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.
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Spacing Between Manholes 2
- Thread starter eghtesady
- Start date
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2
- #2
There is no standard. Spacing depends on pulling tension and sidewall pressure as you have indicated. Maintaining slope for drainage may limit spacing in flat terrain. You may also have to limit the distance based on thermal expansion of the ductbank. Thermal expansion puts pressure on manhole walls unless there is provision for movement. In either case, if there are heavily loaded power cables that produce a lot of heat, the distance may be limited by manhole wall strength or expansion joint distance limits. Standard reel lengths may be a limitation with some cables. There are often also physical location constraints such as the need to avoid other underground utilities. If you don't have 4-360° bends, you don't have to limit the distance for this. Manholes are expensive and extra cable splices reduce the circuit reliability. It makes no sense to arbitrarily make the spacing small just to avoid some calculations.
- Thread starter
- #4
jghrist;
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
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
You need to get the manufactures limit for maximum allowable sidewall pressure. A wire shielded XLP redstripe cable can take more sidewall pressure that an epr cable with a tape shield.
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
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.
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.
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.
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
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.
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.
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.
You also must consider the allowable sidewall pressure for the duct you are using if it is non-metallic. Cables have been pulled through the walls of bends in PVC duct. If you have a questionable pull and have an option as to which direction to pull, calculate it both ways. Direction can make a big difference. To minimize tension pull in the direction that puts the most bends closer to the pulling end.
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
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
eghtesady,
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.
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.
SUBSTATION EARTHING-132kV
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.
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.
jbartos
Electrical
- Jan 15, 2000
- 6,440
Suggestion: Visit
for more information and sample calculations
for more information and sample calculations
All good points to consider.
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.
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.
Here is an story about manhole separation. Turned out to be about 300 meters for a 20Kv cable, but that isn't the point of the story:
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..."!
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..."!
Here's a link to a pretty good write up on underground installation.
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"
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"
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