Still working thru this info.. But I think I'm following (at a distance, lol). I don't have my code book with me today, and finding the info online is more than a little confusing (bits and pieces).
Basically, unless I can prove otherwise, my terminals are rated at 75*. I don't have the actual table 110.14., but does the terminal amp ratings differ from conductor amp ratings at the same temp (75*). For example: 500mcm @ 75*= 380a.. is this the same for the terminals?
I guess so. Basically, the terminal on a 400a breaker would have to be large enough to accept at least 600mcm. Is this the basic idea here?
Jim C-
This forum helps me remember how little I actually know. And am always grateful how freely everyone shares.
I fully agree. Thanks guys.
When selecting conductor the ampacity must meet two conditions.
First it must meet the terminal temperature rating. Your terminals are 75C. So no matter what other adjustments apply your minimum conductor size can never be smaller than size determined using 75C ampacity table.
Second if you are using 90C conductors you can start ampacity adjustments for number of conductors in raceway or ambient temperature using 90C ampacity tables, but if your final conductor selection @ 90C ends up being smaller than the conductor necessary for termination temp rating you must still use the larger conductor necessary for termination temp.
So, I don't think there is a standard 760 amp breaker avalible... So does that allow me to go back to using the 500 cable? Its at 800a, not over.
The next size up rule is not part of conductor ampacity selection - it is part of overcurrent device selection. This information is in 240.4 The over 800 amp level is where we have to have a conductor with an ampacity equal or greater than the overcurrent device setting.
If we have an overcurrent device rated 800 amps or less, we only need to size conductors to the load at minimum and can protect them at the next higher standard overcurrent device rating. These standard device ratings are mentioned in 240.6
A fairly common example of this is 4/0 aluminum conductor. It has an ampacity of 180 amps @ 75C. It can be protected by a 200 amp overcurrent device because that is the next higher standard device size, but the load on the conductor can not be greater than 180 amps to do this. If the load were 185 amps then you must use a larger conductor even though you can still protect it with a 200 amp device.
You have a similar situation with your 500 KCMIL copper conductors. They are only good for 380 amps, but can be protected by a 400 amp device as long as the load on the conductors is 380 or less, and in the parallel situation you have 760 amps of conductor - next size up is 800, but load on the conductor can not be over 760.
To parallel three 500's for a 1200 amp service is not permitted though - because of the "over 800 amp rule".
3 500's = 1140 ampacity and there is no standard rating of 1140 standard overcurrent devices are 1000 and 1200. So we either need larger conductors for 1200 or can protect these conductors at 1000. Second option is not going to work if the load is over 1000 though.