Coatings for buried rigid electrical conduit
prairiescl
| Posted in General Discussion on
Hi all,
I am burying a 25′ run of 3/4″ galvanized rigid electrical conduit to the garage.
Should one use teflon pipe tape at the joints?
For corrosion resistance, I plan to coat the assembled conduit with ashphalt driveway sealer and fiberglass drywall joint tape. Is there any unintended consequence like the sulfur in the ashphalt reacting with the zinc caoting to cause fast corrosion that the bare conduit?
Replies
prairie
Do you have an aversion to using pvc conduit?
Yeah...what the others said.
PVC is the way to go. WAY easier, more forgiving, waterproof... the list goes on.
Why reinvent the wheel? Do you think this is the first time conduit has been buried?
Everyone else needs to listen up, as well.
The advantage to using rigid is that you need not bury it quite as deep. A deeper trench can be a big issue in difficuly soil.It still needs to be buried, most likely 18" deep.
PVC pipe is nice, but you have to go 24" down with it.
How pipe rusts in the ground depends, in part, on your soil. Most likely, the 'rust zone' is within the first six inches of burial; deeper than that and there's not enough oxygen for rust to form.
This is why we have 'pipe wrap tape,' a very heavy vinyl tape that you wrap around the pipe. It's available in plumbing stores, as plumbers are required to use it when they go underground. It's amazing how well it works at preventing rust.
No, you do NOT wrap the threads with teflon tape. We're not the least bit worried about water leaking into the pipe. You might, though, coat the threads with anti-seize compound. (Find at the auto-parts store). That will help keep things from rusting together.
There is also a plastic-coated pipe made for corrosive applications, but that is really overkill, and expensive. You even need special tools to work it.
Thanks for the advice.
So are we talking about regular EMT here?
The OP said "rigid" -- not
The OP said "rigid" -- not EMT.
It should be noted that any sort of conduit -- plastic or metal -- needs to be sleaved where it passes through concrete or rises out of the ground.
Correction
DanH wrote:
It should be noted that any sort of conduit -- plastic or metal -- needs to be sleaved where it passes through concrete or rises out of the ground.
Both rigid PVC and metal conduit can be embedded in, and emerge directly from, poured concrete. No sleevee is required or needed.
PVC must be Sch80 in this case.
This is not what I've read.
Where are you reading that (need to sleeve RNC coming out of concrete)
Interesting
DanH wrote:
This is not what I've read.
I'd be interested to see what enforcable document you read this in.
Before we answer we should have found out a little more. Is this going under a road or other vehicular traffic?
If not, PVC can be down 18".
Is this a single 120v GFCI circuit on a residential property?
If so it can be 12".
As Amish Electric says "Rigid" or Intermediate" metal conduit can be shallower, in this case 6". That is not thinwall EMT, which usually should not be buried at all. You might get away with it in the desert but anywhere that it rains will see it rust out pretty fast.
NEC table 300.5 will define this better.
Coatings for buried rigid electrical conduit
This is a 25' run of rigid galvanized steel conduit 220 vac going from the main structure to a sub-panel in a detached garage. It is on a residential property, in the Chicago area. It is not a GFCI circuit, though I could do that. The garage has a narrow (14") concrete walkway under which the conduit must pass. At both the house and the garage end, the conduit rises out of the ground, not concrete. I have read nothing to suggest that the conduit must be sleaved where it rises out of the ground.
Pipe wrap tape and bury it deep- below the frost line. I think that's about 30" for you.
couplings
Should I use NPT plumber's couplers or the straight rigid couplers? I would imagine the above-ground applications have no water-tight requirement, just mechanical connection.
There is no water tight expectation underground. It is a wet location and conduits always seem to end up collecting water, even solvent welded PVC.