Overload and overcurrent motor protection sizing

Overcurrent protects the wire, overload protects the motor.
Over load and over current protection is simple....and sometimes not. For me it is easy to think of it this way. The overload protects the motor, the overcurrent protects the wire. Because the time/current curves are very far apart from each other, don't ever think that one will adequately protect for the other.

Many electricians size the wire from the starter to the motor based on the ampacity of the overload protection which would be smaller than the overcurrent protective device. This is not correct for the fact that a short circuit or ground fault would probably obliterate the overload.Motor As an example, I watched while a pump panel burnt itself up when the motor feeds shorted together. The panel was fed by a 50 amp Inverse Time Circuit Breaker without instaneous trip. The overloads were sized for 8 amps, and #14AWG wire was used to feed the motor from the pump panel. When they shorted, it burnt up one of the overload clips and it started arcing. The heat of the arcing spread to the other phases until the whole panel was crispy. I called over the radio for a lineman to disconnect at the nearest overhead reclosure and the damage was repaired. But the point is that the electrician who installed it, thought the #14 wires were more than adequately protected by the 8 amp overload clips.

This concept is a good foundation to start with to learn all there is to know in this subject. I pass it on to my apprentices a lot.

One more peculiar thing with motors is that you don't want your overcurrent protection device to trip unnecessarily during the starting i.e. when the motor takes the inrush current for the first few cycles. So let's say if you are putting a fixed characteristic type device, a fuse, then you will end up having a fuse with much higher rated current than the rated current for the motor. This is more so for the smaller ratings. This fuse will not serve the purpose of protection during overload. For that you will have to use other protective device say a bimetallic overload relay. You can also get a combination of short circuit/instantaneous protection and overload protection in circuit breakers like Motor protection Circuit Breakers and Moulded Case Circuit Breakers. It all depends on adapting to the required i2t curve. Huge current less time...less overload more time...

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10/8/2016 6:34 AM
Why can't we use overcurrent protection (ANSI DEVICE 51 with an I.D.M.T curve) as an overload protection for motors?
10/8/2016 6:40 AM
Depending upon the class of the motor and the relay characteristics, you might be able to use the relay for motor overload protection, but motor protection typically includes much more (ground fault, unbalance, loss of phase, jab, undercurrent,...). A dedicated motor protection device and motor circuit protector (or contactor for medium voltage motors) is the best way to go.

I should also add that if the motor is VFD driven, typically a standard breaker is used to protect the VFD and the VFD is programmed to protect the motor. If the VFD has bypass capability then the bypass path would need standard motor protection devices.