Spacing Between Manholes
Spacing Between Manholes
(OP)
Can any one tell me if there is a industry standard for spacing between manholes connected together with duct banks? Or do we need to calculate the required cable pull tension and side wall pressure for every installation if we do not exceed 4x90=360 degrees of total bend between the manholes? Any info is appreciated.






RE: Spacing Between Manholes
RE: Spacing Between Manholes
RE: Spacing Between Manholes
Very good points. You deserve a star. I do need a little more clarification. In many specifications I have seen that the pull tension and side pressure calculation is not required when the spacing between the manholes is limited to 400 feet when the duct run between the manholes is considered to be straight provided that there is no more than 2x90 degrees of bend where the cables are limited to 350 MCM/15 KV. What is your opinion of that? Do you belive if the cable size was 500 MCM / 5 KV it would more or less be acceptable without any calculation (due to thiner insulation)? Under what conditions would you say we should perform the pull tension and side pressure calculation? Any suggestions and/or referances are appricated. Thanks
RE: Spacing Between Manholes
The big factor in sidewall pressure is the radius of the bends not the number of bends.
Calculating pull tension is not that difficult.
Rome Cable has the formulas on their site
http://www.romecable.com/Catalog.htm
or you can get them from Okonite or other cable manufacurer. They fit into Excel easily.
Here's a free one that works well - you have to put the links together yourself.
http://www.electrician.com/electa1/ductwirepull.html
If you pull throught a vault the radius of the sheave you use in the vault will exert a sidewall pressure on the cable. It will not add tension.
If this is an important project I would buy the program PullPlanner from polywater.
http://www.polywater.com/pullplan.html
It easy to use and documents the results. If you have an owner or QC department that wants documentation it's a good tool.
At $99 it cost less than 20' of cable. Polywater may even give you a copy if you buy enough of their cable lubricant.
RE: Spacing Between Manholes
Okonite used to have a good book on cable installation practices that explains the required calculations. I don't see it on their web site. You might ask a sales rep.
See also http://www.okonite.com/engineering/bending-ratios.html
RE: Spacing Between Manholes
I would just go ahead and make the calculations. They are not difficult and if you would like there is inexpensive software available as noted in other posts. I'm not sure what you mean by "...considered to be straight provided that there is no more than 2x90 degrees of bend..." If the duct is straight, how can there be 2x90 degrees of bend. The location of the bend makes a difference also. As noted by alehman, the tension (and thus the sidewall pressure) is different depending on the direction of pull, mainly because of bend location. The 360° of bend limitation is an NEC requirement to limit pulling tensions but should not be a substitute for calculations in long ductbank runs or heavy cable.
RE: Spacing Between Manholes
I did earthing measurement, fall of potential measurements on 6 substation in my area and I got values less than 1 eg R1=0.89,R2=0.90,R3=0.82 & R4=2.27-one Substation,so my problem is how do I conclude with this,does it mean that earth resistsnce is Ok.I did most of the measurements arround April and May.Do I have to worry about the outcomes.
RE: Spacing Between Manholes
RE: Spacing Between Manholes
http://www.kerite.com/catalog/catalogfiles/pulling_tensions_power.htm
for more information and sample calculations
RE: Spacing Between Manholes
RE: Spacing Between Manholes
Another thing to consider is that cable comes on reels and is limited in length. From what i remember reels come in 500'-0" lengths. So in addition to the calculations i space manholes aprroximately every 400'-0" before i need a splice.
RE: Spacing Between Manholes
During a visit to a refinery under construction, the site electrical engineer boasted about how his cerew were able to "pull" the cable a distance of 600 meters... in one piece. This discussion took place as we "walked" the route of the installation.
About midway there was a manhole, and I noticed a splice. I asked "If you pulled the cable in one length, then why is there a splice in it?" He said "Well the plans called for a splice, so..."!
RE: Spacing Between Manholes
http://www.usace.army.mil/publications/armytm/tm5-811-1/
Go to chapter 7 and there it is.
You probably thought all that money that went to the pentagon was wasted, but if go to this site you see all kinds of army manuals on construction listed. Some of which are pretty good.
You can download manual on everything from "Childrens Outdoor Play Areas" to " General Design Criteria to Facillitate the Decommissioning of Nuclear Facillities"