Conduit bends 4

[a10jp]

I understand that in most common pratcice, you should not run more than two 90deg conduit bends without a pull box. Is there a relationship as well with the conduit size? It seems obviously if you run a few #10 cables in a 4-in conduit, it seems perfectly doable to have more bends than 2-90deg. Any comments?

 

[davidbeach]

There are pulling calculation programs. Number of bends is really a bad metric of pull difficulty. Two bend can be too many for some runs, while four bends can be pulled no sweat on other runs. Depends on radius of bends, size of conduit, what is being pulled in, and direction of pull.

 

[REDDOG]

NEC permits 360 degrees of bend between pull boxes

 

[a10jp]

When I did design sometime ago, I show the pull box at required locations such as for change of direction and for access purpose. Now here I am at the field, I realised the electrican kinda knows how many pull box he needs, and the condition may not be straightforward like in the drawing. There are numerous obstacles and beams in the crawlspace, for example, the conduit has to cross and no, they do not follow the (2) 90-deg bend restriction. But the cable pulls thru with no problem.

 

[waross]

The code restriction is 4 90 deg. bends not 2, and offsets should be counted as part of the total degrees.
yours

 

[a10jp]

Can you substitute pullbox with screw-in type coupling? These screw-in couplings has 8 screws to tighten the joints, which you can unscrew and hammer out the section and access the cables inside. In a way, it is accessible, so can you sue that in place of a pull box to satisfy the code?

 

[nightfox1925]

I agree with Waross, the offsets are to be counted as part of the total degrees whether they are 45 or 90 degrees. These are code requirements and should be strictly implemented.

In addition, you may have to consider also drainage (using universal drain/breather) requirements at the lowest points of the conduit runs and bends as well as expansion couplings at long straight conduit runs outdoors.

GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata

 

[waross]

Hello 02101972;
I am not much in favour of expansion joints. I avoid them whenever possible. I can almost always use a 90 deg. bend or even a pair of 90's in a long straight run for expansion.
I have often wondered why the code addresses expansion of the conduit with no thought to the expansion of the cables in the conduit.
An expansion joint tends to concentrate the relative movement between the cables and the conduit at one point. I avoid that when I can.
respectfully

 

[BJC]

a10jp
The Steel Tube Institute has a book on conduit that may be helpfull. Google "teel Tube Institute"
Okonite has the basic pull calc on their website.
I use to have an Excel sheet that I got from a local Okonte rep. It was good but it was slanted towards their cable.

As for a proceedure I have used criteria such as "all pulls longer that 200' ", all pulls with more than 270 degrees of bend" , "all medium voltage pulls" require engineering review and may need a calculation.
IF your doing a lot of calcuations I recommend Polywaters Pullplanner program. Cheap and good.
One caution on pull points and J-boxes. I have seen runs that had 270-360 degrees of bend and then a mogul "C" condulet as a pull point followed by more bends. The condult meets the criteria for a pull point but when the run was pulled the wire went straight through the condulet. It was in effect useless for pulling but did allow the conduit run to meet the criteria for bends. Electricans don't like to pull cable out of a run, it's hard to do and messy with all that pull lubricant on it. With THHN or other building wire I don't say much, but with expensive or hard to replace wire you have to insist.

Another thing you see from time to time is a perfectly designed duct bank with all long radius bend etc. Then a contractor pulls it with a 6" diameter shieve on the "to" end. Somewhere they buy a 6" shieve with an extension that will fit in a 4-6" duct. The smallest diameter radius at the point of highest tension.
If your writing a proceedure be sure to address radius in conduit runs and in temporary rigging devices (shieves)

 

[bimr]

The reason that NEC requires a maximum of 360 degrees of bend between pull boxes and calls out a maximum number of number of wires in the conduit is that you will be able to pull the wires through without damaging the wires.

Nothing more and nothing less.

Check the The Steel Tube Institute handbook as BJC (Electrical)suggested.

 

[davidbeach]

bimr, that is absolutely not true. It is very easy to set up a pull that meets all NEC requirements but cannot be pulled without damage to the conductors. It is also very easy to set up a pull where 720 degrees does not produce excessive tension or side wall pressure. The four bend rule is a cop out to allow setting a standard that does not require calculation. The NESC, on the other hand, is much more practical:

341.A.2 Pulling tensions and sidewall pressures on the supply cable should be limited to avoid damage. Note; Manufacturers' recommendations may be used as a guide.

 

[bigbillnky]

a10jp

One other point to consider when planning/calculating conduit and cable pulls is that certain size cables in certain size conduits can allow single strand conductors to wedge tight together. This normally happens at 90* bends. #4 THHN in 1 1/2" conduit is an example.

Bigbillnky,C.E.F.....(Chief Electrical Flunky)

 

[a10jp]

Thank you all for the excellent advice.

To BJC, thanks for the "rule of thumbs" and special pulling considerations.

And to bigbillnky, I did not know you can allow single strand conductor to wedge together. In fact I orginally thought wedging conductors at bends can create problems. But I will keep that in mind.