I believe that the boxes you describe (12" wide, 10" high, 8" deep) with the wires that you describe (2x 4/0 + ground in, 2x 4/0 + ground out, 2x 10 + ground out) could reasonably be used for a safe installation. However I've got to chime with everyone else: it won't meet the NEC.

Consider: if this were a 'cabinet' with internal permanently mounted terminal lugs connecting for connecting the wires, this box would be large enough to provide all the necessary 'wire bending space' needed for 4/0 wires. (See article 312) But this is not a cabinet; it is a junction box.

Additionally, your 3" conduit is rather oversized for the conductors in question. 314.28(A)(3) permits conduit bodies or junction boxes to be smaller than that ordinarily required by a given conduit, provided that the junction boxes be 'listed for and permanently marked with the maximum number and maximum size of conductors permitted.' My guess is that the particular boxes that you've found are not so listed...but I bet that they are large enough that they could be listed if you wanted to pay the right testing lab [Linked Image] My _hunch_ is based on the size in comparison to wire bending space in article 312, and size of an appropriate box for the _minimum_ conduit which would hold these conductors.

Question: Why use 3" conduit? By oversizing the conduit, you are forced to oversize all the other components. If we presume that you have to pull 3 4/0 conductors (two circuit conductors and an oversized ground conductor), then the minimum conduit size is 2". If you were to use 'compact' conductors, then you may even be able to use 1.5" conduit...though this depends upon which insulation you are using, and pulling the conductors could be a real pain. Depending upon circuit amp rating, the ground may be smaller than 4/0, freeing up some space.

But if you use smaller conduit, then you can use smaller conduit bodies and junction boxes, without any sort of interpretation or UL evaluation. If the oversize conduit is to provide future expansion capability, then the undersized boxes will prevent this. So why not properly size the conduit, and then properly size the smaller junction boxes.

-Jon