Calculating the Size of Pull-Box Needed

[Vankirk Electric SR. PM]

We are doing the electrical on a mixed use, multi family project in Marina del Rey, CA.

Scenario:

My superintendent and I are attempting to size pull-boxes correctly for the unit subfeeds. We have MC cable feeders running from the group metering, located in the parking deck, to pull boxes, and then transitioning at the pull-boxes to SER cable that will be run up through the concrete podium deck and through the wood structure portion of the project, and then to the unit panels.

We have looked through the NEC, but can't seem to locate this scenario for sizing the pull boxes. I only see sizing based on conduit size entering the pull boxes, not MC cable or SER. Also, how do we factor for all the splices we will have in those pull boxes? Each splice will be an insulated Polaris connector.

Thanks for the help.

 

[Dennis Alwon]

I would size it base on the KO that is used for the cables. I am not sure that is correct but that is what I would do.

 

[cpinetree]

Along with Dennis' answer you might make up your joints in the shop and then measure the space needed.

Using the dimensions of the connectors required to terminate MC and SER you likely will have a box that is tight to work in with the polaris taps.

 

[jumper]

I would size it base on the KO that is used for the cables. I am not sure that is correct but that is what I would do.

I think it is correct based on 314.28.

When transposing cable size into raceway size in
314.28(A)(1) and (A)(2), the minimum metric designator
(trade size) raceway required for the number and size of
conductors in the cable shall be used.

 

[Vankirk Electric SR. PM]

Thanks everyone for your responses.

I think you are all correct, in that I should just transpose the required KO size for the cable, into conduit size. The 314.28 code reference makes sense, and I think applies to our situation.

What do you guys think about all the splices in the pull-box? Increase by about 25%? I really don't think the code will speak on this portion. I also doubt that an inspector could gig us, as long as we size it to the minimum required by NEC, and then increase it some for the Polaris splices.

 

[Vankirk Electric SR. PM]

Along with Dennis' answer you might make up your joints in the shop and then measure the space needed.

Using the dimensions of the connectors required to terminate MC and SER you likely will have a box that is tight to work in with the polaris taps.

Good idea. That would give us an idea of how much space they will really take up.

 

[jumper]

Bigger is always better when splicing large conductors.

 

[infinity]

I think it is correct based on 314.28.

Is that really the same? 4-500's in an MC cable with a 2" connector will not fit in a 2" raceway.

 

[kwired]

Is that really the same? 4-500's in an MC cable with a 2" connector will not fit in a 2" raceway.

I will second that, needs to transpose to same raceway size one would use for the conductors involved, they pack them into a tighter space in a cable assembly then what code lets you put into a raceway.

And if you are going to splice them code minimum sized box isn't always going to be large enough, you find that out the hard way sometimes.

 

[Vankirk Electric SR. PM]

I will second that, needs to transpose to same raceway size one would use for the conductors involved, they pack them into a tighter space in a cable assembly then what code lets you put into a raceway.

And if you are going to splice them code minimum sized box isn't always going to be large enough, you find that out the hard way sometimes.

Hmmm. Ok, so the general consensus is to size the box based on the required EMT raceway size for the conductors being used. We will have a range of conductor sizes, from 1/0, up through 4/0 (3C with Ground). If we plan on just sizing the box for all 4/0 conductors (2.5" KO size) entering/exiting, that should give us plenty of space to work with the Polaris splices.

 

[infinity]

Hmmm. Ok, so the general consensus is to size the box based on the required EMT raceway size for the conductors being used. We will have a range of conductor sizes, from 1/0, up through 4/0 (3C with Ground). If we plan on just sizing the box for all 4/0 conductors (2.5" KO size) entering/exiting, that should give us plenty of space to work with the Polaris splices.

Logically that would make sense since with raceways the pull box is sized according to the trade size of the raceway so an equivalent size for cables would allow the same amount of room.

 

[jumper]

Is that really the same? 4-500's in an MC cable with a 2" connector will not fit in a 2" raceway.

Oops my bad. My brain was thinking the cable connector entry would be the same as a conduit entry.:ashamed1:

 

[Dennis Alwon]

I agree also. The cable connector does not have to follow a 40% fill-- my bad too.....Calculate the size of the raceway that would be needed for those conductors and then use that size.

 

[JFletcher]

Good idea. That would give us an idea of how much space they will really take up.

I second this suggestion. Stiff cable and large Polaris connectors are not nearly as compactible as smaller wire and wire nuts.