newbie - MCC Question

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titan01

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Hi, This is my first time posting in the forum.
I just started an entry lvl ee position at a firm.. trying to understand some works of power engineering. Don't have much exp in power engineering.

working on a review of submittal for a MCC panel.

Excuse me if I am asking the question in a the wrong way. I am not too familiar with the right terminology just yet.

I have a MCC with 3 vertical busses. I know that each vertical bus must not exceed 300A? and the entire unit must not exceed 600A from what I recall.

How would I go about calculating the if it has exceeded or not. There are motor starters that are spec'd out along with breakers and a meter. However, the size of the motors we are going to use are still unknown... Should I just add up the trip amperes for the breakers?

Thanks.
 
take a look at your singleline diagram and specs. What is you horizontal bus rating? what is your main breaker size? Sometime vertical bus is not required unless the specs call for it.
 
Since this is a review of the submittal... I'm actually trying to match the single line up with the layout of the MCC panel to make sure everything is ok. (if that makes sense to you?)

I believe the horizontal bus rating is 600A.

The main breaker has a 400A rating but has a 250A trip... On the one-line it shows a 250A breaker is needed... would having a 400A rating breaker with a 250A trip make sense?
 
Go with a 600a vertical bus for the main break section, and other sections to be 300A. What is your biggest load(s) on a single section of MCC?
 
titan01 said:
Hi, This is my first time posting in the forum.
I just started an entry lvl ee position at a firm.. trying to understand some works of power engineering. Don't have much exp in power engineering.

working on a review of submittal for a MCC panel.

Excuse me if I am asking the question in a the wrong way. I am not too familiar with the right terminology just yet.

I have a MCC with 3 vertical busses. I know that each vertical bus must not exceed 300A? and the entire unit must not exceed 600A from what I recall.

How would I go about calculating the if it has exceeded or not. There are motor starters that are spec'd out along with breakers and a meter. However, the size of the motors we are going to use are still unknown... Should I just add up the trip amperes for the breakers?

Thanks.
Click to expand...


Horizontal bus and vertical bus can be bought in different ratings - I am assuming from your post that the job spec for this lineup called for 600A horizontal and 300A vertical bus. If you have a 250A main breaker it is pretty clear that you won't overload 300A or 600A bus, but the main could always be upsized in the future so you still want to calculate the loading. Adding up the rating of the circuit breakers and motor circuit protectors doesn't tell you anything useful - you need to add up the actual loads that you are feeding, and an allowance for any spaces you may have in the vertical section for future buckets.

Something seems to be out of whack in your design process. For you to be checking MCC drawings against the single line without the exact motor loads known is pointless. At this point you could check to make sure that you are getting the correct aux contacts in the bucket, the specified amount of terminal strips in the bucket, and any auxilliary relays that were specified. For you to really review the single line against the manufacturer's proposal requires you to know what the exact loads are, or at least make some guesses about the HP ratings and build the equipment to the guesses - that is a decision the owner might make to keep things moving.

The main breaker that is called out is probably right. Molded case breakers are made in a handfull of frame sizes, and within each frame size different rating plugs can be installed to get different trip ratings for the assembly. On drawings at my plant what you have might be labeled "400 AF, 250 AT" meaning 400 amp frame size with a 250 A trip unit installed. Using that notation on the single line lets people know in the future that to increase the breaker size all they need to do is buy a new rating plug for the existing breaker rather than buy a whole new breaker.
 
Does 600A horizontal bus mean 600A for the whole MCC unit?

In the first vertical bus I have:
400A rating - 250A trip
300A scale meter
3 pole CB - Dual 30/60A Trip
3 pole CB - Dual 15/15
3 pole CB - Dual 30/30

2nd vertical bus:
2 HP Motor Starter
10 HP MS
0.5 HP MS
20 HP MS
20 HP MS
20 HP MS

3rd vertical bus:
3 pole CB - Dual 70/30
3 pole CB - Dual 15/15
7.5 HP MS
2 HP MS
40 HP MS

Since I don't have the actual load #'s, wouldn't adding up the ratings be a good estimate?
 
yes, 600 hoz. bus will feeds all 1-600a and 2-300a vertical buses. Based on your load information, the 300A vertical bus for the section 2 and 3 should be fine.
 
Are you saying that the first vertical bus is not 300A?

Also, isn't the 250A trip too small for that whole MCC unit?
 
I'm not sure what you mean by that question.

The main breaker is rated 400A and has a 250A trip on it. If that is what you are asking?
 
Oppss, I meant 400AT/400AF. My advice is to size your MCC main breaker to max out your capacity of the MCC. But at the same time, you have to look at the existing/or new upstream feeder and breaker sizes.
 
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