Elec – Conduit from exterior panel
My copy of the NEC is in storage too far away to bother to retrieve, so need a little help. I have an exterior main panel on a stucco wall. I need to run conduit down from the panel to go into the crawl space underthe house. Is EMT generally allowed in exterior applications (this is in the mild climate of the San Francisco Bay Area), or do I need to run pvc or heavier conduit for the exterior? (I have never worked with PVC conduit but have bent a fair amount of EMT.) What is the best way to transition from PVC to EMT?
EMT is not considered weather tight or suitable for wet/damp locations.
Use the PVC and transition after an LB through the wall with a female threaded adapter and a metal box connector threaded into it (either compression or set screw type). From there run your EMT if you want. No need to make bends in the pvc unless you have an offset, saddle or need to kick it one way or another to accommodate some obstruction. I like schdl. 80 for outside runs where it might get damaged. It is more expensive than schdl. 40 but the peace of mind is worth it. I also fasten pvc more often than normal emt or imt. Schdl 40 will sag more on horizontal runs than schdl. 80.
I *think* that EMT can be used outside with approved fittings, but I don't have time to look it up.Just one caustion about transistion from PVC to EMT. At one end the EMT has to terminate in a metal box so that it can be grounded. Otherwise you can end up with a hot piece of conduit if there is a fault.
Here in LA, EMT is used outside with compression fittings. But not with setscrew fittings. It's allowed, but unless impact damage is an issue, PVC is better. (PVC doesn't rust, so it looks better for longer.)
Edited 8/18/2006 3:36 pm ET by JohnSprung
EMT is indeed approved for damp locations, but the fitting must be corrosion resistance, and compression fittings would be the only way to go on an exterior application.
After 20+ years of doing electrical work, I can say I have never seen an emt fitting that didn't rust. Even the conduit will rust at the hanger locations, given enough time. The galvanizing on it is so thin that a few knocks or scratches is all it takes.
Just a personnel preference I guess, but I would run pvc or alum. IMT.
BTW I could find no requirements for bonding grounds for EMT and metal J boxes, conduit and such. Only that they may be used as a equipment grounds if continuous. Pulling a ground wire is pretty much standard now days. It is a lot safer than trusting everyone in the future to maintain the unbroken continuity of a metallic conduit system.
I think it's unambiguous that EMT and metal boxes have to be bonded (ie grounded).
2005 NEC article 250.4 says, "Non current carrying conductive materials enclosing electrical conductors or equipment. . . . shall be connected to earth...."
EMT is most certainly considered appropriate for damp, wet, and outdoor applications.
Compression type fittings have been upgraded, so ones exist now that are truly 'raintight.' They are adequate in wet locations for everything but entering a panel from above, where some sort of "hub" should be used.
Since walls are typically not smooth, and you need some room where you will enter the wall, I like to mount my pipe a bit off the wall, using short pieces of strut.
Where I turn to enter the wall, depending on the number of wires, I will either use am "SLB" type conduit body, or a junction box.
The secret to transitioning from one type of conduit, to another, is to either change at a junction box, or find a threaded fitting somewhere.
My copy of the NEC is in storage too far away to bother to retrieve, so need a little help.
FYI, the 2005 NEC can be accessed online at:
No printing or downloading permitted, but at least you can read it in circumstances like this.
Thanks, BarryO. Looks like a promising site. Unfortunately, it seems to be incompatible with my firewall and/or anti viral or some other safeguard program. It will take me a while to sort things out before I can view it.In the meantime maybe you can give me some hints here. For household circuits, I usually run single stranded wires in EMT. However, in my current situation I have a crawl space that is at most 18" and generally less. Running EMT would be more of a chore than I would care for. Is Romex generally considered acceptable in a dry crawlspace, stapled to the underside of the floor? I would need either rigid conduit or EMT to lead the Romex into and out of the crawlspace. For a 12/3 Romex, what size conduit is appropriate. I assume it would need to be larger than that specified for three 12 gauge conductors because of the extra insulating value of the sheath. Thanks
Even though your crawl space is dry now, I would use romex rated for underground installation (presuming you can use romex in this situation). There is no paper in it.I've never been clear on the transistion issue. You could run conduit and single conductor to a junction box, and then connect the romex inside the box. But I imagine you can run the romex in conduit (I have it in my garage, 12/2 in 1/2", so I imagine 12/3 in 3/4" would be fine). It is a question of heat build up. But also you need some type of bushing at the end of the conduit to prevent rubbing and cutting into the romex.
It's legal to run wiring in crawlspaces, but you better make d*mn sure it's critter-proof. Don't believe anyone who tells you that animals don't bother to chew on cables. I have some pieces of racoon-munched coax I've saved as an example.
Type NM is OK in a crawlspace that's "normally dry", any where it's also OK to have untreated wood should be alright.
It's perfectly OK to run NM in conduit. There's no heat build-up issue with a single run of NM: it's already derated below what the insulation actually is (ampacity ratings assume 60 degree ratings, but the insulation is actually 90 degree). It's OK to embed NM in an insulated wall, for example. That's a much worse situation, heat-dissipation-wise, than being run in conduit.
As for conduit fill, measure the diameter along the larger axis. Use that to compute the area of a circular cable with the same diameter as this. This area can then take up no more than 53% of the cross-section area of the conduit.
If Romex is allowed, then is it safe to assume that MC cable would also be allowed? That should guard against the hungry varmits, although buying it these days will rapidly gnaw through one's bank balance... (Actually, I have a several hundred feet of 12/2 stashed from when Home Base went under and sold it at 75% off, but unfortunately what I now need is 12/3...) And thanks for your shared knowledge.
Sure, MC is even allowed in wet locations, if the conductor insulation is listed for wet locations, or the jacket or covering is impervious to moistrue.