Switchboard Rating

Status
Not open for further replies.
A

Alwayslearningelec

Senior Member
Location
NJ
Occupation
Estimator
So looking at job that is budgetary and not fully designed.
The main in this board is 225A breaker set at 225 trip. The branch loads are 100-225 amp frame and 20A to 125A trip.
The trip rating is what matters, correct?
How can I determine those trip ratings wont cause an issue with the 225A main breaker? Thanks
 

Attachments

  • Capture.PNG
    Capture.PNG
    243.6 KB · Views: 39
Without knowing the loads it's hard to guess. The 125 amp trip could be for a motor that draws considerably less when running.
 
The 250A bus rating for a standard panelboard would likely not be able to accommodate 225A frame branch breakers.

If it is a custom SWBD, and the breaker configuration isn't the concern, and you need selectivity between the main and branch, then you will need to know the available fault current and look at the tables, but it is is anywhere close to 65kA, you will have trouble.
 
electrofelon said:
Yes I could see the main tripping before or at the same time as a branch. Is that what you were concerned with in the OP?
Click to expand...
If code required selectivity isn't required, who cares? You would be surprised how many 20 and 30A branches are not selective with 100A and 200A MCB's in many installations, but if not code required, so what?
 
ron said:
If it is a custom SWBD, and the breaker configuration isn't the concern, and you need selectivity between the main and branch, then you will need to know the available fault current and look at the tables, but it is is anywhere close to 65kA, you will have trouble.
Click to expand...

Actually the frame of the breaker usually determines its instantaneous pickup point, which is often a wide band of 7-10X for molded case breakers, after that the curve tends to flatten out up to the AIC of the breaker. I would expect a 250A frame device to overlap a 150A device at roughly 1400A. If coordination was required I would have started with a 2X frame size difference - 150 frame branch being feed by a 400A main.
 
ron said:
If code required selectivity isn't required, who cares? You would be surprised how many 20 and 30A branches are not selective with 100A and 200A MCB's in many installations, but if not code required, so what?
Click to expand...

I was just asking if that is what the OP was concerned about, I wasnt clear on what he was asking. Besides I think it certainly can be a concern even if not code required. Recently had a 25 KVA transformer that the client was turning on and off frequently and it would often trip both the 100 branch and the 150 distribution breaker and thus kill a bunch of offices with it, so yeah that was a concern.
 
petersonra said:
what difference does it make? It is mostly a form factor thing.

Switchgear on the other hand, is a different beast.
Click to expand...

As I said, it doesnt matter for the OP, I was just curious. The smallest switchboard I have seen had an 800A bus. The only practical difference I can think of between panelboards and switchboards is that there is not requirement that a switchboard be protected by an OCPD at its rating.
 
Status
Not open for further replies.
Top