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I'm putting electrical conduit underground and there seems to be mixed opinions on whether to use primer along with the PVC cement. There are videos showing the use of primer, and videos not using it.

In the electrical aisles at Lowes or HomeDepot they display Oatey Medium Gray PVC cement but not primer.

The directions on the Oatey medium gray cement do not say anything about using primer.

The Oatey web site for this product type has a video using primer, but it's just some generic video.

I understand that primer is necessary for plumbing PVC because it's important to have a high quality watertight seal, but that is not as important for electrical conduit.

Which makes me wonder how much moisture can electrical conduit tolerate running THWN wire? Seems like you would not want standing water in electrical conduit, but maybe it does not matter and the conduit is solely for protection thus the joints need not provide a completely water-tight joint.

It wouldn't matter if primer was used along with the cement, right? It would make for a better seal. But from a practical point of view, why bother if it is not necessary.

Is there some condition where you would need to use primer, like around a pool?

Thoughts?

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  • Every outdoor conduit is defined as being a wet location. Full of standing water is pretty much a normal, expected, and allowed for by other points in code (wire insulation must be wet-rated) condition. But it's not about watertightness of the joints, it's about strength of the joints.
    – Ecnerwal
    Apr 23, 2022 at 21:49

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NEC 110.3(b) settles the question

...as always. This requires you follow the instructions and labeling for the approved equipment you are using. Which UL approves as part of approving the equipment. This also defines the scope of their testing, i.e. they haven't tested it for off-label uses and cannot vouch for its safety.

UL does not approve videos. They approve sheets of paper full of obtuse language and disclaimers. I suppose if you buy a carton of PVC conduit the sheet would be in there, but most stores sell it by the stick. But it should also be on the Web.

You are correct that watertightness of conduit is a running joke. It will never be watertight because it will never be airtight. Either direct access or barometric pressure changes will draw ambient air into the pipe, which is underground and cooler. Condensation then occurs. All the water protection is in the wire jacket, all the more reason not to damage it... part of the reason for the illegality of assembling conduit over wires.

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  • The installation instructions for Carlon PVC conduit do not mention primer. I'm interpreting your answer to mean that using primer is a code violation - is that correct? tnb.ca.abb.com/en/pdf-catalogues/fittings-and-conduit-systems/…
    – Mark
    Apr 24, 2022 at 19:39
  • @Mark yeah, that's what it sounds like to me. Although It's ambiguous enough that the AHJ could make the final call, they'd probably say "I'll give a waiver for that" LOL.
    – Harper - Reinstate Monica
    Apr 24, 2022 at 21:46
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I have made many PVC pipe joints in a sprinkler system and a few buried electric conduit joins and never used primer; no problem. I did lightly sand ( 400 grit) the surfaces. Laboratory testing( Amoco proprietary) showed UV deterioration of the plastic surface could be a problem for joint integrity/

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