I’ve got to run some wiring in a commercial space for a buddy. Running a branch circuit from an electrical box down to below the floor deck, then across the underside of the floor assembly and then up through the deck. Building is all wood construction.
All of the existing wiring in the space goes UP from the box to the ceiling, then over to wherever and down, and all of that is BX (flexible metal sheathed) wiring. I was wondering when/if conduit is required instead of BX. For me, its just way faster easier to run BX, but I’ll do conduit if I have to.
Thanks.
Replies
The flex is probably more expensive than the conduit on a per foot basis. But then you won't need the expense of a bender.
If the deck is located outdoors, the flex will leak like a colander. You can get water tight EMT conduit fittings nowadays. But still get some condensation. Conduit will provide better -- more solid -- protection and tend to avoid the tendency of forming too many and too tight of bends.
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A few general comments:
1) Real electricians like pipe. It looks neater, is easier to pull wire through, and you can pull any sort of wire you like. Under certain conditions, the conduit can also serve as the ground path.
2) Flex (spiral wound flexible metal conduit) has its' uses, but it's generally a real pain to pull wires through it. You mearly always need to have a ground wire. Still, you can pull any wires you want in it.
3) MC (metal clad cable - looks like flex, but already has the wires in it) is quite verstile, but you're limited to the wires that come with it. Quite handy, though, when you need to fish around various obstructions.
4) If the previous guy went 'up' rather than 'under,' there's probably a reason. Check twice.
I witnessed a retrofit of wiring in our head office a few years ago, which is a simple wood-framed building. I was surpised to see the guys pulling miles of BX; every single branch circuit was BX. I asked why, and got the answer that that was what the engineer spec'd.
I still don't know why this would be the case (as opposed to good ol' NMD), but there may be a similar spec for your building.
Scott.
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I guess the question is: Code-wise (2005 NEC) is there a requirement to use conduit where flex would be disallowed? Indoor, dry installation.
The NEC has a section devoted to each wiring method, defining uses allowed and uses not allowed. For example, ordinary 'flex' is generally not allowed in wet locations. The scope of this question is a bit broad, and we don't know the location .... so you need to read the code yourself to see if there is a code reason for a particular job.
NM, or "Romex" has generally not been allowed in commercial buildings. While the NEC has loosed a bit in this regard, local rules and trade practices still frown upon it in commercial, or exposed, locations.
I still strongly suspect that the building being discussed has MC (metal clad) or AC (armor clad) cable, rather than flex. One would have to see the fittings, as well as inside the junction boxes, to tell. From the outside, the products look similar.
As to why the engineer specified something, you'd have to ask him. I am not aware of any location where pipe cannot be used as a general wiring method in place of any of the "flexible" methods.
I guess the question is: Code-wise (2005 NEC) is there a requirement to use conduit where flex would be disallowed? Indoor, dry installation.
Code-wise, is there a requirement that electrical work in your area is done by a real-life licensed electrician?
Generally, NM is not allowed in commercially rated structures, BX is not allowed, and MC has taken over for BX. MC is allowed to substitute for EMT where it is protected
In commercial work, the electrical types I know like pulling 14/2 MC for switch legs, as it is only the one pull, with is much simpler feeling through metal studs whether in new or retrofit situations.
That, means teaching one method to the new hands, too. Along with not having to pull wires separately, too.
It can be disconcerting at first, to see the mix of EMT & MC laid out i nthe ceilings, but once you see the "why" of it, it cam make a lot of sense.Occupational hazard of my occupation not being around (sorry Bubba)
To further define the definition;
There really isn't anything called BX these days although the name stuck among people out of the trade.
AC cable is a spiral metal wound "cable" that comes with wire in it and a thin bonding strip. The armor is OK to be used for the ground. This is steel and a "dry location' only wiring method.
MC is similar to AC but it has in internal "green wire" ground and a plastic internal wrapper (why the red bushing is optional). These days it is usually aluminum armor with THHN/THWN wire in it and can be used in wet locations. The armor is generally not suitable for grounding although it is grounded.
There is at least one company making MC with a thick internal aluminum grounding wire and the armor can be used as the ground. This wire is just internal bonding and gets cut off at the termination.
FMC (AKA Greenfield) is just the empty armor.
EMT is thinwall conduit what uses set screw connectors or compression connectors.
IMC is thicker conduit and RMC is thicker than that coming in schedule 40 or 80. These generally have threaded joints.
You select the various conduits based on the potential for "physical damage". EMT is if you expect a small exposure, schedule 80 RMC is for places where it will get beat up pretty bad.
In commercial I usually see EMT in new construction and MC cable for retro work, simply because it is easier to fish. FMC is usually only used for short whips since it is hard to get wire stuffed through if any length is involved. It is also limited to 6' as a grounding method.
Edited 11/30/2008 5:36 pm ET by gfretwell
I would think of the flex for shorter runs (to devices) off of the conduit. Seems like a good appication would be running conduit up above a drop ceiling then running flex to all the lights.
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I guess that's why they do it. That is the "whip" I was talking about ;)