Breaker for Skid

[timm33333]

Industrial plants have packaged skids, which have some motors and some other non-motor loads like lighting, chemical loads, etc. These skids have their own local distribution panels which have individual breakers for each motor and non-motor load. In ETAP these skids are modeled as "static loads", and it is possible to group the loads by type: for example 80% motor-load and 20% non-motor load.

We have to supply the feeder from MCC to a skid, the skid has 80% motor-load and 20% non-motor load. How should we size this breaker (in MCC) for this feeder; should it be sized for 100% motor load, or for 100% non-motor load? Thanks.

 

[waross]

80% motor load plus 25% of the largest motor plus 20% non-motor loads.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[timm33333]

Thanks Waross. But the individual motor breakers in the local distribution panel of the skid would be sized for 250% of motor full load current (NEC Table 430.52). This means that if there is a large motor, then the motor breaker would be larger than the upstream feeder breaker which is sized for 125% of full load current. Is it OK that the downstream motor breaker (in local distribution panel of the skid) is larger than the upstream feeder breaker (in MCC) ?

 

[jraef]

But the individual motor breakers in the local distribution panel of the skid would be sized for 250% of motor full load current (NEC Table 430.52).

Correction; the NEC says you CAN (MAY) size the breakers for the motor loads at NO MORE THAN 250% of the motor FLC, that does not mean that they SHOULD be (although I'll concede that they likely WILL be.) The reason that is allowed is because the motor controller is ostensibly acknowledged to also provide for protection against running thermal overload, at far far less than 250% of FLC, so the breaker is really only expected to provide short circuit protection anyway. The allowance of it being as MUCH as (but not over) 250% is to allow for Thermal Magnetic breakers that do NOT offer adjustable instantaneous magnetic trip settings. So although most now do, it harkens back to when it was commonplace to have a T-M breaker with zero adjustment dials on it at all. So for example if the Mag Trip was factory set for 400% of the breaker thermal trip rating (which was commonplace on some), allowing the T-M breaker to be sized at 250% of the motor FLC resulted in the mag trips to be in the neighborhood of 1000% of FLC, which if you read the footnotes and exceptions, can still remain in the realm of acceptability as short circuit protection. Now days with many (if not most) T-M breakers coming with adjustable mag trips up to 10X the thermal rating of the breaker, it's a better idea to use a T-M breaker closer to the motor FLC size and adjust the mag trip setting to provide appropriate protection.

So in essence, this means the breaker sizing for an individual load within a "skid" has virtually no direct relationship with the sizing of a feeder breaker for the entire skid. That feeder must protect the CONDUCTORS that are going to the skid, and you will see that the rules for sizing the feeder breaker will align with the rules for sizing the minimum conductor size for that feeder. It could then very well be that if you did end up with a situation where the feeder CB ends up sized to be smaller than the motor branch breaker, that you might end up with nuisance trips happening on that feeder breaker. That's where a "coordination study" comes in handy, to avoid unpleasant surprises.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington