FMC Maximum Size Article 348.20

[ee213]

In earlier code versions, flexible conduit with 5 inch size could have been used and then code limited it to 4 inches. Why was this change made? Some types of flexible stainless steel (not interlocked armor) piping has been used for flexible steel conduits on numerous projects at various sizes very acceptably with good integrity. If one is using 5 and 6 inch conduits, the same sizes of flexible should be allowed if it meets integrity, strength, and its other intended purposes without damage to cabling or conduits. These other than interlocked cable are also provide excellent sealing against liquid intrusion when they are welded or used with good threaded fittings.

I currently work on a project that that has the 1987 code as its code of record on a power plant where the code is being implemented by a foreign country. The owner is questioning whether the 5 inch flexibles should be removed due to the exclusion in the 1999 and later codes. I know that code does not apply to power plants, but this question requires a response as to why the size limitation. I can understand not wanting to use large interlocked armor conduit, but not other high quality configurations of steel flexibles.

Does anyone have reasons other than integrity and strength of the flexible for supporting and handling pulling cable,including sidewall pressures, without damage to either the conduit or cables? Any background or other information on this subject would be much appreciated.

Thank you very much in advance.

 

[tbonse]

The only reasoning I could come up with is that if a section of conduit were to bend at a later date (post-installation) that excess stress may be applied to either the insulation or the conductors themselves particularly with larger conductors.

I came to this conclusion after thinking about how the strands of a large conductor bend in relation to each of its neighboring strands. When the conductor is bent, each of the strands bend with a different radius, but because the arc-length of each strand has not changed you get separation between the strands or the slipping of one strand against another. While this is unavoidable during installation, it should definately be avoided (if possible) after installation.

Now let's look at each event starting with the separation of the strands. When the strands separate from each other when the conductor is being bent, we introduce stress on the insulation for that conductor radiating out from the conductor at that bend. This stress on the insulation may result in damage to that insulation, thus ruducing its effectiveness. If this happens to multiple conductors in close proximity to each other where you have a high ampacity (why else would you need a 5" conduit) arcing between the conductors (or from a conductor to the conduit, which is bonded at the ends, right) producing high heat and melting the insulation on the other conductors.

OK, second scenario (assuming the first one didn't happen). If those same conductors are then permitted to rub against one another (as the conduit is flexed) the insulation may become worn away and permit arcing either between conductors or from a conductor to the conduit.

Third scenario, the strands within the conductor rub against each other with friction between the strands causing some to wear way. Now you have a reduced cross-sectional area for your conductor, causing additional heat at this point, which may eventually result in the failure of the insulation and finally arcing (again).

Different thought this time. The reasoning may be that the reduction in the wall thickness or the fact that the conduit is now a single overlapping helix with a far smaller cross-sectional area than would be present in the minimum sized grounding conductor for any grouping of conductors which might require a 5" conduit in the first place (based upon ampacity).

 

[infinity]

If the code of record is the 1987 NEC then IMO you should follow that code. If you want to pick and choose later code changes and add them to a 20 year old code you have essentially defeated the purpose of having a specific code of record in the first place. The 4" maximum size restriction for FMC appears in the 1996 NEC. Since that has been around for ten years I'm wondering if anyone still makes FMC larger than 4".