1,000' pull in 3" Sched 40 pvc

Status
Not open for further replies.
ivsenroute

ivsenroute

Senior Member
Location
Florida
Just got some preliminary information that tomorrow I am inspecting 2 separate 1,000 foot runs of the above for a 12.5kvac feed to a 150kVa xfmr.

Anyway they are down approximately 48" in a trench and have the 4" standoffs approximately 10' apart. The 3" is suppose to get concrete encasement of a 4" minimum top and bottom per specs.

I have 2 concerns, questions:

1) Maximum space between supports is really a moot point since it is buried raceway and will be concrete encased, HOWEVER, with the standoffs at 10' apart I am concerned that the middles with dip down below the specified encasement but I don't see how I can make the horizonal supports apply. Again, just a concern. How do you set up these runs for encasement?


2) There is no exclusion for 352.44 as far as expansion fitting are concerned but what good will they do once the concrete is encased?


The fact that there are 1,000 feet between pulls in a straight run concerns me with the expansion fittings, even at the minimum 5 degree change in temp.

The 2005 NEC applies
 
1) You could go with the 360 degree rule, dips every 10' for 1000' of pipe is a lot of degrees.

2) Expansion fittings underground.....seriously???
 
Last edited:
Well its fine but we always install at 8 foot centers in ground and there will be no dipping in the conduit if they pour the mud from the sides of the trench meaning not directly on top of the conduit .

We always use EB duct on high voltage never sch 40 but you can if you want.

One thing when your pouring concrete over a duct bank you have better tie wirer and stake it down tight meaning hammer in some rebar every 20 foot because if they dont do it when you pour the mud the conduit will float up out of the trench higher .

Just one more thing before you pour a duct bank heres a trick on duct banks to help lets say you had 50 x 4 inch conduits open up one side meaning the taped ends it lets the air out and the day before the pour fill the top runs with water and i mean lots of water this will get the air out and the water will hold it down .


I kinda do this everyday so trust me on this one .

By the way no one ever installs expansion fittings even at 1800 feet what temp its underground
 
Last edited:
1) In my experience, pvc conduits want to float while being encased, so not only do they have to be stood off the trench bottom, but they have to be "nailed" down in some fashion to keep them from floating to the top. Suffice it to say the dips between 10' supports will be countered by the conduits bouyancy during encasement.

2) Thermal expansion and contraction of PVC conduits is typically within the expansion/contraction range of concrete. Wherever the encasement is monolithic, there is no need for expansion joints.
 
Last edited:
ivsenroute said:
1) Maximum space between supports is really a moot point since it is buried raceway and will be concrete encased, HOWEVER, with the standoffs at 10' apart I am concerned that the middles with dip down below the specified encasement but I don't see how I can make the horizonal supports apply.
Click to expand...
I would think floating would be more of a concern than dipping. Do the supports support (sorry) tying down?

You may need to halve your spacing to porevent movement either up or down.
 
ohmhead said:
Well heres one of our duct banks http://i611.photobucket.com/albums/tt195/stringking/image0-1.jpg

Tie downs lots of them this is before the new digital cameras of today sorry for the picture . but notice the typical rebar tie downs .
Click to expand...


Do they install all of the conduits to provide structure so no rebar is necessary? :D

Sorry, I could not resist.

I was involved in very similar installations. I was the guy in the trench, I went home high every day. :cool:

We used the PVC 'chairs' and made possitively sure of the tie downs. Some of our jobs were very long runs, with multiple cement trucks lined up...they waited for no one.
 
Well Pierre the fun is after the pour when we blast out the water in the top rows with 125 lbs air compressor and watch the 30 foot of water shoot out of the top of the conduits onto the other trades .


And yes i think the glue gets pretty heavy in the trench but more so when we measure and install jet lines .

And when were in a manhole sucking in lines the sniffer goes off every 10 minutes from the pvc glue fumes in the taped conduits .:D

We use a concrete pump truck that duct bank was lots of mud .

Take care
 
Last edited:
ohmhead said:
Well Pierre the fun is after the pour when we blast out the water in the top rows with 125 lbs air compressor and watch the 30 foot of water shoot out of the top of the conduits onto the other trades .


And yes i think the glue gets pretty heavy in the trench but more so when we measure and install jet lines .

And when were in a manhole sucking in lines the sniffer goes off every 10 minutes from the pvc glue fumes in the taped conduits .:D

We use a concrete pump truck that duct bank was lots of mud .

Take care
Click to expand...

Just remember paybacks are ---
 
Years ago, during one of the coldest winters I can remember, we were working on a very long run from the street to the back end of the complex being built.
The Rigid Metal Conduit had been installed during the spring before. We were trying to get a string blown in, but the conduit had an ice block in it. The GC brought in his truck sized compressor. I was in the room where the conduit entered the building. We had walkie-talkies (no cell phones back then, just pagers). I was to tell them when it blew clear.
Well...I heard the plug break clear, but did not realize until too late to get out of the room. I got completely cover in ice and water. By the time I walked out to the street where the compressor truck was, I was literally frozen solid and was walking like a robot.
The only good part was, they let me go home early that day. :D
 
with 10-5" PVC in a duct bank, we had frames every five feet--over 3 miles all told. lots of frames... the floating is the issue with larger size PVC. We had no expansion joints in the whole project, but manholes every 500'.

BTW, thanks for the entertaining mental picture, pierre. Could that have been one of those winters in the 70's? I think I speak for the rest of the forum when I say that we're generally glad that you didn't get hit in the head with a solid chunk of ice. Coulda been worse. :)
 
Last edited:
Status
Not open for further replies.
Top