Overload Heater Size Chart – A Complete Guide
Properly sizing an overload heater is essential for motor protection and ensuring the longevity of electrical systems. Overload heaters prevent motors from overheating by breaking the circuit when excessive current flows through them. The right overload heater size chart helps in selecting the correct heater for specific motor ratings. This article covers everything you need to know about overload heater sizing, including selection criteria, calculations, and an easy-to-follow heater size chart.
Table of Contents
Table of Contents
What is an Overload Heater?
An overload heater is a critical component of a thermal overload relay. It is designed to protect electric motors from excessive current by sensing heat generated by electrical resistance. If the current exceeds a predetermined limit, the overload heater triggers the thermal overload relay to disconnect power and prevent motor damage.
The overload heater is typically installed within a motor starter circuit, ensuring that motors operate within safe temperature limits. Selecting the right heater size is crucial because an undersized heater may cause unnecessary tripping, while an oversized heater may fail to protect the motor.
Understanding the Overload Heater Size Chart
An overload heater size chart provides a reference for selecting the correct heater element based on motor full-load current (FLC). It helps electricians and engineers determine which heater number corresponds to specific motor ratings.
Factors Affecting Heater Selection
Motor Full Load Current (FLC) – The full-load amperage of the motor determines the heater size
Motor Voltage and Phase – Single-phase and three-phase motors have different heater sizing requirements
Service Factor – Motors with high service factors may require different overload settings
Ambient Temperature – The operating temperature of the environment affects heater selection
Duty Cycle – Continuous or intermittent motor operation affects heat generation
Overload Heater Size Chart for Motors
The following chart provides a general reference for selecting an overload heater for different motor sizes. Always check manufacturer specifications for exact ratings.
| Motor HP (Horsepower) | Full Load Amps (FLC) | Overload Heater Number | Typical Heater Current Rating (A) |
|---|---|---|---|
| 1 HP | 1.6 – 2.5 A | H1 | 2.5 A |
| 3 HP | 3.7 – 5.5 A | H3 | 5.5 A |
| 5 HP | 7.0 – 9.0 A | H5 | 9.0 A |
| 7.5 HP | 11.0 – 14.0 A | H7 | 14.0 A |
| 10 HP | 16.0 – 20.0 A | H10 | 20.0 A |
| 15 HP | 23.0 – 28.0 A | H15 | 28.0 A |
| 20 HP | 30.0 – 35.0 A | H20 | 35.0 A |
| 25 HP | 37.0 – 45.0 A | H25 | 45.0 A |
| 30 HP | 48.0 – 58.0 A | H30 | 58.0 A |
Note: The above values are general estimates. Always verify with manufacturer charts before selection.
How to Select the Correct Overload Heater
Step 1: Determine Full Load Amps (FLC)
Check the motor’s nameplate to find its FLC rating. This value is crucial for selecting the correct overload heater.
Step 2: Match the FLC with the Heater Size Chart
Using an overload heater size chart, find the heater number that corresponds to the FLC range.
Step 3: Consider Service Factor and Ambient Conditions
- If the motor has a service factor above 1.15, use 90-95% of the nameplate FLC
- If the motor operates in hot environments, adjust the heater selection accordingly
Step 4: Install and Test
Once the correct heater is selected, install it in the thermal overload relay and test the circuit to ensure proper functioning.
Thermal Overload Relay and Overload Heater Relation
An overload heater works in conjunction with a thermal overload relay. The relay detects excessive heat generated by the heater and trips the circuit when necessary. The thermal overload relay symbol is often used in electrical circuit diagrams to represent this protection system.
Types of Overload Relays Used with Overload Heaters
Bimetallic Overload Relay – Uses a bimetal strip to detect heat and trip the circuit
Electronic Overload Relay – Uses sensors to monitor motor current and protect against overload
Differential Overload Relay – Protects against phase imbalance and unbalanced loads
For detailed calculations on motor circuit components, refer to the Cable Size for Motor Calculator to ensure proper wire selection.
Common Issues and Troubleshooting in Overload Heater Selection
1. Heater Tripping Too Frequently
- The heater may be undersized – check the motor FLC and compare with the overload heater size chart
- Excessive ambient temperature causing false trips
2. Heater Not Tripping Even Under Overload Conditions
- The heater may be oversized, allowing excessive current to flow
- Faulty thermal overload relay or incorrect relay settings
3. Motor Overheating Even with Proper Heater Selection
- Poor ventilation in the motor enclosure
- Incorrect installation of the thermal overload relay
Conclusion
Using an overload heater size chart ensures the correct selection of heaters for motor protection. Proper heater sizing prevents motor failures and electrical hazards, increasing the efficiency and lifespan of industrial systems. Always verify the motor full-load current, match it with the heater size chart, and consider ambient conditions before installation.
By following these guidelines, you can ensure reliable motor operation while preventing overload damage.
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- Overload Setting for Motor
- Difference Between Star Delta and DOL Starter
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