4000 A switchboard

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Working up a quote for a new project.

It will have about a 3 MVA transformer (4160 D to 600/347 wye) feeding a switchboard with I think a 4000 A air breaker and bus work to feed four pieces of equipment downstream that will each have their own 1200 A MCCB.

Is there any advantage to making the neutral/ground bond at the transformer as opposed to in the switchboard?

I have asked the switchboard manufacturer to make the air breaker LSIG. Does this cover the requirement to have GF protection for equipment when the OCPD exceeds 1000 A? At present the one line I have from the engineer that laid it out shows two 1600 A fuses but no GF protection at all. Pretty sure it is needed.

Would it make sense to have GF on the 1200 A MCCB as well?

The switchboard vendor is suggesting cable bus to make all the connections. I am not familiar with this product. I have always seen ladder style trays used. Any opinions on cable bus?

Outdoor, if that matters any. Cost matters, but not as much as it might.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
I think that cable bus would be a bit cheaper than tray with cables. Cable bus is an engineered system that uses single conductors with supports every 18-30" feet for short circuit bracing. They give you a cable layout to minimize the inductive reactance losses. The frame is much like typical cable tray, but heavier duty for the few cable bus systems I have installed.

They do engineer it using the free air ampacities, so if you are connecting directly to a breaker or other equipment, you will have to tell the cable bus supplier to use the Table 310.16 ampacities and not those in 310.17. Most often these cables terminate on some type of bus assembly so the free air ampacities are fine for that application.
 

ron

Senior Member
Is the 4000A SWBD 3Ph 3W? Then there is a huge benefit to doing the N-G bond at the transformer, because then you don't have to bring the neutral downstream of the transformer (guessing the service disconnect is further upstream) and you don't have to worry about neutral CT's.

The LSIG on the main takes care of the code requirements, but you can add it at the 1200A breakers too optionally if you want for selectivity, but it is designer's choice.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Is the 4000A SWBD 3Ph 3W? Then there is a huge benefit to doing the N-G bond at the transformer, because then you don't have to bring the neutral downstream of the transformer (guessing the service disconnect is further upstream) and you don't have to worry about neutral CT's.

The LSIG on the main takes care of the code requirements, but you can add it at the 1200A breakers too optionally if you want for selectivity, but it is designer's choice.
I need the neutral downstream so it will be a 4 wire switchboard.

There is no "service" disconnect. It's being fed by a generator so the generator will have relaying to trip it's cb that will serve to disconnect the xfmr and switchboard.

I don't have to worry about neutral CTs as that is part of the generator package.
 
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