ELECTRICAL MOTORS AND CONTROLS 1

[lawdy]

I HAVE A WESTING HOUSE CONTROL CENETR WITH 100A ADUSTABLE INSANTANEOUS TRIP BREAKER INSTALLED FEEDING A THIRTY H.P. THREE PHASE 480V INDUCTION TYPE B ENERGY EFFIENT MOTOR NEED TO CHANGE TO A FIFTY H.P. OF THE SAME TYPE WHAT SIZE BREAKER DO I NEED ?

 

[dpc]

The 100 A mag-only breaker should actually work for a 50 hp 460 V motor. It's really too big for the 30 hp.

The feeder is probably too small, however.

 

[l3city]

I would use at least a 150A/3P for a 50HP motor due to higher starting currents than a 30Hhp motor. Check the new breaker you end-up using is compatible with your existing WestingHouse equipment. Increase the feeder size.
This is just a recommendation, do your own calcs and compare them. See NEC Table 430.52 for example.
Regards

 

[lawdy]

the NEC aticle 430.52 states that the over current protection for a instantaneous trip type breaker ueased with three phase type B motor FLA shall be increased by a factor of 1000% this i know but that seams drastic for the size of the breaker to be sized that high.this would make it a 1000a breaker.unless the adjustment for the trip setting compinsates for this htus the o-1000 range on the adjustment dail.respectfuly not my excpertice in this feild

 

[dpc]

I'd agree that a 150 A MCP is probably a safer bet for the 50 hp motor, although a 100 A MCP with the high instantaneous range should also work.

These mag-only breakers used in motor starters are available with different instantaneous ranges for the same nominal breaker size.

To be absolutely safe, the instantaneous range should be adjustable up to about 1500 A.

 

[GerryMoore]

For a 50HP, 460V motor Design B the FLA is 65A.
The max. inst. setting = 11 x 65 =715A
You are allowed to go up to 17 x 65 =1100A if the 715A is proven to be too small for the starting current.

A 100A Square D Mag. only CB will work nicely. I assume your brand is similar, you'll have to verify that.

 

[powerjunx]

lawdy, notice NEC provides minimum requirements. In designing, MCC, a designer itself has a discretion to extend the feeder CB rating where designer thinks of load growth factor and simaltanuity ratio.

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