Thermomagnetic Breakers per NEC Calculator – NEC

Thermomagnetic breakers are essential protective devices designed to prevent electrical overloads and short circuits. Calculating their ratings accurately ensures compliance with the National Electrical Code (NEC) and guarantees system safety.

This article explores the NEC-based calculation methods for thermomagnetic breakers, providing detailed formulas, tables, and real-world examples. Readers will gain expert-level insights into selecting and sizing breakers per NEC standards.

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  • Calculate breaker size for a 30A continuous load with 120% adjustment.
  • Determine thermomagnetic breaker rating for a 50A motor circuit per NEC.
  • Find breaker size for a 100A feeder with 125% load factor.
  • Calculate short-circuit current trip setting for a 20A breaker.

Comprehensive Tables for Thermomagnetic Breakers per NEC

Table 1: Standard Thermomagnetic Breaker Ratings and Corresponding Ampacity

Breaker Rating (Amps)Typical Wire Gauge (AWG)Maximum Continuous Load (Amps)NEC Reference
1514 AWG Copper12NEC 240.4(D)
2012 AWG Copper16NEC 240.4(D)
3010 AWG Copper24NEC 240.4(D)
408 AWG Copper32NEC 240.4(D)
506 AWG Copper40NEC 240.4(D)
604 AWG Copper48NEC 240.4(D)
1001 AWG Copper80NEC 240.4(D)

Table 2: Typical Thermomagnetic Breaker Trip Curves and Characteristics

Breaker Rating (Amps)Instantaneous Trip Range (x In)Thermal Trip Range (Seconds at 135% In)NEC Reference
155-10x30-60 secNEC 240.6(A)
205-10x30-60 secNEC 240.6(A)
305-10x30-60 secNEC 240.6(A)
505-10x30-60 secNEC 240.6(A)
1005-10x30-60 secNEC 240.6(A)
Load Current (Amps)Breaker Size (Amps)Calculation BasisNEC Reference
1215Load × 125%NEC 210.20(A)
1620Load × 125%NEC 210.20(A)
2430Load × 125%NEC 210.20(A)
3240Load × 125%NEC 210.20(A)

Essential Formulas for Thermomagnetic Breaker Calculations per NEC

Accurate sizing of thermomagnetic breakers requires understanding and applying NEC formulas. Below are the critical formulas with detailed explanations.

1. Breaker Sizing for Continuous Loads

NEC 210.20(A) and 215.3 require that the breaker rating be at least 125% of the continuous load current.

Breaker Rating (A) ≥ Load Current (A) × 1.25
  • Breaker Rating (A): The minimum ampere rating of the thermomagnetic breaker.
  • Load Current (A): The continuous load current in amperes.

Example: For a continuous load of 24A, the breaker rating must be at least 30A (24 × 1.25 = 30).

2. Short-Circuit Current Trip Setting

The instantaneous trip setting protects against short circuits and is typically a multiple of the breaker rating.

Instantaneous Trip Current (A) = Breaker Rating (A) × Multiplier
  • Multiplier: Usually between 5 and 10 times the breaker rating, depending on manufacturer and application.

This setting ensures rapid disconnection during fault conditions.

3. Thermal Trip Time Calculation

The thermal trip element responds to overloads and is inversely proportional to the overload current magnitude.

t = k / (I / In – 1)
where:
t = trip time (seconds)
k = constant (depends on breaker design)
I = actual current (A)
In = breaker rating (A)
  • t: Time to trip in seconds.
  • k: Manufacturer-specific constant, often between 10 and 30.
  • I: Actual current flowing through the breaker.
  • In: Breaker nominal rating.

This formula models the inverse time characteristic of thermal trip elements.

4. Wire Ampacity Check

Breaker size must not exceed the ampacity of the connected conductors per NEC 310.15.

Breaker Rating (A) ≤ Conductor Ampacity (A)
  • Conductor Ampacity: Maximum current the wire can safely carry.

This ensures the breaker protects the wiring from overheating.

Real-World Application Examples of Thermomagnetic Breaker Calculations

Example 1: Sizing a Breaker for a Continuous Load Circuit

A commercial lighting circuit has a continuous load of 20 amps. Determine the minimum thermomagnetic breaker size per NEC.

  • Step 1: Calculate 125% of the continuous load.
Breaker Rating ≥ 20 A × 1.25 = 25 A
  • Step 2: Select the next standard breaker size equal to or greater than 25A.

The next standard breaker size is 30A.

  • Step 3: Verify conductor ampacity.

Assuming 10 AWG copper wire with ampacity of 30A (NEC 310.15), the breaker size is acceptable.

Result: Use a 30A thermomagnetic breaker with 10 AWG copper conductors.

Example 2: Determining Instantaneous Trip Setting for Motor Protection

A 50A thermomagnetic breaker protects a motor circuit. Calculate the instantaneous trip current range.

  • Step 1: Identify the multiplier range (5 to 10 times breaker rating).
Instantaneous Trip Current Range = 50 A × 5 to 50 A × 10 = 250 A to 500 A
  • Step 2: Confirm with manufacturer specifications for exact trip settings.

This range ensures rapid disconnection during short circuits while allowing motor startup currents.

Result: The breaker will trip instantaneously between 250A and 500A.

Additional Technical Considerations for Thermomagnetic Breakers per NEC

  • NEC 240.4(D) Standard Breaker Sizes: NEC mandates standard breaker sizes to ensure uniformity and safety.
  • Continuous vs. Non-Continuous Loads: Continuous loads require 125% sizing, while non-continuous loads can use nominal ratings.
  • Coordination with Upstream Devices: Breakers must coordinate with upstream protective devices to prevent nuisance tripping.
  • Temperature Correction Factors: Ambient temperature affects conductor ampacity and breaker performance; NEC 310.15(B)(2) provides correction guidelines.
  • Breaker Interrupting Capacity: Must be equal or greater than the available fault current at the point of installation (NEC 110.9).

Understanding these factors is critical for safe and code-compliant breaker selection.

Authoritative References and Further Reading