Motor disconnect

[Davebones]

Have a 150 hp 480V 3 Phase motor . I understand the the current draw is 180 amps . I figure at 115 % gives us 207 amps ( 430.110 ) for size of disconnect .Am I correct to understand I can't use a 200 amp fusible switch even if I mark it "do not disconnect under load " ??????

 

[iwire]

Have a 150 hp 480V 3 Phase motor . I understand the the current draw is 180 amps . I figure at 115 % gives us 207 amps ( 430.110 ) for size of disconnect

In my opinion you have to use a 400 amp switch.

even if I mark it "do not disconnect under load " ??????

You can't be serious right?

 

[augie47]

Not only can you not use a 200 amp-, I believe whren you check 430.110, 110.3, and the manufacturer ratings, you will find you muts use a 600 amp switch for 150 hp.

 

[iwire]

Not only can you not use a 200 amp-, I believe whren you check 430.110, 110.3, and the manufacturer ratings, you will find you muts use a 600 amp switch for 150 hp.

Square D shows 250 HP using TD fuses at 480 volt with a 400 amp rated switch.


Model H365

 

[augie47]

Square D shows 250 HP using TD fuses at 480 volt with a 400 amp rated switch.


Model H365

Bob, My error. I had looked at 240v by mistake

 

[iwire]

I had looked at 240v by mistake

Ah, that would do it. :smile:

 

[infinity]

Would a 200 amp fuse even hold under starting conditions for a motor with 180 amp FLC?

 

[iwire]

I love this place so many different ways of looking at the same question. :smile:

Rob, I had not considered that issue, I think the answer is no, not likely for across the line starting.

 

[Davebones]

It will with time delay fuses .I've been here 9 yrs and there's a test stand with 3 150hp motors fed with 200 amp disconnects ,each fused at 200 amps time delay fuses . These are across the line starters in the circuits . I put " do not open under load " labels on them when I first saw them. Besides that they have run with really no problem .I wonder if they justified 200 amp switches by going to ( 430.109 E ) ????????

 

[johnnebr]

I believe(to lazy to look right now)yhat it must be rated in horse power.

 

[brian john]

Bob, My error. I had looked at 240v by mistake

Just driving the job in the bucket no profit this time.

 

[Twoskinsoneman]

Can one of you guys help me out? I certainly iwire's link where the manufacturer calls for a 400A disconnect for the 150hp motor but where is the code to back it?

I see this that says what type of disconnect...

(1) Motor Circuit Switch.​

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A listed motor-circuit switch
rated in horsepower.​

(2) Molded Case Circuit Breaker.​

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A listed molded case
circuit breaker.​

(3) Molded Case Switch.​

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A listed molded case switch.​

(4) Instantaneous Trip Circuit Breaker.​

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An instantaneous
trip circuit breaker that is part of a listed combination​

motor controller.

So hp rated if using (1)
But 430.110 says it only has to be rates 115% of FLC

Is the point basically IF your using a "Motor Circuit Switch" is has to rated for the hp of the motor and the amp rating of the disconnect is not really pertinent as long as it is at least 115% FLC?

 

[Davebones]

I believe in my situtation they looked at name plate which showed 167 amps .They must of figured 115% of 167 which comes up to 192 amps .This allowed them to believe a 200 amp bus switch would be fine .I believe they looked at " 430.109 E " and figured 200 amp isolating switch was fine . They should have looked at 430.A1 that calls for using table 430.250 which shows a 150 hp amp rating at 180 amps .Figuring 115% of 180 comes to 207 amps .A 400 amp would then be required by code .Even though 200 amp disconnect is not correct the motors still start with no problem with the time delay fuses !!!!!!!!!