1200 A in (2) 3-1/2" conduits?

[George Stolz]

Two runs of 6 x 400 THWN-2, 1 x 400 bare stranded.

Ambient of 59F would get you way over the mark. Use 90C terminations on both ends.

 

[iwire]

Use 90C terminations on both ends.

Bingo, and I am surprised Smart did not bring this up as we have talked about it before. Run the conductors at the 90 rating, something like this.

(The j-boxes would also have to be suitable for 90C)

 

[Smart $]

Bingo, and I am surprised Smart did not bring this up as we have talked about it before.

Because derating puts the 90?C conductor ampacity below 75?C termination temperature limitations.

 

[Smart $]

Ambient of 59F would get you way over the mark.

Actually, six 350's as I posted earlier would do without engineering supervision if under NEC 2011 edition. The correction factors have been increased under 2011 (based on 40?C rather than 30?C), and lower temperatures have been added. Under 2008 and earlier, 1.04 per Table 310.16 is the best ambient correction available without engineering supervision.

 

[tkb]

Six 350's comes in just under 1200 at 1165A with ambient correction of 1.04 for 21-25?C applied and 80% for 6 ccc's. Underground ambient unknown for OP'er... however would still require engineering supervision under 310.15(C) for approval. This would permit two 250's for non-ccc neutral and come in just under 40% fill.

How is that allowed and still comply with 240.4(C)?
My calculation comes out with the same size of 400kcmil for ambient of 100% or 104%.

Using an ambient of 15c will calculate 112%.
That would allow 350kcmil

 

[Smart $]

How is that allowed and still comply with 240.4(C)?
My calculation comes out with the same size of 400kcmil for ambient of 100% or 104%.

Using an ambient of 15c will calculate 112%.
That would allow 350kcmil

Engineers aren't limited to 2008 Table 310.16 ampacities if they use the Neher-McGrath method of 310.15(C)...

Only need 107.4% ambient correction to allow 350's.

300/350/.8=1.074

 

[hardworkingstiff]

Any suggestions?

If you have the space available, you could donate the existing conduits to the PoCo and have them install their transformer closer to the building. Or, get the PoCo to change the delivery voltage to 480Y/277 volt (I'm assuming here you have a 208Y/120 source existing) and you install transformers and disconnects at the building then re-feed the existing equipment. You could use the existing wiring possibly. You would not have to cut/patch the parking lot.

Just a different approach.

 

[George Stolz]

Actually, six 350's as I posted earlier would do without engineering supervision if under NEC 2011 edition. The correction factors have been increased under 2011 (based on 40?C rather than 30?C), and lower temperatures have been added. Under 2008 and earlier, 1.04 per Table 310.16 is the best ambient correction available without engineering supervision.

While I agree the table has been expanded, it is still based on 30C. 310.15(B)(16) is still based on 30C, as is it's companion T310.15(B)(2)(a). None of the suggestions I've seen so far require engineering supervision, they've all been playing with 310.15(B) that I've noticed. I figured 59F might be an optimistic ambient, so I based my claim on the 400s.

 

[Smart $]

While I agree the table has been expanded, it is still based on 30C. 310.15(B)(16) is still based on 30C, as is it's companion T310.15(B)(2)(a).

2011 Table 310.15(B)(2)(b) is based on a 40?C ambient. But its really a moot point, so I'll not go there.

Yes, Table 310.15(B)(16), formerly Table 310.16, is still based on an ambient of 30?C as is Table 310.15(B)(2)(a). The expanded ambient table permits using lower than 21?C (70?F) ambient temperatures, which were not available under Table 310.16 of NEC version 2008 and earlier.

None of the suggestions I've seen so far require engineering supervision, they've all been playing with 310.15(B) that I've noticed. I figured 59F might be an optimistic ambient, so I based my claim on the 400s.

So my suggestion don't count???

My first post was on using 350's, and would require 310.15(C) engineering supervision to be compliant under 2008 and earlier editions of the NEC. Under 2011 edition, engineering supervision is not required for compliance because of the noted 310.15(B) expansion.

350's at 59?F ambient and derated 80% for 6 ccc's are good for 313A under 2011 (75?C termination temperature limitation would reduce that 313A to 300A). 300 ? 4/? = 1200A/?

 

[George Stolz]

I was too lazy to look back, I was going from memory of what I had skimmed. That works too.

 

[zbang]

Install another raceway, use directional boring to minimize how much parking lot needs demolished if that is what it takes.

Assuming paralleled conductors, how do you match the lengths? It's certainly possible, but the 3rd pipe will probably have a different length, so you could end up with 20-30' of extra coiled up in a (large) box at one end.

 

[kwired]

Assuming paralleled conductors, how do you match the lengths? It's certainly possible, but the 3rd pipe will probably have a different length, so you could end up with 20-30' of extra coiled up in a (large) box at one end.

Depending on the installation not all 3 pipes are always same length anyway. Why is it going to be 20-30 feet difference?

 

[zbang]

Depending on the installation not all 3 pipes are always same length anyway. Why is it going to be 20-30 feet difference?

I have no idea. Assuming that the initial installation is compliant, then the conductors in the existing pipes are probably matched (310.4(B)). When a third pipe is added, likely as not it will be a different length which will require matching the new conductor's length to the old (as well as the type, etc). Sure it's possible, but not a slam-dunk. If the new is shorter than the existing, you can just coil up the excess at one. OTOH if the new pipe is longer than the existing, there may be a problem. All I'm sayin' is that it might not be as simple as just pushing in another conduit. (I could be wrong, too. It happens a lot.)

If 240.40 exception 2 applies, then they don't have to be connected at the load end and the parallel matching goes away.

 

[kwired]

I have no idea. Assuming that the initial installation is compliant, then the conductors in the existing pipes are probably matched (310.4(B)). When a third pipe is added, likely as not it will be a different length which will require matching the new conductor's length to the old (as well as the type, etc). Sure it's possible, but not a slam-dunk. If the new is shorter than the existing, you can just coil up the excess at one. OTOH if the new pipe is longer than the existing, there may be a problem. All I'm sayin' is that it might not be as simple as just pushing in another conduit. (I could be wrong, too. It happens a lot.)

If 240.40 exception 2 applies, then they don't have to be connected at the load end and the parallel matching goes away.

All the other options so far involve removing existing conductors and finding ways to either replace or reinstall them. If they are removed they can be measured.

Might be good idea to remove them if horizontal boring anyway just in case there would be a mishap with installation of additional raceway - that way conductor is not damaged.

If old conductors were not going to be reused then there is no problem at all.