PVC run underground - multi question ...

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spizeii

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Hello all,

I need to run approx. 2000' of Sch 80 PVC in a 36" deep trench - 3 parallel 4" pipes (I realize I can use smaller pipe but prefer the larger) - containing 3 sets of 350 kcmil THHN (1 set - 4 wires/pipe) 3ph/208/1200A.

Question #1: Not 100% sure if it is code, but I always practice putting a pull box or conduit body every 100' when I run pipe in a building or tunnel or similar. Is it a requirement to have a underground p.b./'manhole' every 100' or is it just for pulling purposes when using this practice and do I need p.b.s every 100' for this run?

Question #2: How much Voltage Drop can I expect for running this long distance? Do I need a boost XFRM in the run to get my Voltage up at the end?

Thanks!
 
Re: PVC run underground - multi question ...

1. Pull-boxes are not required by the code.


2. I estimate between 30 and 50 volts dropped at a load of 1200A. Is that your actual load? Are 3-350 kcmil conductors good for 1200A?
 
Re: PVC run underground - multi question ...

Actual load in the neighborhood of 700-800A.
My mistake - going to use 1000A of the 1200A main service - probably coming off a 1000A C.B. to feed 'sub service' at other end of site.
 
Re: PVC run underground - multi question ...

Make sure you use THWN or RHW.

I would be thinking more like 500' between pull points.

Be sure to use steel 90 sweeps, large radius ones would be a good idea.

If you do not normally do this kind of pull you might consider subbing it out to someone with the equipment to do the job.
 
Re: PVC run underground - multi question ...

Steve, there would be no reason to concrete these 90's, this is not the same as high pressure water lines.

Spizeii, out of curiosity, what is the reason for Schd 80 below ground?

Roger
 
Re: PVC run underground - multi question ...

I have that 2000' or so to go underground for there will be yard traffic and the pipe must be burried for that reason.
 
Re: PVC run underground - multi question ...

For what it is worth, we use grade DB in a 36" deep trench (in other words, about 30" of cover). Grade DB is a utility grade that has a thinner wall than schedule 40. We use it under streets, alleys, parking lots, etc. for protection of our primary cables.

What I am saying is that schedule 40 should do fine at 36" of cover. BTW grade DB is not permitted by the NEC. :D
 
Re: PVC run underground - multi question ...

Spizeii, my question is why the Schd 80, as Charlie says in his post above, Schd 40 would suffice. (But stay with the metalic 90's as Bob said)

BTW, Why 36"?

Roger
 
Re: PVC run underground - multi question ...

You don't have to worry about bonding the metal sweeps correct?
 
Re: PVC run underground - multi question ...

metallic elbows do not have to be bonded if isolated by 18" of earth, 2005 NEC section 250.80. For a 36" radius sweep, thats a 4.5 ft deep trench.
 
Re: PVC run underground - multi question ...

I was under the assumption that electric had to be run in SCH 80 by code and Sch 40 could not be used.

Just curious; why would I run DB pipe if "BTW grade DB is not permitted by the NEC"?? I always do everything by code and that is the reason for my questions.

Also, the 36" is being used because the loader usually digs that deep anyway (that's the bucket used). I realize that my min. is 18" but I do see in table 300.5 that under streets, highways, driveways, roads, alleys & parking lots it's 24"??

Last, I always use galvanized elbows when coming out of the ground and run galv. from there on. I don't usually encase it in concrete unless it's in a high traffic area or somewhere the loader can hit it.
 
Re: PVC run underground - multi question ...

By spizeii: Do I need a boost XFRM in the run to get my Voltage up at the end?
Click to expand...
The only time this would work is when the load current will never vary as in a single load that stays the same. as if the load changes the voltage will change and can rise and damage equipment. the only other way to over come voltage drop other than over sizing the conductors is to step up the voltage at the beginning of the circuit and then step it back down at the end of the circuit.

Sch. 80 is only required where the conduit is subject to physical damage in normal use. And I wouldn't think digging under ground would be normal unless it is in the middle of a farm field. :eek:
 
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