Flexible metallic (FMC) maximum length?

aj164
aj164 Solar Expert Posts: 122 ✭✭✭
in Advanced Solar Electric Technical Forum #1
I've installed a few PV systems using flexible metallic conduit (FMC) through the attic to meet the "metallic raceway" requirement for DC conductors inside the building. Lengths range from 20 to 50 feet. These systems passed inspection, but they are rural.

Now, an electrician is telling me FMC is intended for equipment connections (short runs), and the maximum allowed length is six feet. I am certain I read somewhere that this maximum length somehow doesn't apply to PV, but now I can't find a reference. John Wiles' various publications say EMT and FMC meet the requirement, but he doesn't mention a length limit on FMC.

Anyone happen to know about this or have a link?

Thanks -
Adam
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Comments

  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #2
    Re: Flexible metallic (FMC) maximum length?

    ask him to show you what he thinks is said about it and under what conditions. if he can't show you specifics for pv applications against using longer lengths then ask him to read what wiles said (you have it ready for him) and by all means be cordial and say you want things to be right, but that you have some doubts he can rectify by showing you. i admit i am not familiar with this so i can't say one way or another. hopefully others may throw in their 2 cents also.
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  • Solar Guppy
    Solar Guppy Solar Expert Posts: 1,989 ✭✭✭
    #3
    Re: Flexible metallic (FMC) maximum length?

    Flex is intended for hookup and not long runs, I seem to remember 6ft maximum as well.

    Also, metal flex I believe is not the same protection wise as EMT, EMT is solid and keeps any fires inside the EMT, flex is not sealed and is intended to protect the wire from damage, not fires from escaping from within

    The intent of EMT inside a structure is fire/ shorting protection as the PV source has no breaker from the panels, different intents

    As far as I know you MUST use EMT, inside a structure as of the NEC2005 updates.
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  • aj164
    aj164 Solar Expert Posts: 122 ✭✭✭
    #4
    Re: Flexible metallic (FMC) maximum length?
    Solar Guppy wrote: »
    As far as I know you MUST use EMT, inside a structure as of the NEC2005 updates.

    Well, that's not what I wanted to hear. :D
    I wish Wiles had clarified his point about FMC.
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #5
    Re: Flexible metallic (FMC) maximum length?

    For my install (Los Angeles County, CA, USA), the conduit was required, not to contain a fire, but to prevent some future hapless owner taping into the wires, thinking that they were 120/240 VAC. (actually 450VDC) The conduit was an extra layer of Darwinism protection.
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  • Lefty Wright
    Lefty Wright Solar Expert Posts: 111 ✭✭
    #6
    Re: Flexible metallic (FMC) maximum length?

    The lenght requirement is about a grounding conductor. When using metalic flex for hooking up lighting a ground wire is not required if; length not more than 6 feet & branch circuit breaker not larger than 20 Amp.

    Or that's the way it was. I haven't checked the latest code changes.

    When using longer lengths of flexable metal conduit you must use a grounding conductor.

    Because of the cost I can't think of a good reason to use it in long lengths. EMT is cheap and easier to pull through.
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  • aj164
    aj164 Solar Expert Posts: 122 ✭✭✭
    #7
    Re: Flexible metallic (FMC) maximum length?
    Lefty Wright wrote: »
    The lenght requirement is about a grounding conductor. When using metalic flex for hooking up lighting a ground wire is not required if; length not more than 6 feet & branch circuit breaker not larger than 20 Amp.

    Or that's the way it was. I haven't checked the latest code changes.

    When using longer lengths of flexable metal conduit you must use a grounding conductor.

    THAT is what I had read somewhere, but I cannot remember where I read it. The reason for the length limit had to do with using the conduit as a ground w/o a grounding conductor.
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  • Lefty Wright
    Lefty Wright Solar Expert Posts: 111 ✭✭
    #8
    Re: Flexible metallic (FMC) maximum length?

    There are also some other considerations.

    Flex must be secured every 4 feet. EMT only once in every 10 foot length.

    The type of connectors that screw into the inside of flex limit the number of conductors you can pull. It is difficult and sometimes impossible (depending on the type of insulation on the wire) to pull more than three 10AWG wires through the connector.

    This is a bit off topic, but, I use EMT in my attic because of rodents. They like to chew and PVC conduit is one of their favorite chewies. I don't know how they get in but they often manage. Usually in the fall when they store acorns.
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