NEC 314.71 Manhole dimension requirements

[cpoole]

Have a UG ductbank with 28 each 6 inch conduits entering a manhole with 84 each 15 kV volt single conductors (3 cables per conduit) with 1.7 inch overall diameter of each single cable.

Need manhole dimensions required to meet requirements per NEC 314.71;
1) For a straight pull manhole
2) For an angle pull manhole at 90 degree cable routing however the 90 degree angle manhole cable does not have to be pulled in continuously at one time. The cable could be pulled in straight with a 200 foot tail outside the manhole and the remaining 200 foot tails pulled in at later 90 degree angle into the building.

 

[davidbeach]

Infinitely large, ;-)

That's really too much for one manhole. That's 14 circuits worth; it's really ugly when there's a fault on the lowest run in a manhole and there's only 5 or 6 circuits in there. I can't imagine what will happen with 84 cables (hopefully it's actually only 42 cables and not 84 cables plus 42 splices) in a single enclosure.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[bacon4life]

Wow, I start to get nervous when more than 4 sets of 15 kV cable share a vault. Although I don't have any NEC advice for you, I am curious what kind of application this is for.

 

[waross]

2) For an angle pull manhole at 90 degree cable routing however the 90 degree angle manhole cable does not have to be pulled in continuously at one time. The cable could be pulled in straight with a 200 foot tail outside the manhole and the remaining 200 foot tails pulled in at later 90 degree angle into the building.

Have you done this before?
Yes, it may be possible, I have seen this done many times with TECK cable and tray cable in cable trays.
But there are pitfalls.
What do you do when you get to the last 10 feet and find that you have a 1/4 or 1/2 twist in the cable.
That's tough enough to deal with with TECK cable.
It may be a real problem with 15 kV cable.
What is the bend radius of the cable, compared to the diameter of the manhole cover?
The real problem will be guiding the last loop of cable down the hatch without scraping it on the side of the manhole opening.
I remember pulling a tail of 3c 15 kV TECK on the side of a building with 20 feet of tray to work with.
We got it done but we had problems and the cable was slightly damaged.
Do you have a good cable splicer available to repair the damage that is inevitable?

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[7anoter4]

First, the distance between 6" ducts has to be 10" so a ductbank of 7 rows and 4 columns is 72" height and 42" wide.
Second, the NEC Art.314.71 is referring to handholes not manholes.
I think the manhole dimensions are as per NESC [2007 the edition a have with me] 323. Manholes, handholes, and vaults B. Dimensions
Any way these are the minimum dimensions.
In the manhole you have to follow the recommendations for pull boxes as per NEC Art. 314.71 indeed, but you need open cable trays in order to pull the cables.
In my opinion, you need 7 rows of cable trays. For Straight Pulls the cable tray length shall be 48*1.7=81.6" [approx..7 feet]
For Angle or U Pulls the first cable close to the corner- for instance- will be at least 36 times of 1.7 [62"] and it has to add 10" for the second and so on.

 

[Gr8blu]

In addition - cable type/construction determines minimum bend radius. Per 2014 NEC Article 300.34:
1. Single or multiple conductor cable without metallic shielding > 1 kV requires minimum bend radius = 8X overall cable diameter
2. Single conductor cable with shielding > 1 kV requires minimum bend radius = 12X overall cable diameter
3. Multiconductor cable with individually shielded conductors > 1 kV requires minimum bend radius = 12X conductor diameter OR 7X overall cable diameter (whichever is GREATER)

Thus if your cable is unshielded, min bend radius = 8*(1.7) = 13.6 inch. If shielded, min bend radius = 12*(1.7) = 20.4 inch.

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