guidance in the placement of electrical conduit pull boxes

[sjgordon]

I am a civil engineer working in land development (large retail commercial site planning). As with most civil's my electrical knowledge is enough to make me dangerous. I rely on my colleagues experienced in electrical engineering for advice. On my current project I coordinated with our local electrical monopoly to run power from switch cabinet to transformer for the building. They could not give me too much reasoning as to the layout of the route. They only mentioned they had used a computer program to determine the location and number of pull boxes. This to me is an unfortunate trend I have seen lately, someone using software that the don't understand (VERY VERY SCARY). Anyways, that is an entirely different issue.

My question is some design guidance as to the placement of pull-boxes. The power must be run through the parking area, so all the pull boxes must be traffic rated, which are considerably expensive. I understand that there are many issues that must be considered when running the route avoiding other underground utilities, landscape islands, etc.

In order to come up with a design that is cost effective and not going to drive the electrician crazy, could someone please advise me where to look to find basic design criteria.

Much Thanks

 

[dpc]

From a EE perspective, the issue may be the pulling tension when installing the cable. If this is a long run or a route with several turns, they probably calculated a pulling tension that the cable would see and compared this with the maximum allowable pulling tension and sidewall pressure that the cable is rated to withstand. Pullboxes are put in to allow the cable to be pulled in incrementally rather than having to pull through the entire route.

This can be fairly easily calculated by hand, but there several software programs that do this is as well. There should be quite a bit of flexibility in the location of the pullboxes, although probably not the quantity of pullboxes.

If you go to the Okonite website (

http://www.okonite.com),

you should be able to find basic engineering info about cable installation.

This info from Polywater may also be of interest

http://www.polywater.com/2000tech.html

 

[rbulsara]

Is this for high voltage cables?

It appears so, and if so most common rules of thumb are:

Caution: This all from top of my head..

No more than two 90 degree bends or equal between two pull boxes (handholes or manholes). Three would be stretching it, although may be permissible specially if the conduits are oversized.

For MV cables you can generally have 600 to 800 feet of a continuous run without splices (between pull boxes). I beleive it is difficult to get greater than 1000' of MV cable in one piece.

So try avoiding bends, shoot for straight lines. In many cases gentle bends (long radious) or two 45 degree rather than one 90 and such will help.

If you have access talk to a good electrical contractor.

 

[EEJaime]

The above guidelines listed by rbulsara are good. Oftentimes, when designing a campus type facility, hospital, high school, community college, etc..., it is important to note what the overall master plan is. Pullboxes are not only useful in allowing the pulling of cables without adverse impacts to the insulation, but are also access points for future extensions, additions, etc.... For medium voltage systems, (less than 25kV), it is a good rule of thumb to go 600-800 feet, with added boxes for multiple bends, tees, etc.... Where there is low voltage distribution, 600V and below, this is pushing it since there are probably multiple sets of large cables such as 500kcmil, etc.... In that case we like to keep the pulls to a maximum of 500-550 feet. Additionally, some of our clients' IT infrastructure people demand a pullbox for copper and fiber telecom cable at 250-300 feet max. So there are many factors going into the design. I would suggest getting someone with specific knowledge to lay this out for you.

I have recently completed a project with 5kV emergency power loops, 12kV normal power distribution loops and telecommunications distribution which consisted of up to 60 4" conduit in concrete encased ductbanks and low voltage systems, (security, energy management, ATS controls, CCTV, Fire Alarm, MATV, etc...), for a new hospital project. This is major construction and proper planning, placement and supervision is critical.

I performed a review on a claim for a project where a pullbox was deleted in the field and we had a 12.47kV feeder fail where a fourth 90 degree bend was added without approval. It gets very costly and can be dangerous.

I wish you much luck with your project, but would sincerely suggest you obtain someone with experience in this area.

 

[rbulsara]

EEjamie said it better...

 

[BJC]

You get in the wrong midset when you think "Pull Boxes" for 12 KV systems.
Think vaults and keep swimming pools in mind not boxes. Start out with duct runs between vaults that you could see through then raise them up in the middle for drainage. Put as few bends as possibel in the system, keep them long radisu and near the "from" end of a pull.
A pull tension calculation program can help you in the design. Polywaters "Pull Planner" software is cheap and good. They may even give it to you if your going to buy lots of their product.
For a large campus I would use two seperate system laid out in "trains". This may sound like a nuke thing and it is but the fire at Bellagios in Las Vegas is a case of what can happen if you put all your electrical system in one vault.

http://www.reviewjournal.com/lvrj_home/2004/Apr-15-Thu-2004/news/23660454.html