating (In)

The core IEC rule is Ib <= In <= Iz, where Iz is the cable's ampacity.

The breaker rating (In) must be higher than the load current (Ib) but lower than the cable capacity (Iz).

Example: For Ib=801A and Iz=1200A, a 1000A breaker (In) is a perfect choice.

Step 4: Verify Short-Circuit Capacity

The breaker must safely interrupt a fault. Check two ratings:

  • Icu: Ultimate breaking capacity. Must be > system fault level.
  • Ics: Service breaking capacity (often 75% of Icu).

Example: If the fault level is 50 kA, select a breaker with Icu >= 50 kA. Choosing 65 kA provides a good safety margin.

Step 5: Check Selectivity

Coordination ensures only the breaker closest to the fault trips, keeping the rest of the system online.

This is an advanced step that requires checking the manufacturer's time-current curves.

Step 6: Apply Derating

This critical step adjusts for real-world conditions. See the detailed section on derating factors below.

Derating Factor Reference Tables

Ambient Temperature

Ambient TempFactor
Up to 40°C1.00
45°C0.95
50°C0.90
55°C0.85

Cable Grouping

Cable GroupingFactor
No Grouping1.00
2-3 Circuits0.80
4-5 Circuits0.70

Altitude

AltitudeFactor
Up to 2000m1.00
3000m0.99
4000m0.96

Understanding Derating Factors

A circuit breaker's capacity is rated for ideal lab conditions. In the real world, factors like heat, altitude, and cable bundling reduce its effective rating. You must apply derating factors to ensure safety.

Ambient Temperature

Heat is a primary concern. A breaker installed in a hot environment (e.g., a poorly ventilated electrical room) cannot dissipate its own heat effectively and will trip below its rated current. A derating factor corrects for this.

Example: A 100A breaker at 50°C may only be able to safely handle 90A (100A x 0.9 factor).

Cable Grouping

When multiple current-carrying conductors are bundled together in a single conduit or tray, their collective heat raises the local temperature. This prevents individual cables and the breaker from cooling efficiently, requiring a derating factor.

Example: Grouping 3 circuits might require reducing the effective capacity by 20% (x0.8 factor).

Altitude

At higher altitudes (typically above 2000m), the air is less dense. Thinner air is less effective at convective cooling, meaning heat dissipates from the breaker more slowly. A derating factor is necessary to prevent overheating in these conditions.

Example: At 4000m, a breaker's capacity might be reduced to 96% of its rating (x0.96 factor).

Quick IEC Compliance Checklist

Design Current (Ib)

Start with the actual current your load will draw.

Breaker Rating (In)

Choose a rating slightly higher than the design current (Ib).

Cable Capacity (Iz)

Ensure the cable ampacity is greater than the breaker rating (In).

Short-Circuit (Icu)

Verify Icu is greater than the system's fault level.

Apply Derating

Account for temperature, grouping, and altitude.

✅ NEC Compliance Quick Checklist

Calculate Load Current

Load current calculated correctly (3-phase formula if applicable).

Continuous Load Sizing

Continuous load sized at 125% of load current (NEC 210.20(A), 215.3).

Conductor Ampacity

Conductor ampacity sized per 100%/125% rules (NEC 210.19(A), 215.2).

Protect Conductor

Breaker rating does not exceed conductor ampacity (NEC 240.4).

Interrupting Capacity (AIC)

Breaker interrupting capacity meets or exceeds expected fault current.

Apply Derating

Derating applied for ambient & bundling conditions.

Select Standard Size

Choose the next standard size up from the calculated value (NEC 240.6(A)).

Circuit Breaker Size Chart (Quick Reference)

This chart provides recommended standard breaker sizes for different load types based on a 400V, 3-phase system with a 0.9 power factor. Use the calculator above for precise calculations based on your specific system voltage and parameters.

Load (kW) Continuous (125%) AC / Heat Pump (175%) Welder (200%) Motor (125%)

Frequently Asked Questions (FAQs)

What is the 80% rule on breakers?

The 80% rule is a safety standard from the NEC stating that a circuit breaker should only be loaded to 80% of its maximum rating for continuous loads (those running for 3+ hours). This prevents overheating and ensures the breaker operates safely within its thermal limits.

How to size a 3-phase breaker?

First, calculate the full load current using the formula: I = Power / (1.732 x Voltage x Power Factor). Then, multiply this current by a safety factor (e.g., 125% for continuous loads) and select the next standard breaker size equal to or greater than the result.

What is the 125% rule for breakers?

The 125% rule is the inverse of the 80% rule and is used for sizing. It requires you to size a breaker to be at least 125% of the continuous load's current. For example, a 10A continuous load would require a breaker of at least 12.5A, so you would choose the next standard size, which is 15A according to NEC standards.

What is 80% of a 40 amp breaker?

80% of a 40 amp breaker is 32 amps (40A x 0.80 = 32A). This means a 40A breaker can safely handle a continuous electrical load of up to 32 amps without overheating, according to the NEC 80% rule.

Related Power Calculators

Star Delta Contactor Size Calculator

Calculate contactor sizes for star-delta starters.

Try Calculator

Fuse Size Calculator

Determine the correct fuse size for circuit protection.

Try Calculator

Solar Panel Fuse Size Calculator

Calculate fuse sizes specifically for solar panel installations.

Try Calculator