310.16:75c or 90c

[Al Pike]

I have a 400a 277/480v service.I have one 4in RGS going from the back of my outside POCO CT cabinet to the inside SE, ATS.

There will be 4 parallel sets in my conduit plus a ground.

Can I use the 90c column and parallel 250mcm, 290x2x70%=406a or do I have to use the 75c with 350mcm, 310x2x70%=434a. I'm thinking 75c because of the outside CT cabinet is in a wet location.

Also am I wrong to count the neutral ?

And I have to run a EGC to the CT cabinet, do I have to parallel a EGC or can I run one #3 to the CT cabinet? 250.122 (400a=#3 EGC)

My grounding will be done in the ATS with a 2/0 grn. 250.66 note1

 

[raider1]

You can't use the 90 degree column for the final ampacity due to the fact that I am not aware of any service equipment that is rated at 90 degrees C.

110.14(C) limits the ampacity of the conductors to that of the terminals that the equipment that the conductors are terminated on.

Chris

 

[raider1]

And I have to run a EGC to the CT cabinet, do I have to parallel a EGC or can I run one #3 to the CT cabinet? 250.122 (400a=#3 EGC)

If the CT cabinet is located on the line side of the service you need to bond it in accordance with 250.92 and if you are using a bonding jumper it needs to comply with 250.102(C). (Table 250.122 is not used for bonding jumpers on the line side of a service)

Chris

 

[infinity]

For derating purposes you can use the ampacity of the conductors based on the insulation type and listed in table 310.16. Since you're in a wet location your THHN/THWN would only good for 75?C. If you use XHHW-2 or THWN-2 you can derate from the 90? C column in 310.16. I'm curious at to why you have an EGC on the line side of the service disconnect?