Conduit Bank

[hanksmith]

Currently working on the design of a concrete encased electrical conduit bank, have to install 600V feeds and about 1200A, currently looking at3/phase of 750kcmil.

My question comes with the conduit bank and how many bends I can have before it starts getting hard to pull the cables through, the run I have is only about 40m or 130ft. Right now I am thinking I will have a 90 at the start of the run and a 90 at the end, will the installing contractor be able to pull this cable?

Does anyone have any links to literature, I might need something to justify the need of a pull pit?

Thanks
Hank

 

[dpc]

http://ecmweb.com/mag/electric_simple_calculations_cable/

This might get you started.

 

[BJC]

This is worth the $99.

http://www.polywater.com/pullplan.html

 

[davidbeach]

Pull calc software as suggested, and there are others, and look at all runs from both ends. There will be runs that can be pulled in only one direction.

 

[dpc]

And some that can't be pulled from either end.

 

[davidbeach]

True enough. And if you find a point that can only be pulled into, but not out of you need to figure on splicing the conductors there.

The NEC limit of 360[°] is also high arbitrary; there can be pulls where one 90 is enough to make the pull impossible while in other cases a pull through 720[°] would work just fine.

 

[eststation]

I dont like bends at all. If you think about it the bends take time and or money to make! So do what it takes for labor cost and job cost to stay as low as possible. God I would hate to do work behind davidbeach with his 720degree bends. Have you ever heard of a pull box or a junction box? Also invest in grc LONG sweep 90's. Its less of a quick turn for the wire which makes everyone happy!

 

[davidbeach]

The 720[°] would have been in two consecutive hair pin turns plus more twists in a roadway. The whole thing would have been done with straight lengths of PVC, with a turn radius of 200 feet or more, all on a 4% slope. Calc'ed out as an easy pull downhill and an impossible pull up hill. Contractor decided to put in an extra pull vault to avoid that much curvature; lucky for him there was actually someplace to put the vault, a marginal location at best and one of the very few possible vault locations I didn't use. A few vaults were forced into the roadway when there were no road side locations for 1000's of feet. Tried to limit all the pulls to 2500' or less, wound up with one of about 2700'. When it finally came time to pull in the conductors, one #1/0 25kV per 2" conduit, no pull went badly and the longer pulls generally went easier than the shorter pulls (there was a reason why they were shorter).

On the other hand, I designed a rather nasty run in a hospital remodel with pull points as often as possible and then the contractor proceeded to pull the whole 7 x 90[°] run as a single pull figuring there would be more risk to the cable from piling it up on the floor at each pull point than pulling it all the way in one pull.

 

[resqcapt19]

When you do the pull calculations make sure that you look at both the sidewall pressure and the pulling tension. The sidewall pressure is most often the limiting factor, but it can be reduced with the use of long radius bends.

 

[waross]

Often changing the ratio between duct diameter and cable diameter is more effective in changing sidewall pressure than changing the bend radius.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[resqcapt19]

Bill,
I don't have my Southwire Power Cable manual (the only document that I have that shows the pulling calculations) here at home, but I don't recall the ratio between conduit and cable diameters having a big impact on the side wall pressure. As I recall it is just the pulling tension coming out of the bend divided by the radius of the bend in feet. What is the formula to take the diameter ratio into account?
Don

 

[resqcapt19]

Bill,
I had some time and found the pulling calcs in the Southwire book. The ratio tells you what configuration the wire will take in the conduit when it is pulled...either cradled or triplexed. If it is cradled the side wall pressure will be reduced as compared to triplexed. I also found that the side wall pressure calc is a lot more involved than just the pulling tension divided by the radius in feet when there is more than on conductor.
Don

 

[waross]

Hi;
I don't have the calculations handy, but the point that I was trying to make is that there is a range of diameter ratios that tend to jam badly in bends.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

Interestingly, the worst fill for jamming is very close to the 40% maximum fill allowed by the NEC. In applications where the NEC does not apply it can be easier to pull a run that is filled beyond 40% than it is to pull one slightly under 40%.

 

[itsmoked]

davidbeach!!!! You know that's what I've seen! Amounts that seem rational don't make it and ones that seem impossible do. I'd just never made that (il)logical leap.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[davidbeach]

Find a copy of Okonite's guide. You'll have all the esoteric pulling information you could ever want.