Switchgear busbars sizing

AlBerton said:

Is there any standards reference for this?

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Electrical codes?
AHJ?

AlBerton said:

What is your opinion?

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Is this homework?
Do it yourself dreaming?

waross said:

Electrical codes?
AHJ?

Is this homework?
Do it yourself dreaming?

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I thought this was a place where professionals could discuss, but looks like someone has time to waste trying to be witty...

I would assume manufacturers would consider the worst case or most conservative case when designing switchgear - that the main bus can handle all the attached load. This would be common on a main tie main system - two 1200A mains with a 1200A tie supplying <1200A of load. Your situation is a bit unusual.
I’m sure there’s a standard out there but not being in the switchgear business I don’t know it offhand.

wcaseyharman said:

I would assume manufacturers would consider the worst case or most conservative case when designing switchgear - that the main bus can handle all the attached load. This would be common on a main tie main system - two 1200A mains with a 1200A tie supplying <1200A of load. Your situation is a bit unusual.
I’m sure there’s a standard out there but not being in the switchgear business I don’t know it offhand.

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Hello, thanks for your contribution. Yes, probably some switchgear manufacturer would be the more indicated person.
By the way, the main bus can handle all the load; the point is that with several generators in parallel, whatever section of the bus bars you consider, it will never be crossed from the current correspondent to the full load.
If instead you have 2 transformers and a tie breaker NO, with the 2 transformer normally working at 50%, you may have only one transformer available and the tie breaker closed, so in that case the current in the bus bar section close to the incomer will be almost the one correspondant to the full load.
We could deduct that the correct sizing corresponds anyway to the power available on half bus bar.

Electrical codes:
Sum of the loads plus 125% of the largest load.

AHJ; Authority Having Jurisdiction:
Will these be sized as two independent bus bars, at 2012.5 Amps or one bus at 412.5 Amps?
Whatever says the AHJ.
Will you be allowed a deviation from the code to consider the spacing of sources and loads?
Whatever says the AHJ.

Is this homework?
Do it yourself dreaming?
4 x 70 KVA sets in parallel complete with sync gear and a possible load control panel seems an expensive way to go compared to fewer, larger sets.
We work with the information supplied, eg: 50 Amps per motor.
Yes there are exceptions such as extended operation with only a few motors running, and for lightly loaded motors, even though the code is based on full load current.

I don't know of any rule that requires the main busbar to be greater than the sum of the loads. I often see switchgear where the loads are greater than the bus size, these system often have backup equipment such that all loads don't all operate at the same time. I've also seen main-tie-main systems where each side had close to the rated bus current connected, meaning the total load could well exceed the bus rating if run through the tie switch.

Your question appears to just be theoretical based on the component sizing hence it gets questioned why you'd do such a thing.

LionelHutz said:

I don't know of any rule that requires the main busbar to be greater than the sum of the loads.

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In my code, for motor loads; Sum of the loads plus 25% of the largest load.
Busbars the same as feeders.
Your code may differ Lionel.
CEC vs NEC

Thank you all for all the replies, I will be more specific, it's not theoretical, it was an example suitable for several cases. In this one, I have to make a preliminary study to get a budgetary quotation for a new plant that will receive power on a new 11 kV SWGR with Busbar A-Bus tie-Busbar B from 4 power lines, 2 on busbar A and 2 on busbar B. These 4 lines are synchronized. Incomers are 630A rated and there are several loads. My point is: normal practice would suggest to size the busbars 2500A, but as in worst case the maximum current that will flow in whatever section of the busbars is when you have all power on one side and all loads on the other side, I am considering to size them 1250A so saving money. I was wondering if someone has experience on this - thanks Lionel - and if there is some Standard prescription. Maybe in the NEC - thanks waross - there is something but for Low Voltage, if I recall correctly.