Double ended Switchgear Feeder size

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Roblawro65

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I'm working with a design engineer who is using the the 75c rule, feeding from a 4 wire 200% Nautral System double ended Main-Tie-Main to a Building 90 meters away with the same type M-T-M. the thing i can't get over is we are feeding a 3000AMP X 2 Switchgear with 12 sets EACH 500MCM if we were able to use 90c calculation, it would drop down to 8 sets. These are Rackable Breakers and pure Copper to Copper connections

Why Does the 75c for Device Connection rule Apply?
 
The rule is to wire to the weakest link. In most cases the terminals are only rated 75C so you must use that column in determining you wire size. Now if you use 90C wire you can derate from the 90C column but the final ampacity cannot be larger than the 75C column-- assuming 75C wire and terminals. Obviously if it is an old panel with 60C terminals then you would have to use the first column in T.310.16
 
Problem is 90c terminals are expensive, we had the same problem in a plant on one of those huge air compressors in a polymers plant
 
3000 amp main feeder calculations

3000 amp main feeder calculations

We have not desided on our switchgear yet so the real question is can i (by code) buy 90Degree c terminals for the switchgear so that i can use the 90 degree c amperage table 500MCM 380Amp75C veres 420Amps at 90C this is run in Underground duct bank and is the differance between 11 sets of 500MCM and 8 Sets of 500MCM
 
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