Switchboard

[hugop973]

Hi,
A question about a switchboard rated 3,000 Amps, 480V, 3 phase. When we say in our drawings that the equipment is 3,000 Amps, the manufacturer always provide the equipment with the busbar rated 100% or we have to specify in our drawings, that the bar is "3,000 Amps, 100% rated". Thanks for your help

 

[WoodrowJWeen]

Are you asking whether the buses are 100% rated vs 80% rated like a breaker? Or are you asking whether the bus is rated 3000A to the farthest section of switchboard vs tapered bus which would reduce the amp rating as you get away from the main. Two different issues.

 

[hugop973]

Hi,
I want to know if it is necessary to indicate in the single line drawings, that the bus in the 3000A switchboard is 100% rated like a breaker. I always understood that when you buy a 3000A equipment, the bus is designed to handle 3000A with no problem for indefinite time (100%). Am I right?.

Thanks

 

[jraef]

As WJW indicated, it isn't quite that simple because of the possibility of "tapered bus", which is a somewhat popular way to keep cost down. So in that instance, the ampacity of the bus bar would depend on the location you are talking about, as in how far away it is from the main incoming terminals.

But in general when you ask for 3000A bus, it will be designed to handle 3000A continuously within the temperature limit of the design.

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[hugop973]

Hi,

I don't ask about the tapered bus or vertical buses in the switchboard, I asked about the horizontal bus from the main incoming terminals.

Thanks

 

[rbulsara]

when you buy a 3000A equipment, the bus is designed to handle 3000A with no problem for indefinite time (100%).

.

Not necessarily. It depends on what the equipment is. Bus is just part of an equipment. A 3000A switchboard may not be a 100% rated switchboard without using 100% rated breakers, especially the main, even if it has so called a 100% rated bus.

Buses are always 100% rated so to speak. But their ratings can be determined in different ways. Many spec calls for certain current density (xxx amps/ sq inch). Most mfrs will take exception to that and provide what is called "heat" rated (I believe that is thermally tested) . You have to ask them as to what exactly they mean by that and how do they test it. It is generally is not an issue but there is a difference.

Also while you are not asking for tapered bus, if you do not specify "Fully" rated (not 100%) Main bus, you may get tapered main bus, that jraef indicated.

Most importantly specify in plain English what you want, rather leaving it open to interpretation. It does not hurt to ask for 100% rated bus. Let the mfr. say it is their "standard".



Rafiq Bulsara

http://www.srengineersct.com

 

[rbulsara]

Also thermal rating depends on the ambient. So if the performance is that critical, specify under what conditions the "equipment" is supposed to perform. Spell out the performance you want.

Rafiq Bulsara

http://www.srengineersct.com

 

[alehman]

Or if you want it sized based on current density, say that (e.g. 1000A/sq in fully rated).

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[asco1]

To a UL 891 Switchboard manufacturer, 100% can mean a few different things.
1. Sized and spaced per the manufacturers temperature test results: Most of the current flows along the skin of a bus bar. Therefore you might get a greater current carrying capacity with a lower density, large surface area arrangement.
Size and spacing may also be dependent upon the location of the bus within the switchboard. Cooler bus is more efficient.
2. Less than 3000A = 1000A per square inch.
3. 3000A - 6000A = 800a per square inch.
4. UL 891 charts specific bar sizes, quantities, and spacings for a particular amperage.
e.g.: 3000A = (3) 1/4 x 6" cu bars with .25" spacing.
3000A = (4) 1/4 x 4" cu bars with .25" spacing.
3000A = (3) 1/4 x 5" cu bars with .25" spacing may be used for a switching device.

So, 3000A may look very different form manufacturer to manufacturer. And it may look different within a particular switchboard.

I have received specifications requiring that all bus have a current density rating of 800A or 1000A per square inch.
Or stating a full rated (not tapered) Main Bus with provisions for adding future equipment.

 

[asco1]

To a UL 891 Switchboard manufacturer, 100% can mean a few different things.
1. Sized and spaced per the manufacturers temperature test results: Most of the current flows along the skin of a bus bar. Therefore you might get a greater current carrying capacity with a lower density, large surface area arrangement.
Size and spacing may also be dependent upon the location of the bus within the switchboard. Cooler bus is more efficient.
2. Less than 3000A = 1000A per square inch.
3. 3000A - 6000A = 800a per square inch.
4. UL 891 charts specific bar sizes, quantities, and spacings for a particular amperage.
e.g.: 3000A = (3) 1/4 x 6" cu bars with .25" spacing.
3000A = (4) 1/4 x 4" cu bars with .25" spacing.
3000A = (3) 1/4 x 5" cu bars with .25" spacing may be used for a switching device.

So, 3000A may look very different from manufacturer to manufacturer. And it may look different within a particular switchboard. But it will carry 100% of the current for 100% of the time.
fyi: Even when you are busing or cableing to an 80% rated device, your conductor (bus/cable) must be rated 100%.

I have received specifications requiring that all bus have a current density rating of 800A or 1000A per square inch.
Or stating a full rated (not tapered) Main Bus with provisions for adding future equipment.
Maybe this is your concern??