Conductor Fill for Large Boxes/Wires

[philhurtgen]

What's up, guys!

I'm an apprentice of two years in residential electrical work, slowly building my knowledge of the code, and I have an interesting question, on which I crave your all's excellent insights.

I'm going through article 314 right now, and I've been acquainted with box fill calculations before, but I hadn't ever really considered them for larger boxes/wires--never really had to, as we don't often deal with large installations. In any case, it's clear to me that 314.16 deals with box fill for wires no. 6 or smaller, and I believe I understand what 314.16 has to say about those applications. As I've read on, it seems like 314.28 deals with wires no. 4 and up, and the rules seem pretty straightforward as far as sizing boxes correctly there; what I'm confused on, though, is the question of how many wires we can put into said correctly-sized boxes. The large junction boxes that we use do seem to have the cubic volume listed on them, but I haven't been able to find any directive in the NEC on the volume allowance required for conductors no. or larger--table 314.16(B) just stops at 5 cubic inches for a no. 6 wire. Can you guys tell me what I'm missing here?

 

[Dennis Alwon]

When you get into wire sizes larger than 6 then you aren't dealing with cu.in of wires sizes but rather the size of the conduit being installed. Once you calculate the vol. based on conduit size then you can install as many wires as is allowed for conduit fill.

 

[infinity]

As Dennis stated #4 and larger you use the raceway size to determine the box size. There is no longer a volume calculation but a distance calculation between the raceways based on how the conduits enter the box. For example a straight pull with 4" EMT would require a 32" long box (4"*8=32"). The box could be 6" X 6" wide or 60" X 60"wide. It only needs to be wide enough to get the raceway into the box and its volume is irrelevant.

 

[don_resqcapt19]

When you get into wire sizes larger than 6 then you aren't dealing with cu.in of wires sizes but rather the size of the conduit being installed. Once you calculate the vol. based on conduit size then you can install as many wires as is allowed for conduit fill.

Yet 314.16 says this:

Boxes and conduit bodies enclosing conductors 4 AWG or larger shall also comply with the provisions of 314.28.

I submitted a proposal to delete that wording but it was rejected with the following panel statement.

Although there are no prescriptive requirements in this section that apply to large conductors, the parent language about sufficient size
does apply.

No idea of how you can determine sufficient size other than the rules in 314.28 for conductors 4 AWG and larger.

 

[philhurtgen]

Thanks, guys! Always appreciate the knowledge shared here. I like the easiness of that rule. It's definitely less complicated than the rules for smaller wire sizes.