Range of breakers to include in 1200A panel

[JonnnyDonuts]

I am currently working on a project where we are replacing a 480V 1200A distribution switchboard filled with fused switches (installed in the 80s) with a 1200A distribution panelboard with circuit breakers. The panelboard is serving mechanical loads which were also installed in the 80s.

This replacement panelboard will have a 600A CB, a 400A CB, and 5 30A CBs.

However, another engineer told me that it would be risky to design a panelboard like this. He said the inrush current from the large loads would damage the smaller mechanical units on the same panelboard and recommended installing a subpanel to handle the 30A CBs.

Is this true? I've seen plenty of panelboards where a 400A CB and a 15A CB are on the same 480V panelboard bus, especially in labs; and this is without any electronic breaker adjustments. I don't mind making the change, I just want to know what the right way to go about this is.

 

[jim dungar]

Is this true?

No.

There is no problem putting small breakers into the same panel/switchboard as larger breakers. This is a common practice in most of the US and has been for more than 60 years.

 

[JonnnyDonuts]

No.

There is no problem putting small breakers into the same panel/switchboard as larger breakers. This is a common practice in most of the US and has been for more than 60 years.

Thanks. I knew voltage drop could be a concern given that it affects the whole bus, but I never thought inrush current from a single HVAC unit would have an effect on other breakers.

 

[Jraef]

Thanks. I knew voltage drop could be a concern given that it affects the whole bus, but I never thought inrush current from a single HVAC unit would have an effect on other breakers.

It doesn’t. The person who told you that likely experienced some situation where VD caused by a large motor load on an undersized transformer caused motor loads on smaller circuits to pull high current and trip breakers. But if a system is designed correctly, that shouldn’t happen. It has nothing to do with breaker sizes and if that bad design was the case, the smaller breakers being in a sub panel would have made no difference whatsoever.

 

[kwired]

The person who told you that likely experienced some situation where VD caused by a large motor load on an undersized transformer caused motor loads on smaller circuits to pull high current and trip breakers.

Which those smaller motors would still draw more current when the voltage drops regardless of where they the are connected if supplied from same source.

One issue with say a 30 amp breaker in a 1200 amp panelboard is you may need higher AIC rated breaker than if you maybe would if you ran that circuit from some downstream panel. Often that larger panel only has feeders and not smaller capacity branch circuits leaving it. There can be cases it still worth getting the higher AIC breaker. Of course the withstand rating of equipment supplied and length of circuit factors in as well.