Adjusting LTPU on Breaker (Still 100% rated?)

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minesh21

Senior Member
Location
CA USA
Hi,

I have an interesting scenario. I have an existing installation of a square d 800A breaker (480V, 3P) that is 100% rated and in a substation all rated for 100%. We wanted to install parallel sets of 600 kcmil conductors in order to utilize the full 800A (using 90 degree C wire but sizing based on 75 degree C column). However, we recently found out the existing wire is parallel sets of 500 kcmil (THHN) and the client does not want to do a shutdown to upgrade the wire size. We have to temporarily adjust the long time pick up to 760A in order to protect the 500 kcmil wire (change to 600 kcmil at a later date). Is this still considered a 100% installation where I can fully utilize a continuous load of 760A? Or must I treat this like an 80% breaker (because of the trip dial change) and only utilize a continuous load of 0.8 x 760A = 608A?
 
The 100% rating has to do with the breaker being sized so that it can carry 100% of it's rating for an indefinite period of time. For your 800 A breaker this would be at the LTPU setting of 1 (800A).

So if you lower the LTPU to meet the code to protect the conductors, that does not change that the breaker is a 100% rated at 800A as if can carry a 800A load continuously, it certainly can carry a lesser load continuously.

So you can set the breaker to 0.95 x 800 = 760A and it is still a 100% breaker
 
240.4(B) says 800A or less can round up. So using 800A breaker 100% rated is fine with 2 sets of 500's.

However the calculated load cannot exceed the wire rating of 380 x 2 = 760A
 
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