In this article, I will explain to you how motor overload relay is sized according to National Electrical Code (NEC). I will be sharing with you what I have learned from college and review center when I took registered master electrician.
What is an Overload Relay?
Overload Relay is normally used for motor control as a protective device against overloading. It can be an electronic type or the conventional type that uses a bimetallic strip that will react to the amount of line current passing.
It’s wired in series with the power circuit for current sensing and has usually two contacts for signalling and command.
Why do we need to properly size an Overload Relay?
Overload relay must be properly sized according to motor data on nameplate. Let’s keep in mind that motor has different manufacturers and they test motors according to their standard. They provide ratings according to their standard procedure. That’s why we need to consider the service factor and other essential ratings that the manufacturer is providing to get the proper Overload Relay Size. Undersized overload relay might give false alarm and tripping to production while oversized overload can damage motor as it should trip during fault.
How to size motor overload relay?
Please keep in mind that to size an overload relay, motor nameplate is required and the following should be noted:
1. Full Load Current (FLC/FLA) – Motors Rated Current at Full load
2. Service Factor – is the percent amount over overloading the motor that can handle in small period of time at rated voltage. Normally at 1.15 or 115%
Note: If there’s no SF given in name plate, your multiplier will be 1.15. If 1.15 SF is given, your multiplier will be 1.25.
Let’s say, FLA=20A, SF=1.15 as per motor nameplate.
You have to multiply FLA to SF to get the OL Ampacity
- = FLA x SF
- = 20A x 1.25
- = 25A
The result which is 25A will be your basis on selecting overload range. Means you can buy greater than or equal to 25A rated motor overload relay but you have to set it on your computed value.
What if you cannot see FLA on motor name plate? You can use the NEC table of motor HP with corresponding standard FLA depending on type of motor you’ll use.
Please take note that the function of motor overload is from the word itself, overloading. It cannot sense motor heating unless there’s a changes in load that exceeds the motor rating. A thermistor is used to monitor and give feedback for motor temperature especially over-heating which is attached internally to motor for proper sensing. It will give signal to your control to indicate or trip the circuit based on the requirements.
As you can notice and I have observed too, even the current value is at the same level of rated value. Motor Overload will not trip because it’s sized with a service factor considered in your calculations. It will take time to trip as soon as the bimetallic strip reacts or electronic circuit will be triggered.
How motor overload wired to circuit?
As I experienced for a conventional motor starter, main contacts are usually connected in series with each line going to the load.
In Star-Delta starters, it usually in series with one set of three phase of a phase conductors.
The auxiliary contacts give the feedbacks to control circuit if an overload is sensed. Usually it is in normally closed contacts that will open/trip when overload occurs.
The normally open auxiliary contact is also a feedback mostly used for lamps or signalling.
Both auxiliary contacts can be used as PLC input to provide feedback to controller during overload fault.
Nowadays, some companies limit the use of Overload Relay as there are Thermal-Magnetic Circuit Breaker with overload features available in market. It can functions as overload relay and trip the motor in the event of fault.
Motor Overload Relay can be tested manually by its local switch to test its functionality. Once you pressed the switch, you should check if auxiliary contacts also switched with the use of continuity test. This will ensure that motor overload relay will give proper signalling in the event of fault. It also has reset feature to go back to normal contact position used during testing.
Motor Overload Relay normally installed after main contactor. In Star-Delta starter, it is installed either below Main or Delta contactor. It can be wired, terminated and installed in a rail. Some have small busbars attached to it for easy installation.