MCC main breaker ratings less than Bus rating

[philly]

This may be an obvious answer but I have seen a lot of MCC's recently that had a main breaker rating which was less than the bus rating. For example a 400A main breaker on an 800A MCC. Or a 1000A breaker on a 2000A MCC.

I would think that if you had an 800A MCC, you would eventually maybe want to use its full capacity so why not make the main breaker an 800A breaker (Even though the current load calculation only called for 400A) If the feeders are protected by the upstream feeder breaker than the main breaker becomes somewhat irrelevant in terms of cable feeder protection anyway and is just there to protect the bus.

I'm thinking that maybe this is a cost thing, with people only wanting to pay for the bare minimum for what is needed at the time. Or maybe it has something to do with the different bus ratings of MCC's that are available since they are only available in certain increments. For instance even if you have a 400A load, the smallest MCC you can buy is 800A.

Just curios to hear others thoughts.

 

[petersonra]

Why do some people still insist on silver plated bus in MCCs?

People do all kinds of things that make little sense viewed from the outside.

Sometimes it is something obvious like what bus sizes are actually available.

Other times, it is just that someone 50 years ago decided that all MCCs should have a 400A main on them. I ran into that one time. I had to split an MCC into two parts because I needed more than 400A and could not get them to budge on their 400A main rule.

Going from a 400A to a 800A feeder is a big chunk of change if you only need the 400A feeder. An 800A main takes up more space than a 400A main too.

 

[GoldDigger]

A lower size main breaker is also often used for residential in conjunction with a solar PV installation, in order to meet the 120% rule for bus sizing relative to main breaker plus PV backfeed.

 

[Jraef]

It costs a lot of money to test and maintain certifications of MCC bus for short circuit bracing / withstand, the cost of the copper itself is almost inconsequential. So there is almost no difference in cost between 600A and 800A bus in an MCC, and 600A is generally the smallest available. So if all you need is a 400A main, you are going to get 600A bus anyway, then because it will cost only a few dollars more for 800A bus, people often go for it.

Now, someone putting in 2000A bus and using a 1000A Main, well that's just wasteful. But it also might depend on the FRAME rating of the breaker they call for. For example if the spec on the above calls for the main to be a Power Circuit Breaker with a 2000A Frame, but a 1000A trip plug, changing later to 2000A would only require changing the trip plug.

 

[Cow]

I just had to order a section for an older AB centerline MCC that has a 400 amp main with a 600 amp aluminum horizontal buss. Fortunately, the AB salesman was on his game when I called him to order it and he had me verify the existing buss, I'm glad I checked! Up to this point I had never seen an AB MCC with an aluminum buss. And hopefully, not ever again! 800 amp copper for me only please!

 

[templdl]

The MCCs that I sold the minimum was 800a horizontal and 300a vert and sn plated cu bus with ag plated available as an option.
In the bidding process if you only require a max of a 400a ampacity the minimum ampacity for an MCC is 800a a 400a main breaker is specified because the MCC is being feed with a transformer and cable capable of supplying that load. To size them based upon the 800a would be illogical causing needless addition expense.
MCCs were one of my favorite products to work with. If you knew how they were built you could be very creative with them.

 

[mjmike]

I recently had dealings with an old SquareD MCC that used copper tubes for the vertical bus. they were coated with black plastic shrink wrap. Assume the plastic coating is either removed before a new bucket is added or the bucket itself punches thru the plastic. First time I ever seen the copper tubing as a bus.

 

[Jraef]

I recently had dealings with an old SquareD MCC that used copper tubes for the vertical bus. they were coated with black plastic shrink wrap. Assume the plastic coating is either removed before a new bucket is added or the bucket itself punches thru the plastic. First time I ever seen the copper tubing as a bus.

A-B vertical bus is round as well. 300A is a tube, 600A is solid.
It's more expensive, but stronger under fault conditions than flat bar.

 

[Tony S]

A consideration is, will the plant incoming supply match the full rating of the bars?
Cabling for 800A instead of 400A is going to be expensive.
An MCC’s of a standard size will be type tested so no need for further testing.
Will the MCC ever use 800A?

Two of the above simply come down to economics.

 

[philly]

So as an example lets say I was installing a new MCC and the load calculation for the loads on the MCC totaled 350A. It sounds like the minimum MCC that is available is an 600A MCC but for a small difference in cost an 800A MCC can be purchased. Lets say I go with either an 600A or 800A MCC, with either one it sounds like I would get a 400A main breaker based on my load calculation. When determining what size main breaker to use should consideration for future MCC loading be taken into account?

If I knew that I would never add future load to the MCC to increase it beyond 400a then would it make sense to just use the 400A breaker. Likewise although I have a calculated load of 350A now but know it is likely that additional loads may be added in the future, does it make sense to go with a larger main breaker maybe 600A or 800A?

I guess the same future load considerations hold true for the upstream feeder breaker as well. For instance if my MCC is 800A and my load calculation is only 350A and I know that I will not add any loads in the future it seems like it would make sense to use a 400A feeder breaker with only one set of 500MCM. Why spend the money on a 800A breaker and larger (or parallel) cables to match the capacity of the MCC when you will never use it? On the other hand if you know that it is likely that load will be added in the future than you may want to spend money on the front end to have your feeder breaker and cables match the capacity of the MCC?

It seems like the additional costs of feeder breaker size and cables could be a significant cost factor when determining how to feed an MCC based off of existing load calculations.

 

[Jraef]

So as an example lets say I was installing a new MCC and the load calculation for the loads on the MCC totaled 350A. It sounds like the minimum MCC that is available is an 600A MCC but for a small difference in cost an 800A MCC can be purchased. Lets say I go with either an 600A or 800A MCC, with either one it sounds like I would get a 400A main breaker based on my load calculation. When determining what size main breaker to use should consideration for future MCC loading be taken into account?

If I knew that I would never add future load to the MCC to increase it beyond 400a then would it make sense to just use the 400A breaker. Likewise although I have a calculated load of 350A now but know it is likely that additional loads may be added in the future, does it make sense to go with a larger main breaker maybe 600A or 800A?

I guess the same future load considerations hold true for the upstream feeder breaker as well. For instance if my MCC is 800A and my load calculation is only 350A and I know that I will not add any loads in the future it seems like it would make sense to use a 400A feeder breaker with only one set of 500MCM. Why spend the money on a 800A breaker and larger (or parallel) cables to match the capacity of the MCC when you will never use it? On the other hand if you know that it is likely that load will be added in the future than you may want to spend money on the front end to have your feeder breaker and cables match the capacity of the MCC?

It seems like the additional costs of feeder breaker size and cables could be a significant cost factor when determining how to feed an MCC based off of existing load calculations.

It didn't used to be that way, but it is now I suppose. But just because you put in an 800A frame breaker with 400A trips, doesn't mean you HAVE to feed it as if it were 800A. You can put in the CONDUIT and not run the cable, that's relatively cheap. Then if you ever DO need to go to 800A, you just parallel some new conductors in those conduits, and the cost of that new copper is in THAT project's budget.