# CB Instantaneous Setting



## JRaef (Mar 23, 2009)

All2226 said:


> I am currently working on an application that contains two motors that are identical. Ordinarily I strictly deal with a single motor only. Both motors are 3 phase 40HP with a Full Load Current of 66 for each motor. Each motor is landed on a Eaton vaccum starter and overload. I am feeding the breaker with 480V. The CB that has been ordered is an Eaton E2J 250 3pole 600VAC with instantaneous trip settings from 500 to 1000. Normally NEC 430.52 is where I look to find how I need to set my instantaneous trip setting, I am curious if I need to take the same approach as I am familiar with or do I need to calculate this application differently? If so, could you share the way you determined the setting. As this being the first time I have two motors to take in consideration. Thanks


So you have just the one breaker feeding two non-combination starters then?


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## All2226 (Jan 19, 2014)

Yes, 480 feeding the breaker and coming out of the breaker feeding the first motor contactor. From the first motor contactor I have jumpers coming off the top feeding the second motor's contactor. Each motor has a start push button dedicated to them.


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## JRaef (Mar 23, 2009)

Ok, so keep in mind that the NEC is not really intended to allow this, but does not specifically restrict it either, it just makes the rules clear for each motor BRANCH circuit protection. You have created a quasi feeder /branch situation. That breaker is a feeder because it branches below it, i.e. feeder tap rules, but it is a branch because it is the last protective device before the load. Not the intent of the NEC.

So you need to look at that single breaker as if it is one breaker feeding one motor, ie a motor branch circuit protection device. In a motor circuit, the OL relay is taking care of the long term over current protection, but the breaker is providing the short circuit and ground fault protection not only for the leads to the motor, but for the motor winding conductors too. Hence the strict rules on sizing. If the sizing works for both motors by looking at each one individually (which I doubt) then the rules remain the same as far as settings go. So the breaker cannot be greater than max% allowed of *one* motor FLA, and if you can prove that the single motor will not start with the lower settings, you can turn the breaker mag trips up to the values shown in the exceptions, depending on the motor type (on my iPad so I don't have my NEC here). Then be prepared that if one motor starts while the other is running and the breaker trips, but you are already at the max setting, it isn't going to work.

Realistically you should have one breaker as a main (feeder) sized at 125% of the largest FLA + total remaining FLA, then each motor branch circuit should have its own separate breaker or other SCPD.


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