Interesting, as I have called out many of these and just worry about the wire sizes that can feed through (upwards of 250MCM main wire size on the brand I like), yet as I have known the mains are rated for only 125 amp.
I may be wrong, but I have kept to the following:
1- I have been using the 75 degree column only for any wire ampacity installed. This is probably not due to rating of the feed thru lugs which are likely rated for 90 degree, but due to the rating of the mains (the wire that feeds the disconnects) and is according to Mfg. literature.
2- I have assummed, maybe incorrectly, that the 125 amp mains is protected by the overcurrent devices in the pedestal and the feeder rules.
3- When I am over 250 (255 amp on the 75 degree of 250 MCM) amps on a feeder or using larger wire than 250 MCM, I tap out of handholes (grade level pull boxes).
It is possible that I have been having these installed incorrectly and there are overheating issues that I can not conceive. If so, the terminals are just oversized to accept large conductors due to voltage drop and we need to tap out of a pull box until the connected load (due to outside tap rules) is below 125 amp. This does not sound logical.
Our best source of correct information will be UL or other testing lab. If you are concerned enough, ask and let us all know, especially if I am wrong.
Shane
