Thermal Relay Selection Calculator – IEC

Selecting the correct thermal relay is critical for protecting electrical motors from overload damage. This calculation ensures optimal safety and efficiency in motor control systems.

Understanding the IEC standards and applying precise calculations for thermal relay selection prevents costly downtime and equipment failure. This article covers formulas, tables, and practical examples.

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Calculate thermal relay setting for a 15 kW, 400 V, 3-phase motor.
Determine relay current setting for a motor with 22 A rated current.
Find appropriate thermal relay for a 7.5 kW motor operating at 50 Hz.
Calculate overload relay setting for a 30 A motor with 1.15 service factor.

Common Values for Thermal Relay Selection According to IEC Standards

Motor Power (kW)	Rated Current (A) – 400 V, 3-Phase	Thermal Relay Setting Range (A)	Typical Service Factor (SF)	IEC Standard Reference
0.75	1.7	1.4 – 2.2	1.0	IEC 60947-4-1
1.5	3.0	2.4 – 3.6	1.0	IEC 60947-4-1
2.2	4.2	3.3 – 5.0	1.0	IEC 60947-4-1
4.0	7.6	6.0 – 8.5	1.0	IEC 60947-4-1
7.5	13.5	10.5 – 15.0	1.0 – 1.15	IEC 60947-4-1
11	18.5	15.0 – 22.0	1.0 – 1.15	IEC 60947-4-1
15	24.0	20.0 – 28.0	1.0 – 1.15	IEC 60947-4-1
22	32.0	28.0 – 36.0	1.0 – 1.15	IEC 60947-4-1
30	41.0	36.0 – 45.0	1.0 – 1.15	IEC 60947-4-1

Fundamental Formulas for Thermal Relay Selection – IEC

Thermal relays protect motors by tripping when current exceeds a set threshold for a specified time. The selection depends on motor rated current, service factor, and relay adjustment range.

Rated Motor Current (I_n): The nominal current the motor draws under full load, typically from the motor nameplate.
Service Factor (SF): A multiplier indicating permissible overload capacity, usually between 1.0 and 1.15.
Thermal Relay Setting Current (I_set): The current at which the relay trips, adjustable within a range.

1. Calculating Thermal Relay Setting Current

Iset = In × SF

Where:

I_set = Thermal relay setting current (Amperes)
I_n = Motor rated current (Amperes)
SF = Service factor (dimensionless, typically 1.0 to 1.15)

This formula ensures the relay setting accommodates permissible overloads without nuisance tripping.

2. Selecting Thermal Relay Range

Thermal relays come with adjustable current ranges. The selected relay must cover the calculated setting current.

Imin ≤ Iset ≤ Imax

Where:

I_min = Minimum current setting of the relay (Amperes)
I_max = Maximum current setting of the relay (Amperes)
I_set = Calculated thermal relay setting current (Amperes)

The relay must be capable of adjusting to the calculated setting current within its range.

3. Motor Full Load Current Calculation (If Not Provided)

When motor rated current is unknown, it can be estimated from motor power and voltage:

In = (P × 1000) / (√3 × V × η × cos φ)

Where:

P = Motor power (kW)
V = Line voltage (Volts)
η = Motor efficiency (decimal, e.g., 0.9)
cos φ = Power factor (decimal, e.g., 0.85)

This formula calculates the approximate full load current for three-phase motors.

4. Thermal Overload Relay Trip Time

Thermal relays trip based on the thermal time constant and overload current magnitude. The trip time can be approximated by:

t = k / (I / Iset)2

Where:

t = Trip time (seconds)
k = Thermal constant of the relay (seconds)
I = Actual current (Amperes)
I_set = Relay setting current (Amperes)

This inverse square relationship reflects faster tripping at higher overload currents.

Detailed Real-World Examples of Thermal Relay Selection

Example 1: Selecting Thermal Relay for a 15 kW, 400 V, 3-Phase Motor

A 15 kW motor operates at 400 V, 50 Hz, with a service factor of 1.15. The motor nameplate lists rated current as 24 A. Select an appropriate thermal relay.

Step 1: Calculate the thermal relay setting current.

Iset = In × SF = 24 A × 1.15 = 27.6 A

Step 2: Identify a thermal relay with an adjustable range covering 27.6 A.

From the table above, a relay with a range of 20.0 – 28.0 A is suitable.

Step 3: Verify relay selection.

The relay setting of 27.6 A fits within the 20.0 – 28.0 A range, ensuring proper protection without nuisance tripping.

Example 2: Calculating Thermal Relay Setting for a Motor Without Rated Current

A 7.5 kW, 400 V, 3-phase motor has no rated current listed. The motor efficiency is 90%, and power factor is 0.85. The service factor is 1.0. Calculate the thermal relay setting.

Step 1: Calculate the rated current using the formula:

In = (P × 1000) / (√3 × V × η × cos φ) = (7.5 × 1000) / (1.732 × 400 × 0.9 × 0.85)

Calculate denominator:

1.732 × 400 × 0.9 × 0.85 = 530.4

Calculate current:

In = 7500 / 530.4 ≈ 14.14 A

Step 2: Calculate thermal relay setting current:

Iset = In × SF = 14.14 A × 1.0 = 14.14 A

Step 3: Select a thermal relay with a range covering 14.14 A.

From the table, a relay with a range of 10.5 – 15.0 A is appropriate.

Additional Technical Considerations for Thermal Relay Selection

Beyond basic calculations, several factors influence thermal relay selection and performance:

Ambient Temperature Compensation: Thermal relays are sensitive to ambient temperature. IEC 60947-4-1 requires derating or compensation if ambient temperature deviates from standard conditions (typically 40°C).
Motor Starting Current: High inrush currents during motor start-up do not cause relay tripping due to thermal time delay characteristics.
Coordination with Circuit Breakers: Thermal relay settings must coordinate with upstream protective devices to ensure selective tripping and minimize downtime.
Service Factor Variations: Motors with higher service factors (up to 1.15 or 1.25) allow for higher thermal relay settings, accommodating occasional overloads.
Frequency Effects: Motor rated current and thermal relay settings may vary with supply frequency (50 Hz vs. 60 Hz).

IEC Standards and Guidelines for Thermal Relay Selection

The International Electrotechnical Commission (IEC) provides comprehensive standards governing thermal relay selection and motor protection:

IEC 60947-4-1: Covers contactors and motor starters, including thermal overload relays.
IEC 60947-5-1: Specifies control circuit devices and switching elements.
IEC 60034-1: Defines motor ratings and performance, essential for relay selection.

Adhering to these standards ensures compliance, safety, and reliability in motor protection schemes.

Summary of Best Practices for Thermal Relay Selection

Always use motor nameplate data for rated current and service factor.
Calculate thermal relay setting current by multiplying rated current by service factor.
Select a thermal relay with an adjustable range encompassing the calculated setting.
Consider ambient temperature and derating factors as per IEC guidelines.
Coordinate thermal relay settings with upstream protective devices for selective tripping.
Verify relay trip characteristics match motor starting and running conditions.

Proper thermal relay selection extends motor life, reduces downtime, and enhances system safety.