High Voltage Manhole or pullbox

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steve66

Senior Member
Location
Illinois
Occupation
Engineer
What would be required for a pull point for an underground high voltage run? I have a set of (3) 500KCM cables in a 4" conduit and a spare conduit, and the run is long enough, with enough bends that I think the contractor will need an pull point in the middle of the run.

Would a manhole would be required? How do I size it?

Is there a company that makes precast manholes??

Thanks in advance.
Steve
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
There are a number of companies that make large precast manholes. Often the same companies that make precast manholes for sewer systems or drainage systems make the electrical ones too.

One of the issues with using a manhole is the size of the cover. It has to be big enough to permit a "U" bend of the conductor to pass through it without bending the conductor tighter than its permitted bend radius.
 

eric9822

Senior Member
Location
Camarillo, CA
Occupation
Electrical and Instrumentation Tech
Here is a link to one manufacturer of precast vaults, Google it and you will find more I am sure.

http://www.gillespieprecast.com/product_ElecVaults.html

Like Don says you may still have a problem getting the cable out the manhole and back down without overbending the cable. We had a similar installation not that long ago running through an existing precast concrete vault. We had to make a splice in the vault. Fortunately it was a pretty good size vault so we had plenty of room to add supports to the wall and loop the cable around to make splicing easier.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
One of the issues with using a manhole is the size of the cover. It has to be big enough to permit a "U" bend of the conductor to pass through it without bending the conductor tighter than its permitted bend radius.

That poses quite a dilemma. Are there any other options??

My 500KCM sheilded cable is about 1.5" in diameter. 36 times that is 54". But to get the cable out of the manhole, and back into the manhole, don't I need 2x that? That would be 108", or 9 feet. I'm obviouslly not going to get a 9' cover.

From the link Eric posted, it looks like the top of the manhole is removable. Could one assume they will pull the cable before the top is installed? Of course, if they ever replace the cable, they would have to excavate, and remove the top again.

That's still a 9' long vault. I'm wondering how much space is required for splicing in the manhole. That would have to be less than 9'.
 

eric9822

Senior Member
Location
Camarillo, CA
Occupation
Electrical and Instrumentation Tech
The vault we were dealing with had been in place for about twenty years. The top is flush with grade and surrounded by asphalt. I assume that these things are intended to have the top flush with the grade because otherwise you would need a vertical extension from the top to the manhole at grade level. We very briefly considered removing the entire top but decided against it due to the risk of having something bad happen and damaging the existing cables.
 

don_resqcapt19

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Staff member
Location
Illinois
Occupation
retired electrician
I have never seen a manhole cover that let you install the conductors without bending them tighter than their minimum bend radius. I have made a number of pulls with 350kcmil, 5kV shielded cable in manholes and we just worked the loop down in like you would do with any other conductor. Never gave any thought to bending it too tight. I think it is just one of those rules that are in the code, but is almost always violated when installing cables in manholes.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
I guess a temp. bend while installing the conductors may not be the same thing as a permenant bend. It looks like 24", 30", and 36" are standard cover sizes. I'll spec the larger 36" cover.

Don't the electricians often leave a loop in the manhole to make it easier to pull? It seems like it would be easier to get a simple loop through the opening than an "omega" shaped U bend.

It looks like I size the length of the manhole per 314.71(A) which is 48 times 1.5" = 6' long.

110.73 for over 600 volts is not very clear. For a simple pullbox at 4160V, does the work space need to be met or not? (It's not clear if the phrase "likely to require examination..." only applies to the first sentance in that paragraph, or if it applies to the entire paragraph.)
 

don_resqcapt19

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Staff member
Location
Illinois
Occupation
retired electrician
...Don't the electricians often leave a loop in the manhole to make it easier to pull? It seems like it would be easier to get a simple loop through the opening than an "omega" shaped U bend. ...
Typically the conductors are fed off the spool, though the manhole cover and into the raceway. After you get the conductors out the other end of the raceway, you unspool the rest of the conductors, and feed that end into the raceway going the other direction. At the end of the pull you have to get the cable down into the manhole.
 

