Changing Motor Feeders

I'm not sure how it is in India, but here in the US you are required to size the conductors and the feeder protective devices by the motor nameplate data. If you have the proper size cable but want to use a smaller (lower rated) protective device, that is up to you, but you will increase the likelihood of nuisance tripping. Thermal overload relays do see the starting current as part of the overall "thermal image" of the energy going to the motor, so starting a motor larger than the OL device is set for will indeed increase the risk of nuisance tripping.

Motor overload relays are really there mostly to protect against starting a fire in the motor circuit or damaging the motor windings. They are not the best "load monitor" relay, which is what I interpret your second overload to be used for. For that I would choose an electronic power sensing relay that can be programmed with a time delay so that it does NOT record the starting current. Siemens has such devices in their SIMIREL function relay product line. They have one that monitors "Active Power", meaning the real kW going to the motor, so it will know the difference between a current increase due to a voltage drop, or an actualy increase in the motor shaft load. That would be a better choice for what you are doing. Then just leave the feeder device set for the actual motor nameplate data.

hi

lad for once please document your intension and the reasons so that we may be able to help you, use correct terms, I'm uncertain if your replacing the cables or the contactors or the drive, to me a feeder is a distribution cable to a load.

resubmit this thread please

Hi ally z,

Here feeder is Direct On line starter. The problem is we need O/L setting at 50% of FLC. But according to standards , the O/L range starts from 75%(appr.) of FLC.