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Anonymous Poster

Busbar Sizing / Rising Mains

05/26/2009 6:49 AM

Hi friends, Just happened to view this querry while surfing. Now, I too have fallen in the category. Can some one help? "Dear Sir, I have a quarry regarding the rising mains. I have tower of 61 floors and I am using rising mains to serve the floors. I have chosen 10 floors for each set of rising main. My total load on each rising main at the breaker incoming point is 1000 A. As I move towards the loads the load on rising mains decreases. I am using Cu busbars. It is possible to use rising mains in sections like 1000 A, 800A and then 630 A. If it wise to reduce the sections of busbars. And if Not possible is it due to SC ratings. Similarly if LV panel is concerned having a very long bus bars. Will above have some impact on Bus bar sice of the panels (like why to have same size of bus bars throughout the panel when load decreases as we move away from incomer). "
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PWSlack
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#1

Re: Busbar Sizing / Rising Mains

05/26/2009 8:31 AM
  • Should there be a phase-to-phase fault on the ground floor, what will the short circuit current be?
  • Should there be a phase-to-phase fault on the top floor, what will the short circuit current be?

If the answer to the above questions is the same, then it is not advisable to reduce the section of the busbars as one passes up the building.

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Anonymous Poster
#2
In reply to #1

Re: Busbar Sizing / Rising Mains

05/27/2009 12:13 AM

Hi Slack,

I have read many of your threads and definetly were much useful. You meant to say the busbars are sized in such a way to accomodate SC fault current. How to determine the SC fault current?
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PWSlack
Guru
United Kingdom - Member - Indeterminate Engineering Fields - Control Engineering - New Member

Join Date: Jan 2007
Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain. Kettle's on.
Posts: 32175
Good Answers: 839
#3
In reply to #2

Re: Busbar Sizing / Rising Mains

05/27/2009 3:16 AM

It doesn't matter. Think it through: are the two fault currents in #1 the same or are they different?

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 "Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
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Anonymous Poster
#4

Re: Busbar Sizing / Rising Mains

05/27/2009 2:56 PM

Manufactures of busways allow a reduction in the top sections. Refer to their info regarding what is allowed per the UL listing for the specific busway (or other approved testing lab). Also note that 1000 amps triggers GFI protection for 480 volt systems (protects equipment in case of ground fault) in the US, so you might as well go to 2000 or 2500 amp busway and reduce the number of main feeder switches. Design on paper 2 or 3 different ways and choose the best. Use major suppliers such as Sq D or Siemens reps to help.
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