MCC Connected Load vs Bus Rating

[CuriousElectron]

Greetings,

Is it acceptable to have a connected load be higher than the bus rating of the MCC. I think it is OK, as long as the demand load(coincident load) is lower than the bussing rating. The feeder to the MCC would be sized based on the demand load, not necessarily on the bus rating.

Thoughts?
Thanks,
EE

 

[che12345]

Dear Mr. CuriousElectron
Please tell us where the MCC is located and which [Standards] i.e. IEC or ANSI/NEC is to be complied with. It would be also useful to state the voltage i.e. LV (<1kV) or MV (>1kV). This is to ensure that the opinion would be applicable to your question or need.
Note: IEC may has different recommendations/requirements/practices differ from that of NFPA/NEC or ANSI/CSA.
Che Kuan Yau (Singapore)

 

[waross]

As I understand the Canadian code, the bus and feeders must be sized based on the connected load.
When adding loads to an existing panel or MCC, the demand load may be used to justify the addition IF the demand can be justified to the AHJ by actual demand records.
In the case of adding loads to a main service, producing utility billing showing the monthly maximum demand for the previous year is generally acceptable. A years records will account for seasonal variations.
In the case of an unmetered MCC you would have to install a peak demand meter for a time acceptable to the AHJ.

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

 

[waross]

Dear Mr. Che Kuan Yau.
A hint.
When I click on your handle (user name) I see that you are connected to the internet in Singapore (SG).

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

 

[CuriousElectron]

Hi Guys,

Thank you for the replies. My installation needs to adhere to NEC. We're talking about a 480V system, 3 wire system.

Regards,
EE

 

[jraef]

There is nothing in the NEC directing you to not overload the bus, other than the proverbial "common sense" rule in 110.3 stating that equipment must be suitable for the intended use. But what you are describing is exactly what almost everyone does, they size the bus for the maximum demand of connected loads. But fair warning, most MCC mfrs now use software programs to lay out their MCCs, and those programs do not have demand factor capability, they will just add up the FLA of all units in the MCC and select a bus bar rating to handle them, or give the user a warning when it cannot go high enough (most MCCs stop at 3,000A bus). So if you want to size the bus only for coincident loads, it generally means having the ability to override the software selections, something that sometimes requires intervention by factory engineers.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[che12345]

Dear Mr. waross
Yes, I am a Singaporean and I am in Singapore.
I stated my name and including [(Singapore)] as I had noticed that my name had been used illegally without my knowledge. I hope that it is unlikely another person with the same name (as mine) in Singapore, a very small island at the southern tip of Malaysia.
Kind regards
Che Kuan Yau (Singapore)

 

[che12345]

Dear Mr. CuriousElectron

1. " ... Thank you for the replies. My installation needs to adhere to NEC...."
As NEC is the [law], you have no alternatives but to comply in full.
It is unfortunate that there isn't any provision in the NEC that explicitly answers your question/need.
2. Caution: following are for [information] purposes. They (may not) be applicable.
a) search through UL publications e.g. UL 891 etc...., Note: there is no UL-IEC 61439-x series currently.
b) search through IEC publications (withdrawn)IEC 60439-x series and the (current) IEC 61439-x series. Your question has been [explicitly] answered in both the withdrawn and also in the current publications. The same clause in the said IEC Standards for the past decades.
Che Kuan Yau (Singapore)