Xfmr and Distribution Panel Integral to MCC or Not????

[BigBadTexOU812]

I'm hoping someone could give me some pointers based on their experience(s).

What are some pros and cons of including a transformer and distribution panel inside a vertical section of an MCC?

I know it would free up some wall space which can be prime real-estate for future growth .

In these tough economic times the question on upper managements minds is whether or not it is a more costly option...assuming this is a new order/build (full lineup) rather than adding a section as an afterthought?

Some will undoubtedly say "It's mainly preference;" however, I would like your thoughts on what sells you one way or the other.

Your thoughts are greatly appreciated.

 

[ScottyUK]

MCB panels in switchboards can be problematic for the operation & maintenance staff even if the designer likes them. Here's why I don't like them:

[ul]
[li]The typically high fault levels at a main board require the MCBs to be backed up by HRC fuselinks because MCBs are rarely capable of breaking more than 6kA or 10kA.[/li]
[li]It is often difficult to add new cabling to a distribution panel in switchboard tier, and it can be outright dangerous doing so when the switchboard is energised.[/li]
[li]The flash hazard is frequently high or very high, so adding a circuit may require a board outage or an extended period working in a flash suit.[/li]
[li]MCBs go obsolete faster than most components in an MCC, and replacing a pan assembly can be awkward because they're never a direct swap.[/li]
[li]The cost of panel space in a switchboard is substantially higher than space on a wall.[/li]
[/ul]


Benefits of incorporating it in a switchboard?
[ul]
[li]It looks nice on paper.[/li]
[li]It will probably look nice on commissioning day too.[/li]
[li]The switchboard manufacturer likes them because they make him money.[/li]
[/ul]

 

[rcw retired EE]

With high labor rates, having the main 480V breaker, 15 kVA transformer and 208/120V panel all delivered pre-wired may be cost effective, especially if you have a blanket agreement with the supplier. Sometimes, there is more room for wires than in a narrow panel. I don't like doing it with anything larger than 15 kVA, the transformer takes too much volume and adds to much heat.

 

[FreddyNurk]

ScottyUK has an excellent summary of the situation of an internal distribution panel.

I'm in agreement with ScottyUK here, I'd put a separate feed in the MCC to an external supply panel every time if I had the opportunity. Note, though, that I don't have to deal with 480 to 208/120V issues here.

Prewired external distribution panels are just as easy to get manufactured offsite, and can be replaced separately to the MCC panel if needed. All of this, though, is largely dependent on expected plant life, and I suspect ScottyUK is used to the same lifetimes and maintenance upgrades that I am. For a much shorter term site, the argument might be different.

 

[jraef]

Given that your name implies some connection to Texas, I assume you are thinking in terms of US /NEMA style MCCs. Some of the issues ScottyUK mentioned, although perfectly valid, may be confusing to you if you are not familiar with international equipment standards. Some of those don't translate to us here. Here, anything inside of an MCC must be listed under UL845, which by definition precludes the use of any devices that would not be rated for the fault current at the MCC bus bars, or at least a listed series combination. So those issues are really non-issues for us (assuming someone doesn't "roll their own").

The biggest problem with putting the xfmr and pnlbd inside of an MCC section for us is the limitations it carries. First is that if you have a preference for one particular brand of pnlbd and breaker line in order to have everything match, you may not be able to get what you want in every MCC, specifically BECAUSE of that UL845 listing issue. Let's say for example you have a specific need for using Sq. D (Schneider) NQOB breakers and panels to match existing, but you want an Allen Bradley or Siemens MCC for other reasons. AB and Siemens will not have the Sq D panels listed in their MCCs. The listing process is expensive and specific, so MCC mfrs only use one brand, their own (if possible).

The other issue is size limits. Because everything in an MCC must pass a heat rise test, and transformers put out heat, the size of transformer you can list inside of an MCC is limited, which also indirectly limits the panel board size. Most stop at 45kVA. If that works for you, fine. But if you need 75kVA, it has to go outside of the MCC anyway.

Other than those issues, I've never had any problem with including them in the lineup. One thing I like about it, besides the reduction in field labor, is that if the transformer was on the floor, you sometimes cannot put the panel board above it because of access issues, which means mounting it next to the transformers, doubling the wall space used. In the MCC lineup, the panel board is brought out to the front of the MCC so it can go above the transformer, meaning less wall space consumed.


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