400A 3Ø 120/208V - Service entrance conductors

[cobrahead]

Going from CT/transocket to a 400A fusible disconnect (Square D D325NR) to 400A panel with main breaker inside shop. Single lug per phase in disconnect are rated for (max) 600 kcmil or (max) 2ea 250 kcmil. Note on service disconnect enclosure says not enough room to bend larger than 250 kcmil inside enclosure. Lugs are 'suitable' for 75C conductors.

Dual lugs per phase in CT accept (max) 600 kcmil on each.

I plan to go straight out the top of the CT with dual 3" conduits to a wire way that is 8" x 8" x 48" and then out of the wire way straight down into the disconnect with dual 3" conduits. I also plann to use parallel 250 kcmil CU THHN on all phases and neutral.

Does that sound about right? Overkill anywhere? missing something entirely?

Thanks

 

[infinity]

Parallel #3/0's is good for 400 amps. Parallel 250's might be little overkill.

 

[kwired]

Parallel #3/0's is good for 400 amps. Parallel 250's might be little overkill.

The 3/0 could even be squeezed into 2 inch raceways if desired, 250 maybe not.

 

[texie]

I'd run them all in 1 nipple, not dual between the enclosures. Save material, cleaner, less bonding to do, etc. And if load calcs allow you could even just use 1 neutral conductor with parallel ungrounded conductors.

 

[kwired]

I'd run them all in 1 nipple, not dual between the enclosures. Save material, cleaner, less bonding to do, etc. And if load calcs allow you could even just use 1 neutral conductor with parallel ungrounded conductors.

If short enough you don't need to adjust ampacity. Though if he is running 250 when he could likely run 3/0 it probably fine if adjustments are needed.

 

[cobrahead]

Parallel #3/0's is good for 400 amps. Parallel 250's might be little overkill.

I thought the amperage rating had to be based on the 75C temp in table 310.15(B)(16) since there will be more than 3 CCC in the raceway. The neutral of 120/208V three phase is considered a CCC, right? If so, I have to derate by 80%. That is how I came up with the 250 kcmil size wire. 255A x 2 x .8 = 408A

 

[roger]

The neutral of 120/208V three phase is considered a CCC, right?

Doubtful, see 310.15(B)(5)(a)

Roger

 

[cobrahead]

Doubtful, see 310.15(B)(5)(a)

Roger

Good point. However, I am somewhat concerned that the inspector would point to 310.15(B)(5)(c).

 

[kwired]

Good point. However, I am somewhat concerned that the inspector would point to 310.15(B)(5)(c).

Is the majority of your load non linear? This is mostly only an issue with something like a data center.

 

[roger]

Good point. However, I am somewhat concerned that the inspector would point to 310.15(B)(5)(c).

He might and that is another consideration you need to look at when calculating it, but a neutral is not automatically a CCC.

Roger

 

[cobrahead]

He might and that is another consideration you need to look at when calculating it, but a neutral is not automatically a CCC.

Roger

Agreed. In this case I would rather consider the neutral to be a CCC, just to mitigate that entire discussion with this inspector.

 

[roger]

Agreed. In this case I would rather consider the neutral to be a CCC, just to mitigate that entire discussion with this inspector.

That's up to you but I agree with kwired, it is unlikely that it is necessary.

Roger

 

[cobrahead]

Is the majority of your load non linear? This is mostly only an issue with something like a data center.

Initially there will be hardly any load, just enough to pass inspection and get the meter set. This is an additional service to a light industrial building that wants to be able to add 3 phase motors for air compressors, plasma cutters, welders, etc in the future.

 

[augie47]

Since you are feeding he panel from the fusible disconnect, there is no need for a main in the panel. A MLO panel will save you a few bucks and give you a bit more wiring room.

 

[kwired]

Initially there will be hardly any load, just enough to pass inspection and get the meter set. This is an additional service to a light industrial building that wants to be able to add 3 phase motors for air compressors, plasma cutters, welders, etc in the future.

Unless those items you mentioned are run via inverter drives they are not non linear loads. Even if they are run by such drives they usually don't utilize the neutral, so there still isn't a situation where the majority of neutral load is non linear load.

 

[cobrahead]

Since you are feeding he panel from the fusible disconnect, there is no need for a main in the panel. A MLO panel will save you a few bucks and give you a bit more wiring room.

Correct. I proposed a MLO to begin with. However, the customer requested a main breaker. He wants the ability to kill the panel from inside the building. Also, the customer wants to lock the external disconnect in the ON position... I think a main breaker may technically be required in this case.