Distance, Pullboxes, conduit runs

[Orerofsirhc]

In General, what is the recomended distance I should place pullboxes in conduit runs?

 

[bphgravity]

This issue is completely dependent on all of the conditions of the installation. Good judgment and common sense should prevail...

 

[kingpb]

In General, what is the recomended distance I should place pullboxes in conduit runs?

As often as your cable pulling tension calaculations tell you, so that you will not damage the cable during installation.

 

[hockeyoligist2]

You also have to consider how many bends there are in the conduit.

 

[iwire]

As often as your cable pulling tension calaculations tell you, so that you will not damage the cable during installation.

Is it your opinion that an experienced electrician needs to perform pulling calculations for every wire pull?

If not what is your cut off point?

Only small percentage of the pulls electricians do approach the limits. When I am on a job where pulling tension may be an issue there is usually a very helpful engineer asking questions.

A job I did last spring I requested that I could drop from 4" raceway to 3" raceway, the engineer said sure when you show me how it will effect pulling tensions and prove the fill will be allowable.

Both questions where easily answerd...raceway diameter does not effect the pulling tension assuming the fill is still within NEC limits.

In General, what is the recommended distance I should place pullboxes in conduit runs?

I agree with Byran there is a certain amount of experience that needs to be applied, sometimes experience will tell you it will be a breeze, another time experience may tell you that some calculations need to be done.

In this thread we have no idea what type and size of conductor is being pulled in the raceways so it's imposable to give a real answer.

I'll take a guess that it's 10 to 14 AWG THHN.

 

[mdshunk]

You know that your pulling tension was too high when your rope breaks or your grip pulls off. No, seriously, didn't very old code revisions (60's?) have a maximum distance between pull points? Three-hundred feet sticks in my head for some reason.

 

[infinity]

As Bob said it's almost impossible to answer the question with any type of "rule of thumb". One thing that I would mention is that on vertical straight runs with larger conductors pull boxes may be need a close as every 40 feet for cable support.

 

[hardworkingstiff]

I used to have a link to a pull tension calculator, but it's gone now (the site, not the link). Anyone still have a link they can share?

 

[iwire]

Anyone still have a link they can share?

http://www.electrician2.com/electa1/ductwirepull.html

 

[hardworkingstiff]

Thanks (again) Bob.

 

[iwire]

No problem.

I think it is a good site, glad to share it.

 

[kingpb]

Bob,

I have heard, no proof though, that if you are pulling it by hand, their is no need to do a pulling tension calc, because a normal human (neanderthals aside) you couldn't pull hard enough to exceed the pulling tension limit of the cables.

Now on the other hand if your using the bumper of your truck or the bobcat, which I'm sure none of you have ever done, then you could damage the cable.

Bends are actually where more people get into trouble because of jam and sidewall pressure.

In my opinion, ALL medium voltage cable runs should have pulling tension calcs done, and any other pulls that require non-human assitance.