What are code rule differences between installing MC vs FMC with equivalent conductors?

Question

AFAIK MC (metallic cable) is just FMC (flexible metal conduit) with wires already in it.

However, my understanding is that while MC can just be terminated as-is to e.g. fittings on junction boxes, FMC+wires needs to first be assembled as an empty raceway and only then have the wires pulled through it.

Is this correct, or can one pre-wire FMC before terminating it to boxes? The latter option would be convenient...

Response to answers:

Thanks 3phase, jay and RMD for clarifying my thinking on this!

This all was prompted by my recent use of a segment of 1/2 inch FMC with 90-degree squeeze connectors in upgrading the quality of some outlets in unfinished garage wall that will see continuous use at 15A for EV charging. The 90 degree connectors made the 5x 12ga wire pulling quite a pain.

My thought was that FMC looks the same as MC armor and uses the same fittings, so could I just assemble my own wired FMC "whip" to ease a similar setup? I'm only thinking of "standard" MC, not any of the improved/specialty varieties.

I realize now that:

1. I should think of code as a system to prevent errors rather than thinking of the wiring system as just an object in its completed state.

2. FMC is a raceway with the associated rules to prevent errors in both filling and terminating it, while MC is a cable, so as for e.g. NM, the conductors are a given, and the user just deals with the termination. (notwithstanding that the outer armor may be the same in both)

Thanks again for the helpful input!

Situation that prompted question:

Answer 1

Your assumption about MC vs. FMC is off-base

While typical interlocked-armor MC cable may look similar to a pre-wired FMC whip, the two product classes are quite different in their capabilities:

• Short lengths of stock FMC may be used for grounding, while typical interlocked-armor MC cables require a separate grounding wire in their construction as their armor isn't suitable to serve as a grounding means
• MC cables are always factory-made assemblies, whereas FMC is meant to be pulled through in the field; this means that FMC assemblies have more void space between the armor and the inner conductors than a MC cable does
• Most interlocked-armor MC cable has a plastic binding tape inside it as a function of the cable construction, although many MCI-A cables omit it
• And most importantly, the code category of MC cable encompasses far more cable constructions than what's on the shelf of your favorite borg-box:
  • MC cables with smooth or continuously corrugated (welded) armor
  • Interlocked-armor MC cables with a grounding means integral to their armor (generically known as MCI-A)
  • Industrial control and fire alarm cable constructions
  • PVC jacketed MC cable suitable for wet locations, direct burial, and concrete embedment
  • MC-HL (Hazardous Location) cables that, when installed properly, are suitable for use in Classified hazardous locations that may have flammable vapors, gases, dusts, or fibers in them
  • Armored communications cables used for industrial Ethernet and fiber links
  • and even fire-resistive MC cables (Marmon/RSCC VITALink, Prysmian Draka Lifeline) that can carry power after being seared in a furnace for 2 hours then hosed off at full blast

This means that the two products have very different installation rules. For instance:

• FMC can only be installed in dry locations, ever, whereas MC cable constructions rated for wet-location service can be installed in wet locations (or equivalents)
• FMC is subject to conduit bend limits, whereas MC, being a cable construction, is not limited by pulling constraints
• Outdoor-rated MC can be suspended from messenger wires for aerial runs, something generally not discussed for flexible conduits
• Suitably rated MC cables (generally of the jacketed type) can be used in environments subject to corrosive influences (such as vapors from pool chemicals), which is something FMC can't do either
• MC can be run in elevator hoistways, something FMC is prohibited from doing
• MC cables can also be used in cable trays or even other raceways, neither of which is mentioned as a permitted use for FMC
• and FMC is generally prohibited from being used in fire-rated wiring systems (CIC cables require RMC, IMC, or sometimes a special RTRC)

Answer 2

The difference between installing conductors in MC and FMC is that FMC is approved for pulling wire on site after it's fully installed, not otherwise, and MC is not approved for pulling wire at all. You can't mix-and-match the rules that govern the two things for your own convenience. You can't pretend you have an MC factory in your garage because it's more convenient there.

The installation of conductors in MC is done in a factory. It's not governed by NEC. The conductors are not pulled through the finished conduit. It's not even right to call it a conduit. You can't make it on site. You can't remove wires from it, you can't add or replace them. You can't buy a machine from an MC factory and build your own MC outside the framework of an approved and tested process.

The same goes for prefilled appliance whips. Although they may be composed of something that could have been FMC, their construction in a factory is governed by UL or similar rules, not NEC. You can't do that yourself and you can't modify them.

When you buy empty FMC to use as conduit it is governed by all the rules about using conduit. You can't prefill it. You have to fill it via finished pull boxes because that's how it's designed.

Your statement that prefilling it is "convenient" is exemplary for being unacceptable. Even with a short, straight, run where both ends and their boxes are in view, so you know for sure that pulling after installation would be easy, you can't pull it before installation. It's conduit and you have to follow its installation rules.

This answer summarizes the code perspective: Conduit is designed to protect wire after installation according to design and instructions. Running wire through it during assembly fails to achieve that goal and exposes it to specific new harm.

This answer provides lots more practical reasons not to do it, although those reasons may not apply to a short point-to-point piece of FMC.