MCC over current protection 2

[batman2010]

Hello folks,

I am working on a project that involves (8) 3 phase, 480V, 75HP pumps fed from an existing MCC. We are replacing those pumps with new ones at the same HP ratings. The existing starters in the MCC have circuit breakers as an overcurrent protection rated at 150A. According to the NEC code the pumps full load current is 96A and with multiplying it by 2.5 I get 240A or 250A the next available standard rating. Now the right way of doing it is to use a 3P-250A breaker for each pump but the existing condition of the MCC limits the use of 3P-150A otherwise the whole MCC has to be changed, now I am worried if we order 8 new starters with 3P-150A breakers that those breakers would trip at some point when the pumps start although the current ones didn't trip but maybe the new pumps have different start up curves that the use of 3P-250A is needed? I know 250A breaker is the max allowed by code but isn't 150A a big drop from the maximum(250A) ?

 

[wroggent]

Are you using just a molded case circuit breaker or a motor circuit protector plus a contactor with an overload relay?

 

[batman2010]

THE over current protection is a circuit breaker inside the starter enclosure comes from the manufacturer and it has a contactor with an overload.

 

[jraef]

You are over interpreting things I think, but it's a common mistake.

Chances are, the original starter breakers involved are Magnetic only MCPs (Motor Circuit Protectors), or as the NEC calls them, IT (Instantaneous Trip) only breakers. They are only allowed yo be used in factory built combination motor starters that are UL listed as a complete starter, as is the case with MCC buckets. So when using an IT breaker, you have to read the footnotes and exceptions, but basically, the breaker "rating" becomes simply the continuous current rating of the breaker, not the trip rating, so a 150A IT breaker IS the correct breaker for a 75HP motor starter. IT breakers are not UL Listed, which means they cannot be used in the field other than as rlacement parts. They are UL Recognized components and as such can only be used in combo starters that are themselves going to be short circuit tested and UL Listed as a complete unit, something that can only be done effectively by mfrs who can amortize the extreme expense over thousands and thousands of unit sales.

The sizing rules in the NEC you are referring to are for when a contractor or user builds their own starter circuit in the field, in which case they must use UL Listed breakers, which will then mean Thermal Magnetic circuit breakers. TM are not required to have adjustable magnetic trips and some have fixed mag trips at low values, such as 400%. The 250% rule was added to allow their use, because otherwise the low fixed mag trips would nuisance trip on starting current. But also recognize that this is a Maximum value, not a recommended value. Whatever works at or below that is fine.

Of greater concern now is the motors used on your new pumps. Many new Energy Efiicient motors have extra high magnetic inrush current values, not starting current, but that <1 second instant peak when power first hits the windings and there is no inductance yet. In efforts to improve running efficiency, many new motor designs have loweedr the copper resistance in the windings, which has the unfortunate side effect of allowing higher inrush. That is known to cause nuisance tripping even on IT breakers. The NEC therefore now allows setting of the instantaneous trips to as high as 1700% of motor FLA, but only if DEMONSTRATED that it is nuisance tripping below that. Unfortunately, IT breakers typically have a maximum setting of 1000% of the breaker rating, which means in some cases, the 1700% of motor FLA may excede the 1000% of CB rating adjustment, and trying to get higher may mean going up a frame size. This happens quite a bit at the size breaker you are looking at, and some breaker mfrs need more MCC space for a 250A frame. So if you can get the magnetic inrush values from the motors you are using from the motor mfr, it's a good idea to plan ahead by determining that you may need to go to the 1700% limit, which may mean telling the MCC supplier in advance to use the larger frame breaker to allow getting there. It's much tougher to make all those changes after the fact. Trust me, I know from experience...

"Will work for (the memory of) salami"

 

[batman2010]

Thanks for the clarification, it is very helpful.

MJ