Inverse Time CB Calculation

[Fishn sparky]

Again, jump right to it...
Size an inverse time CB for the below motors:

(1) 20HP / 3PH / 480V
(2) 10HP / 3PH / 480V
(1) 50HP / 3PH / 480V - 5 min varying duty

My calculation:
20HP / 3PH / 480V = 27A
(2) 10HP / 3PH / 480V = (2)*14A = 28A
50HP / 3PH / 480V - 5 min varying duty = 65A (Not including 5 min varying duty as I am not sizing conductors)

=65A(250%) +28A+27A
=217.5A
=225A Inverse Time CB (Next Size Up per 430.52 (C)(1) EXP #1

Correct?

 

[JoeStillman]

Your calculation seems to comply with 430.62(A). Now calculate the feeder ampacity according to 430.24 and see if 4/0 CU is oversized.

 

[david luchini]

=65A(250%) +28A+27A
=217.5A
=225A Inverse Time CB (Next Size Up per 430.52 (C)(1) EXP #1

Correct?

65A * 2.5 = 162.5 (Next size up is 175A per 430.52)

175A + 27A + 14A + 14A = 230A (Next size down is 225A per 430.62(A))

225A is the correct answer, but your method for getting there was off.

 

[Fishn sparky]

Thank you all!

 

[kwired]

Your calculation seems to comply with 430.62(A). Now calculate the feeder ampacity according to 430.24 and see if 4/0 CU is oversized.

That just it, conductor used factors into what you can or can not do for overcurrent protection here, presuming the motors mentioned are the only load served.

430.24 should only require 1/0 copper max for this group of motors, yet it can be on up to 225 amp breaker and still comply with 430.62.

 

[david luchini]

430.24 should only require 1/0 copper max for this group of motors, yet it can be on up to 225 amp breaker and still comply with 430.62.

I would think 430.24 would only require a #1awg cu conductor for this group of motors. (Though the nameplate current of the varying duty motor isn't given.)

 

[kwired]

I would think 430.24 would only require a #1awg cu conductor for this group of motors. (Though the nameplate current of the varying duty motor isn't given.)

Maybe so, I guess I figured them all as continuous duty. intermittent duty motor that large probably kind of rare as well unless it is an OEM motor.

 

[JoeStillman]

Article 240.4 says the motor feeder has to have 225A ampacity if the breaker is 225A - unless you read on to 240.4(G), which sends you to article 430, Part II, for the specific conductor application.

But if you size your conductors according to 430.24 and your breaker according to 430.62, be prepared for the armchair code experts that will accuse you of a code violation.

 

[kwired]

Article 240.4 says the motor feeder has to have 225A ampacity if the breaker is 225A - unless you read on to 240.4(G), which sends you to article 430, Part II, for the specific conductor application.

But if you size your conductors according to 430.24 and your breaker according to 430.62, be prepared for the armchair code experts that will accuse you of a code violation.

If you ran such a feeder and tapped it to four disconnects or combination starters - many may not think much about it.

If you ran same feeder to a panelboard with only the four breakers for the four motors and no other loads, there will be some that will question it every time.

Both installs should be code compliant though.

If someone adds another breaker to the panel even for a pretty light load, it may no longer comply. However same would go for another tap for the disconnects method, it just takes a little more work to add that disconnect than to plug in another breaker.

 

[JoeStillman]

If you ran such a feeder and tapped it to four disconnects or combination starters - many may not think much about it.

If you ran same feeder to a panelboard with only the four breakers for the four motors and no other loads, there will be some that will question it every time.

Both installs should be code compliant though.

If someone adds another breaker to the panel even for a pretty light load, it may no longer comply. However same would go for another tap for the disconnects method, it just takes a little more work to add that disconnect than to plug in another breaker.

Agreed. I would only size a feeder like that if it fed a control panel. Even a wireway with a bunch of combo starters fed off of it can be tapped later on.

 

[kwired]

Agreed. I would only size a feeder like that if it fed a control panel. Even a wireway with a bunch of combo starters fed off of it can be tapped later on.

I'd do it any time you have a specific or limited load situation where there isn't much chance of adding more loads in the future. One example common here, though usually service supplied with multiple service disconnects, is irrigation equipment. Most installations have well, pivot, and likley a fertilizer/chemigation pump and that is it. But if they were typically feeder supplied I would do it this way. Of course we also often have long runs and larger conductors for voltage drop anyway.