Calculate: 800A, 120/240V feeders, 300 ft

[bcm]

Folks,

Somehow, the contractor on my client's Florida project buried (2) 4" PVC conduits without consulting anybody about the conductors required to feed 800A at 120/240V across 300 feet from a manhole to a gang meter center. Now the client is coming to me asking if they can even run 2 sets of conductors together in each 4" conduit and if it requires innerduct, etc.

From table 310.16, I determined that 800A will mean 4 sets of #3/0. If I run a voltage drop calc over 300' takes me to 4 sets of (3) 300 MCM, which I'd normally run 1 set in each of (4) 2.5" conduits.

However, with 3 conductors per set and only 2 conduits, that gives me 6 conductors per conduit. I know derating will be involved which leads me to NEC Table 310.15(B)(2)(a). That means 80% as the adjustment factor. Should I be applying the adjustment factor before or after I do voltage drop calculations? I haven't done something quite like this before, so I'm trying to figure out how I calculate the derating as well. I think the 80% will take me to 640 amps, but I'm not sure how to get that back up to 800A? Can somebody help clue me in?

Thanks!

 

[roger]

Have them run a set of 600KCMIL or 700KCMIL in each conduit

Roger

 

[petersonra]

Folks,

Somehow, the contractor on my client's Florida project buried (2) 4" PVC conduits without consulting anybody about the conductors required to feed 800A at 120/240V across 300 feet from a manhole to a gang meter center. Now the client is coming to me asking if they can even run 2 sets of conductors together in each 4" conduit and if it requires innerduct, etc.

From table 310.16, I determined that 800A will mean 4 sets of #3/0. If I run a voltage drop calc over 300' takes me to 4 sets of (3) 300 MCM, which I'd normally run 1 set in each of (4) 2.5" conduits.

However, with 3 conductors per set and only 2 conduits, that gives me 6 conductors per conduit. I know derating will be involved which leads me to NEC Table 310.15(B)(2)(a). That means 80% as the adjustment factor. Should I be applying the adjustment factor before or after I do voltage drop calculations? I haven't done something quite like this before, so I'm trying to figure out how I calculate the derating as well. I think the 80% will take me to 640 amps, but I'm not sure how to get that back up to 800A? Can somebody help clue me in?

Thanks!

VD is a design issue.

The conductors have to be minimally sized to the calculated load. The calculations almost always significantly overstate what the real load will be so VD is often not an issue.

Most times the 80% adjustment factor does not matter much as you can start with the 90 degC column for that purpose. As this is UG you would need to use THWN-2 or some other conductor listed for getting wet and at 90 Deg C as the conduit will inevitably fill up with water. I think all THWN of this size is typically THWN-2 anyway, but you never know.

IF VD worries you, make the 3 (or 6) CCC in each pipe as large as you can and still fit them in. Don't forget you need a 4th wire in there - the EGC. An 800A OCPD needs at least a 1/0 copper EGC. If you make the CCC larger to account for VD, you will need to make the EGC bigger as well.

 

[bcm]

Have them run a set of 600KCMIL or 700KCMIL in each conduit

Roger

Heh... I got so caught up in derating and voltage drop that I forgot I can use bigger conductors right out of the gate! Since I only would put 1 set in each conduit, that takes away the derating issue. I assume I need to recalculate voltage drop though, right?

Thanks!

 

[bcm]

VD is a design issue.

The conductors have to be minimally sized to the calculated load. The calculations almost always significantly overstate what the real load will be so VD is often not an issue.

Most times the 80% adjustment factor does not matter much as you can start with the 90 degC column for that purpose. As this is UG you would need to use THWN-2 or some other conductor listed for getting wet and at 90 Deg C as the conduit will inevitably fill up with water. I think all THWN of this size is typically THWN-2 anyway, but you never know.

IF VD worries you, make the 3 (or 6) CCC in each pipe as large as you can and still fit them in. Don't forget you need a 4th wire in there - the EGC. An 800A OCPD needs at least a 1/0 copper EGC. If you make the CCC larger to account for VD, you will need to make the EGC bigger as well.

If I use 90 deg C, THWN-2 and need 800A using just 2 sets of conductors in the 4" conduits, that would mean I use #4/0 if I am taking into consideration what you mention about calcs significantly overstating what the real load is and VD not being an issue. I can fit plenty of THWN-2 #4/0 into a 4" conduit, so that would make pulling over the 300' easier (with handholes as required, etc.). When you say EGC, you mean "equipment grounding conductor"? I didn't think I needed that since this is from a utility service point (manhole) to a gang meter bank. Can you expound on needing an EGC?

Thanks for your input!

 

[petersonra]

If I use 90 deg C, THWN-2 and need 800A using just 2 sets of conductors in the 4" conduits, that would mean I use #4/0 if I am taking into consideration what you mention about calcs significantly overstating what the real load is and VD not being an issue. I can fit plenty of THWN-2 #4/0 into a 4" conduit, so that would make pulling over the 300' easier (with handholes as required, etc.). When you say EGC, you mean "equipment grounding conductor"? I didn't think I needed that since this is from a utility service point (manhole) to a gang meter bank. Can you expound on needing an EGC?

Thanks for your input!

I did not realize it was a service. I think the OP said something about a feed that I interpreted to mean this was a feeder.