Thermomagnetic circuit breakers are essential for protecting electrical circuits from overloads and short circuits. Calculating their ratings accurately ensures safety and compliance with NEC standards.
This article explores the thermomagnetic circuit breaker calculator based on NEC guidelines, covering formulas, tables, and real-world applications. Learn how to select and size breakers precisely for various electrical loads.
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- Calculate breaker size for a 30A motor circuit with 240V supply.
- Determine trip settings for a 100A feeder with 3-phase load.
- Find thermal and magnetic trip values for a 50A breaker protecting lighting circuits.
- Compute breaker rating for a 15kW resistive heater on 208V single-phase supply.
Comprehensive Tables for Thermomagnetic Circuit Breaker Ratings and Settings
Below are detailed tables listing common thermomagnetic circuit breaker ratings, trip curves, and NEC-compliant sizing values. These tables assist engineers and electricians in selecting appropriate breakers for various applications.
| Breaker Ampere Rating (A) | Thermal Trip Range (A) | Magnetic Trip Range (A) | Typical Application |
|---|---|---|---|
| 15 | 12 – 18 | 75 – 150 | Lighting circuits, small appliances |
| 20 | 16 – 24 | 100 – 200 | General purpose outlets, small motors |
| 30 | 24 – 36 | 150 – 300 | Medium motors, HVAC equipment |
| 50 | 40 – 60 | 250 – 500 | Large motors, feeders |
| 100 | 80 – 120 | 500 – 1000 | Industrial feeders, large equipment |
| Load Type | NEC Breaker Sizing Factor | Typical Breaker Rating (A) | Notes |
|---|---|---|---|
| Resistive Load (Heaters) | 1.25 × Full Load Current | 15 – 50 | Continuous load, no inrush current |
| Motor Load (NEC 430.52) | 250% of Full Load Current (Magnetic Trip) | 30 – 100 | Allows for motor starting current |
| Lighting Load | 125% of Full Load Current | 15 – 30 | Continuous load, NEC Article 220 |
| Feeder Circuits | 125% of Continuous Load + Noncontinuous Load | 50 – 200 | NEC Article 215 compliance |
Fundamental Formulas for Thermomagnetic Circuit Breaker Calculations
Understanding the formulas behind thermomagnetic circuit breaker sizing is critical for accurate protection and NEC compliance. Below are the key equations and variable definitions.
1. Full Load Current (FLC) Calculation
The full load current is the current drawn by the load under normal operating conditions.
- IFL: Full load current (Amperes)
- P: Power rating of the load (Watts or VA)
- V: Line-to-line voltage (Volts)
- PF: Power factor (unitless, typically 0.8 – 1.0)
- η: Efficiency (unitless, typically 0.85 – 1.0)
For single-phase loads, the formula simplifies to:
2. Thermal Trip Setting (Overload Protection)
The thermal trip protects against sustained overloads by heating a bimetallic strip that trips the breaker.
- Ithermal: Thermal trip current (Amperes)
- 1.25: NEC multiplier for continuous loads (NEC 240.6(A))
3. Magnetic Trip Setting (Short Circuit Protection)
The magnetic trip responds instantaneously to high current surges, such as short circuits or motor starting currents.
- Imagnetic: Magnetic trip current (Amperes)
- K: Multiplier depending on load type
- For motors: K = 5 to 10 (typically 6 to 8)
- For general loads: K = 3 to 10
4. Breaker Ampere Rating Selection
Breaker rating must be equal to or greater than the thermal trip current and comply with NEC sizing rules.
Breaker ratings are standardized (15A, 20A, 30A, 50A, 100A, etc.) and must be selected accordingly.
5. Motor Circuit Breaker Sizing (NEC 430.52)
For motor circuits, the magnetic trip must accommodate starting currents, which can be several times the full load current.
Some manufacturers specify magnetic trip settings up to 8 times IFL for large motors.
Detailed Real-World Examples of Thermomagnetic Circuit Breaker Calculations
Example 1: Sizing a Thermomagnetic Breaker for a 5 HP, 3-Phase Motor at 460V
A 5 HP motor operates at 460V, 3-phase, with a power factor of 0.9 and efficiency of 0.92. Determine the appropriate thermomagnetic breaker size according to NEC.
- Step 1: Find the full load current (IFL).
- Step 2: Calculate thermal trip current.
- Step 3: Calculate magnetic trip current.
- Step 4: Select breaker rating.
Step 1: Calculate Full Load Current
Power (P) = 5 HP × 746 W/HP = 3730 W
Step 2: Calculate Thermal Trip Current
Step 3: Calculate Magnetic Trip Current
Using NEC 430.52, magnetic trip is 250% of full load current:
Step 4: Select Breaker Rating
Breaker rating must be ≥ 6.36 A (thermal) and have magnetic trip ≥ 12.73 A.
Standard breaker sizes: 10A, 15A, 20A, 30A…
Choose a 15A thermomagnetic breaker with thermal trip ~12-18A and magnetic trip ~75-150A, which safely covers both requirements.
Example 2: Breaker Sizing for a 15 kW Resistive Heater on 208V Single-Phase Supply
A 15 kW resistive heater operates on 208V single-phase. Determine the minimum breaker size according to NEC.
- Step 1: Calculate full load current.
- Step 2: Apply NEC continuous load multiplier.
- Step 3: Select breaker rating.
Step 1: Calculate Full Load Current
Step 2: Apply NEC Continuous Load Multiplier (1.25)
Step 3: Select Breaker Rating
Standard breaker sizes near 90.15 A are 90A and 100A.
Choose a 100A thermomagnetic breaker to ensure safe operation and NEC compliance.
Additional Technical Considerations for Thermomagnetic Breaker Calculations
- NEC Article 240: Governs overcurrent protection requirements, including breaker sizing and trip settings.
- Ambient Temperature Correction: Breaker trip characteristics can vary with temperature; derating may be necessary.
- Coordination and Selectivity: Proper breaker sizing ensures selective tripping, minimizing system downtime.
- Breaker Interrupting Capacity: Must exceed the maximum available fault current at the installation point.
- Manufacturer Trip Curves: Consult specific breaker datasheets for precise thermal and magnetic trip characteristics.
By integrating these factors with the formulas and tables above, engineers can design robust, NEC-compliant electrical protection systems.