ampacity protection using 80% of thermal mag breaker rating - AHJ approval?

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mayanees

Senior Member
Location
Westminster, MD
Occupation
Electrical Engineer and Master Electrician
I'm reviewing a design in a small Data Center where 225-amp busway is connected to 250-amp thermal magnetic breakers. I can rationalize that the 250-amp breaker provides protection at the 80% level, or 200 amps, and I've gotten approval before from an AHJ for similar rationale. But this won't likely be inspected, so I'm writing to get opinions from forum members.
I would prefer to see a 225-amp breaker, then there's no question.
I won't reject a design when I think it's actually safe, but there could be other opinions out there that present an opposing viewpoint, which I would really like to hear before passing approval.
Any comments are appreciated.
John M
 

kwired

Electron manager
Location
NE Nebraska
Would you feel the same way about protecting a 225 amp conductor with a 250 amp breaker?

It is really about the same thing.

I possibly would feel that either would likely never have a problem, but that does not change the fact that it is a code violation.

Get your conductor (bus) rated more than 225 and no problem, you can go up with overcurrent protection.
 

mayanees

Senior Member
Location
Westminster, MD
Occupation
Electrical Engineer and Master Electrician
Yeah, thanks David. I know it's pretty cut and dry, but the 80%-trip level on thermal mags always comes to mind for me, but I guess it's not recognized as a legitimate protection level.
Augie, glad to see you back from your recent situation.
John m
 

kwired

Electron manager
Location
NE Nebraska
Yeah, thanks David. I know it's pretty cut and dry, but the 80%-trip level on thermal mags always comes to mind for me, but I guess it's not recognized as a legitimate protection level.
Augie, glad to see you back from your recent situation.
John m

80% trip level of the 250 amp breaker means nothing. You still can only connect 80% of 225 amps (180) of continuous load to this bus as well as the proper 225 amp breaker.

add: no go back to what I said before about if the bus were rated more than 225. If it were rated 226 it could be protected at 250 because it is next size up. It still can only have 80% of 226 connected as continuous load which would be about 181.
 
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mayanees

Senior Member
Location
Westminster, MD
Occupation
Electrical Engineer and Master Electrician
Point taken kwired.
Funny thing is that I just got word from the client that it's already been inspected and approved, so I'll be the bearer of bad news. But your point that the 225-amp bus can only safely carry 180 amps is well taken.
Thanks
 

mayanees

Senior Member
Location
Westminster, MD
Occupation
Electrical Engineer and Master Electrician
.. to your point about a 226-amp rating being able to use 240.6 to get to 250 amps - I have a call in to the manufacturer to see if there's any leeway there. Like I wrote earlier, I hate to reject equipment on a technicality when it will likely be fine in the field, so I'm gonna do whatever I can to look for a way to pass it. But a particular feature of this application is that it's busway, so it can be loaded up with more and more drops, such that the load may be increased to the point that the ocpd function is more critical than it would be if it were a dedicated load.
 

kwired

Electron manager
Location
NE Nebraska
.. to your point about a 226-amp rating being able to use 240.6 to get to 250 amps - I have a call in to the manufacturer to see if there's any leeway there. Like I wrote earlier, I hate to reject equipment on a technicality when it will likely be fine in the field, so I'm gonna do whatever I can to look for a way to pass it. But a particular feature of this application is that it's busway, so it can be loaded up with more and more drops, such that the load may be increased to the point that the ocpd function is more critical than it would be if it were a dedicated load.

If you have motors or other inductive loads you may find an exception that would allow a higher overcurrent device for starting, but sounds like the loads you have this may not happen very easily.

Read 430.62(A). but the majority of the load would have to be a single motor before this will help much.
 

mayanees

Senior Member
Location
Westminster, MD
Occupation
Electrical Engineer and Master Electrician
... ends up that the busway is UL listed 100% rated for 225 amps.
But not an amp more, so I can't technically round up to a 250-amp ocpd.
And the time-current-characterisitc curve for a 250-amp Sq D PowerPact JD breaker plots out to a 250-amp pickup at the 3-hour mark, so technically it can't be rationalized.
Thanks for the input, and for any additional comments.
 

kwired

Electron manager
Location
NE Nebraska
... ends up that the busway is UL listed 100% rated for 225 amps.
But not an amp more, so I can't technically round up to a 250-amp ocpd.
And the time-current-characterisitc curve for a 250-amp Sq D PowerPact JD breaker plots out to a 250-amp pickup at the 3-hour mark, so technically it can't be rationalized.
Thanks for the input, and for any additional comments.

Unless you have 100% rated breaker it still can only have 180 amps of continuous load.
 

mayanees

Senior Member
Location
Westminster, MD
Occupation
Electrical Engineer and Master Electrician
Unless you have 100% rated breaker it still can only have 180 amps of continuous load.

And that's all the more reason for me to not approve the installation.

I'm doing the Fault, Coordination and Arcflash analysis for a new system. I never see load sheets, so I have to protect the system for whatever can be connected to it. And since this is busway, the ocpd is the limit.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
There is no 80% trip level. It is possible that the breaker will trip on an 80% continuous load, but it is also possible that the breaker will never trip at 134% of the breaker rating. The standard requires a trip in one hour or less at 135%, but does not require a trip at less than 135%.
 
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