Circuit Breaker Finagling?

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bcorps

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Evansville, IN
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I am looking at a solution to a problem that for some reason is making me feel shady, but I'm not sure if it really is. I did not design the project, but got saddled with the CA on the job when the Engineer who did left the firm.

I have an existing feeder that was served by a 400-amp fusible switch in a switchboard. The fuse values were not available at the time, and the feeder serves three stacked 400-amp panels. in an old building where replacing the feeder would be difficult, to say the least. The demand load from all available information on the panels totaled 278 amps.

So the old switchboard was replaced with a new, circuit breaker switchboard. A standard 400-amp circuit breaker was put in to feed the existing panel stack. No problem, right?

Wrong. The contractor has discovered that the switch is fused at only 300 amps, and the conduit is only 3" EMT.

Shop drawings have been approved, so I'm maybe stuck with the 400-amp breaker. However, it is an adjustable trip, both magnetic and instantaneous. Is it legal to set the instantaneous trip at 300, and the thermal trip at 375, essentially turning it into a 100% rated, 300-amp circuit breaker?
 
You need to be able to set the breaker at 300A for the long time function.
I have never seen a breaker that allows the magnetic and instantaneous functions to be set lower than long time setting.
 
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Is this a breaker with an Electronic Trip Unit then? Adjustable thermal trips are not allowed as feeders. If it’s an ETU, you can set it to any value lower than the max rating.

But all settings should be coordinated with others in the system. It kind of sounds as though you need a coordination study done here.
 
See 240.6 (B) and (C). It's only legal to base feeder size on a reduced long-time pickup setting if conditions in Para (C) are met. (Restricted access) Otherwise, the full rating of the breaker must be used. I don't understand your concern about the instantaneous pickup.
 
Jraef said:
Is this a breaker with an Electronic Trip Unit then? Adjustable thermal trips are not allowed as feeders. If it’s an ETU, you can set it to any value lower than the max rating.

But all settings should be coordinated with others in the system. It kind of sounds as though you need a coordination study done here.
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It's a Siemens JXD6 breaker, not electronic. Adjustable thermal magnetic.

However, interesting development today. They opened up one of the concrete walls for a new doorway, and found the 3" conduit to this panel stack right in the middle of it. So now, I might get to be the solution to a problem, rather than the cause. I can look down, kick some dirt, and say "okay.....if you really want me to demo and replace that feeder...but it's going to be expensive."
 
bcorps said:
It's a Siemens JXD6 breaker, not electronic. Adjustable thermal magnetic.
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I've never seen a JXD6 with adjustable thermal trips and I worked for Siemens. Adjustable mag trips, yes, but the thermals are either fixed or interchangeable, meaning you can change the trips by replacing the entire trip block. But the "X" in "JXD..." means it is the NON-interchangeable trips... The Interchangeable version would be a JD6...
 
Jraef said:
I've never seen a JXD6 with adjustable thermal trips and I worked for Siemens. Adjustable mag trips, yes, but the thermals are either fixed or interchangeable, meaning you can change the trips by replacing the entire trip block. But the "X" in "JXD..." means it is the NON-interchangeable trips... The Interchangeable version would be a JD6...
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Thanks. I see knobs, I assume adjustable. But it looks like they are going to replace the whole feeder with bigger, so I am off the hook (for now).
 
bcorps said:
Thanks. I see knobs, I assume adjustable. But it looks like they are going to replace the whole feeder with bigger, so I am off the hook (for now).
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Five 500kcmil THHNs fit in a 3" EMT raceway, so a 400 amp feeder is not a big deal.
 
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