Rigid Metallic Conduit vs. Galvanized Water Pipe

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Old 07-06-04, 09:05 AM
James S.
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Rigid Metallic Conduit vs. Galvanized Water Pipe

I was wondering if there is any difference between "rigid metallic conduit" and galvanized water pipe. As far as I can tell, the threads match, and both the ID and OD are very similar. Both appear to be galvanized steel.

I am running 1/2" rigid metallic conduit (underground from house to detached garage), and have mixed in a few pipe nipples from the plumbing department to pass through the walls, etc. The job is permitted and will be inspected. I am the home owner, and am doing the work myself.

Second question - is any type of pipe dope or other compound used to connect the sections of conduit? Since it is running underground, presumably making it water tight would be important.

Thanks very much for any replies !!
 
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Old 07-06-04, 10:24 AM
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If making the conduit water tight is important, then PVC conduit with solvent welded joints would be a better bet. The threads of water pipe and electrical conduit are differtent. The water pipe has tapered threads and the electrical conduit has straght threads. You should be able to mix a few couplings with little problems. I, however, would not try to use water pipe as conduit, as the interior is probably not smooth enough for wires. Also, 1/2" leaves little room for expansion. You might want to run a 3/4 or 1" line just for expandibility. Finally, no pipe dope or thread tape is required for conduit. Good luck on the water tightness, I have seen too many galvanized conduits underground with standing water.
 
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Old 07-06-04, 10:30 AM
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All underground conduit eventually fills up with water, through condensation if not through infiltration. There's no sense trying to stop it, and it's not necessary to try either.

1/2" conduit isn't big enough for hardly anything useful. What exactly are you going to run through it?
 
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Old 07-06-04, 11:09 AM
James S.
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Thanks very much for the replies. Glad to hear that having the conduit fill with water is inevitable. No need to worry about that further. I guess my main question then is whether the galvanized water pipe would somehow violate code (we are still under the 1990 NEC here).

Regarding size, I am planning to run 3 #12 THWN stranded conductors through the 1/2" conduit. This will form a single 20 Amp GFCI circuit feeding a garage door opener, a single outlet, and a light. From what I understood from the code book, the 1/2" conduit could support more than 5 #12 conductors, so we could later an a second 20 Amp circuit if necessary. I think that would be plenty for us - this is a single car detached garage, and we are just trying to meet the city requirements to upgrade our home service to 100 Amp (from 60).

Scott - I cannot use PVC as it will only be buried to a depth of 6" (the reason that it will be only buried to 6" depth is extensive large tree roots). The inspector told me I had to use the threaded rigid metal conduit. From what I recall from the 1990 NEC, conduit threads are specifed to be tapered as well.

Thanks again for the help ! Please let me know if any of the above sounds incorrect.
 
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Old 07-06-04, 11:42 AM
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I just looked at the 99 NEC and you are right, condiut threads are tapered. I am positive that when I was apprenticing with an electrician that they taught me that conduit threads were straight. Learn something new everyday!!!!
 
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Old 07-07-04, 09:42 PM
P Michael
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Rigid conduit is designed to be bent. Plumbing pipe isn't.
~Peter
 
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