Motor Protection Device Sizing — Practical Calculation Guide
Correctly sizing protection and switching devices for an electric motor is essential for safety, reliability, and equipment life. In this article, we’ll walk through a simple, practical method to calculate ratings for:
- MCB (Miniature Circuit Breaker)
- Contactor
- Overload Relay
using a sample motor specification.
Given Motor Data
Assume the motor has the following ratings:
- Motor power: 7.5 HP
- Rated current: 11 A
- Voltage: 415 V
- Power factor: 0.8
- System: Three-phase
We will size the protection devices based on the motor’s full-load current (FLC) of 11 A.
Why Safety Factors Are Needed
Motors draw higher current:
- During startup
- Under load variations
- Due to supply fluctuations
- Because of temperature changes
Therefore, protective devices are selected slightly above or below the rated current depending on their function.
1) MCB Size Calculation
The MCB protects against short circuits and heavy overcurrent.
A common practice is to add a safety margin so that a normal motor start does not trip the breaker.
Calculation
Safety factor = 25%
Increase current by 25%:
Safety current
= 11 × 25% = 11 × 0.25 = 2.75 A
Total current:
= 11 + 2.75 = 13.75 A
The next standard breaker size above this value is selected.
Recommended MCB Size
➡ Use 16 A or 20 A MCB, depending on local standards and starting conditions.
2) Contactor Size Calculation
The contactor must safely switch motor current continuously. Typically, a larger margin is applied.
Calculation
Safety factor = 58%
Extra current:
= 11 × 58% = 6.38 A
Total:
= 11 + 6.38 = 17.38 A
Recommended Contactor Size
➡ Choose a contactor rated 18–20 A or higher, according to manufacturer ratings (AC-3 duty).
3) Overload Relay Size Calculation
The overload relay protects the motor from long-term overheating due to overload. Unlike breakers, overloads are often set slightly below or at rated current to ensure proper protection.
Calculation
Safety factor = 20% reduction
Reduction:
= 11 × 20% = 2.2 A
Relay setting:
= 11 − 2.2 = 8.8 A
However, in practice, overload relays are selected with an adjustable range that covers the motor FLC.
Recommended Overload Range
➡ Use a relay adjustable range covering 9–13 A, and set near 11 A after testing.
Final Device Selection Summary
| Device | Calculated Value | Practical Selection |
|---|---|---|
| MCB | ~13.75 A | 16–20 A |
| Contactor | ~17.4 A | 18–20 A |
| Overload Relay | Adjustable | Range covering 11 A |
Practical Tips
✔ Always check the motor nameplate current
✔ Consider motor starting method (DOL, Star-Delta, VFD)
✔ Account for ambient temperature and cable size
✔ Follow local electrical standards
✔ Use manufacturer selection charts for accuracy
Correct motor protection sizing prevents nuisance trips while protecting equipment from damage. Applying proper safety factors ensures smooth motor operation and longer system life.