Medium Voltage Pull Box Sizing

[jdengr]

Two related questions:

I am laying out an installation of 3-conductor, shielded, 5kv cables (2.15-inch O.D. over sheath), and need to size pull boxes. According to 314.71(A), for straight pulls, the min. length of a PB is 48 x the cable diameter. This makes the min. length 48 x 2.15 = 8.5 feet!! Am i doing something wrong here?

Also, I need to size a cable trench (below grade) for the same cables. Does 110.74 apply to the width of a cable trench? I am bringing the cables into the end of the trench and then out of the side. This would be an angled pull and 110.74 references back to 314.71(B). Using this makes the trench 2.15 x 36 = 6.5 feet wide. This is assuming 1 cable in the trench. This seems a little too wide considering the min. bending radius of the cable is less than 2 feet.

Any help is appreciated.

 

[RUWired]

Two related questions:

I am laying out an installation of 3-conductor, shielded, 5kv cables (2.15-inch O.D. over sheath), and need to size pull boxes. According to 314.71(A), for straight pulls, the min. length of a PB is 48 x the cable diameter. This makes the min. length 48 x 2.15 = 8.5 feet!! Am i doing something wrong here?

Also, I need to size a cable trench (below grade) for the same cables. Does 110.74 apply to the width of a cable trench? I am bringing the cables into the end of the trench and then out of the side. This would be an angled pull and 110.74 references back to 314.71(B). Using this makes the trench 2.15 x 36 = 6.5 feet wide. This is assuming 1 cable in the trench. This seems a little too wide considering the min. bending radius of the cable is less than 2 feet.

Any help is appreciated.

Your 8.5' straight pull box is the minimum required.What kind of trench is going in the ground.Figure 310.60 gives some examples of underground installations, and the tables 310.77-86 gives the ampacities of the cables at those distances.Your trench would only need to be wider than that at the transition points.
Rick

 

[celtic]

This makes the min. length 48 x 2.15 = 8.5 feet!! Am i doing something wrong here?

Have you ever spliced one of those cables?

In the event of a MV cable fault, you WILL need substaintial area to make that splice up....also consider that in the same scenario you will be removing a portion of the damaged cable, again, you WILL need substainial area to set up the pull.

We commonly refer to those long narrow boxes as "coffins".

Also, I need to size a cable trench (below grade) for the same cables. Does 110.74 apply to the width of a cable trench? I am bringing the cables into the end of the trench and then out of the side. This would be an angled pull and 110.74 references back to 314.71(B). Using this makes the trench 2.15 x 36 = 6.5 feet wide. This is assuming 1 cable in the trench. This seems a little too wide considering the min. bending radius of the cable is less than 2 feet.

I do NOT believe 110.74 applies to your installation.
The Title of Part V is "Manholes and Other Electric Enclosures Intended for Personnel Entry,All Voltages" ...is this trench intended for personnel entry?

If your answer is "No, not intended for entry", consider eyeballing 310.60.

 

[jdengr]

The pullbox is only intended as a pull point. It will be surface mounted on an exterior wall, and there will be no splices inside it. This is a special "VFD" cable, and if there is a failure, the entire cable will be replaced. However, if i do need it, where do i get a pull box this long?

The cable trench is only intended for pulling and routing cables. The only personnell entry required will be what is necessary for pulling and racking. The trench will only be 5'-6" deep, not enough for "personnel entry" as in the case of a manhole or vault, so i don't think 110.74 would apply.

 

[celtic]

Give http://www.hoffmanonline.com/ a try.