Conduit in water pipe
I can't seem to locate any Code requirement precluding me from securely supporting a 1-in. rigid metal conduit onto the bottom of a 10-in. insulated metal water pipe for about 40 ft. The pipe is rigidly supported via pipe stands. The client wants to make the conduit less conspicuous by having it securely strapped to the pipe, either inside or outside the insulation. He doesn't want it on the pipe support.
The application is a pumping station that is prone to vandalism. We can't bury it without getting a wetlands permit. No continuous concrete pad exists between the building and the groundwater tanks located outside, so surface mounting is out. The insulation is fiberglass-covered with metal (tin or aluminum). The conduits are for 480V power to an immersion heater and the temperature and level switches.
Beside the portions of the Code that define the support distances, I don't see anything that says not to do it. My instincts tell me that for personnel safety, it should be done. For example, the pipefitter or insulation installer has to come and perform maintenance on the piping. He sees (or worse, doesn't see) the conduit. He doesn't have time or doesn't want to wait for an electrician to show up to disconnect and remove it. He can't perform his task with it attached, so he figures he will disconnect it himself. Maybe he is careful and knows what he is doing or maybe he doesn't.
I do not know of any Code rule that directly addresses your issue. The primary rules for support in your case are found in Sections 300.11(A) and 344.12. Both of these require that the conduit be “securely fastened in place.” They do not say, “directly attached to the structure.” Generally, the maximum support spacing requirement for RMC (rigid metallic conduit) of trade size 1 is 10 ft, but for straight runs with threaded couplings, this spacing is increased to 12 ft. Nothing in the NEC states that you cannot use the same supports for conduit as for other systems. Although Section 300.11(B) generally prohibits the use of conduit as support for anything else, it does not mention the use of water pipe or other systems (other than ceiling grids and wires) for support of conduit. Some plumbing or mechanical codes may cover this. I doubt that the physical load of a size 1 RMC would be significant to the 10-in. water pipe system or its supports. A different argument might be made if the piping used to support the conduit was much smaller.
All of the arguments you make are valid considerations. The argument, however, that the pipefitter or insulation installer might tamper with the conduit supports rather than wait for an electrician could apply to any installation where electrical and other systems are in close proximity. I would add one more consideration: If the conduit was placed within the insulation, the ampacity of the conductors would likely be affected, and that would have to be considered. I think the installation would be safer if the conduit were on the outside and identified in some manner, and I would prefer that the pipe supports be used directly. Those are simply my preferences, however, and I can't quote Code references that would require those installation methods.
Please advise on where we might find criteria for sizing in ground (typically concrete) vaults, used for pull or junction, for all voltages. Are we just stuck with NEC Article 370?
Since you refer to Article 370 for pull or junction boxes, I assume you are in the 1999 or earlier NEC. In the 2002 NEC, Article 370 was renumbered as Article 314. In 1999, Part D was added to Article 370. This part covers manholes and other electric enclosures intended for personnel entry. The same rules are found in Article 314 Part IV of the 2002 NEC.
You do not pose a specific question but I suspect that the information given above will help solve your problems. As a former member of an NEC Code panel and current member of another NFPA committee, I do not want anyone to feel “stuck with the Code.” Public proposals made to the committee can revise the Code or expand it. Information on the Code proposal system is found in the back of the Code book. Additional information is available from NFPA. I will also provide assistance if you have questions about the Code development process.
E-mail your NE Code questions to firstname.lastname@example.org