BJ Conner

Senior Member
Location
97006
The "loop" that has to be pulled back into the valut is technically the "bite" If you have 1-1/2" diameter cable the opening would have to be MBR x 1.5 x 4. IF the MBR is 12 x the diameter the opening is 12 x 1.5 x 4= 72"
You can get vaults with hinged metal plate covers up to 99" wide google up a 676-LA/WA or a 612 LA.
They are on an Oldcastle website. http://www.oldcastleprecast.com/plants/Auburn/products/electric/Pages/electricvaults.aspx

People new to pulling medium voltage think of pull boxes and sizing them from various NEC sections. You want to approach the job thinking about swimming pools and go down from there.
Contact your cable vendor, they will usually help you out. When the job is done and the cable doesn't test out the ususal conclusion is that the cable was bad. Vendors like to get in early and help so it goes right.
The things that will damage your cable are Sidewall Pressure and minimum bending radius. IF you don't know what they are your cable vendor will help you. Buy some long radius and segimented shieves. Buy the Polywater Pull Planner program to check your pull. Buy lots of soap and keep things clean. Use real pull roap ( some thing with ZERO streach - no nylon and NO STEEL CABLE-COATED OR NOT) and lots of it.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
The "loop" that has to be pulled back into the valut is technically the "bite" If you have 1-1/2" diameter cable the opening would have to be MBR x 1.5 x 4. IF the MBR is 12 x the diameter the opening is 12 x 1.5 x 4= 72"
...
Why "times 4"?
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
Typically the conductors are fed off the spool, though the manhole cover and into the raceway. After you get the conductors out the other end of the raceway, you unspool the rest of the conductors, and feed that end into the raceway going the other direction. At the end of the pull you have to get the cable down into the manhole.

It's hard to describe. Maybe this will explain what I'm thinking;

View attachment Loop Pull.pdf

It seems like leaving a loop of wire in the manhole would make it easier to fit the loop through the cover.
 

mkgrady

Senior Member
Location
Massachusetts
When installing MV cables it is customary to pull cables through the manhole not to double back in the other direction. Doubling back is a good way to damage MV cables. If the run has too many bends to pull through the manhole then it should be pulled in both directions and spliced in a manhole. The smallest manholes I worked in were about 8x8x6 with a cover around 30 inches. Getting the cover to be flush with grade would normally include using bricks for a chimney to raise it up to grade if necessary. If making a tight bend in the cable is a concern then use wire shielded instead of tape foil shielded.
 

jimmac49

Member
Location
NY & CT
What would be required for a pull point for an underground high voltage run? I have a set of (3) 500KCM cables in a 4" conduit and a spare conduit, and the run is long enough, with enough bends that I think the contractor will need an pull point in the middle of the run.

Would a manhole would be required? How do I size it?

Is there a company that makes precast manholes??

Thanks in advance.
Steve

Steve are you sure you need a manhole? Polywater has a software program called pull planner that will calculate your pulling tensions, sidewall pressure, etc. I would put your run into the program and see what the numbers are. If you do need a manhole I think a 6'x6'x6' would meet your needs with plenty of room to splice the cables. Elastimold makes a great dead break termination that is waterproof and easy to work with. They are called 'hammer heads' in the trade.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
Steve are you sure you need a manhole? Polywater has a software program called pull planner that will calculate your pulling tensions, sidewall pressure, etc. I would put your run into the program and see what the numbers are. If you do need a manhole I think a 6'x6'x6' would meet your needs with plenty of room to splice the cables. Elastimold makes a great dead break termination that is waterproof and easy to work with. They are called 'hammer heads' in the trade.

Well, I'm just doing the plans and specs. But if the contractor is going to need a manhole, I want to size it and make sure its in the bid package. I don't want to get hit with a change order for a manhole.

Its about a 900' long run with at least (2) 90 degree bends. Maybe with a few offsets if the contractor needs to make grade changes.
 
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