Hi Joe,

You didn't state the group -- so first, check your switches' or enclosures' literature versus the class, division, and group. Some enclosures or enclosed devices require a much smaller minimum distance to the nearest conduit seal (I'm looking at one that says 2 inches if group B, 18" if group C or D).

If you don't have any special minimum requirement and are proceeding according to the code default, it's only necessary for the seal to be within 18" of each box.  If the boxes are side by side and there is only one 18" conduit running between them, only one seal is needed.  Actually, that conduit could be 36" long with a seal in the center (but that's pushing it).

The purpose is to prevent hot gases (products of explosion) from building a pressure wave that can blow the conduit apart -- so, looking at your conduit arrangement, follow the path of the gases from each box.  If you can travel more than 18" (or whatever the minimum is for your particular box, class, division, and group) without hitting a seal, you need to add one.

Good on ya,

Goober Dave

Oops, for got my code citation:

NEC (2008) 501.1(A)(3) "Where two or more explosionproof enclosures for which conduit seals are required under 501.15(A)(1) are connected by nipples or runs of conduit not more than 36" long, a single conduit seal in each such nipple connection of run of conduit shall be considered sufficient if located not more than 18" from either enclosure."

Goober Dave

Dave,

What if both switches are running to a J-box in a single conduit run? See attached. I still need to get with my designer and sort out his intent, he's a bit new to this. Both of the switches shown in the image are sharing a conduit run back to a J-box somewhere else on the skid. Can both of these switches hide behind a single seal, or do we need to do it similar to what he has shown? I understand that we will need a second seal near the box, within 6" according to the manufacturer.  

Joe Hasik,
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• http://files.engineering.com/getfile.aspx?folder=19ce4a98-97bc-4dbd-9eaf-a9

Assuming "[o]nly explosionproof unions, couplings, reducers, elbows, capped elbows, and conduit bodies similar to L, T, and Cross types..." are being used in the diagram, you still have a potential problem.  Although it appears at first glance something like an M.C.Escher drawing to me, what we can't see is the fitting in the vertical conduit run from the "T" conduit body.  If it is another seal, then technically you are Code compliant although it is a far more complex installation than necessary. If the fitting is something else then you don't appear to have a seal within 18" of the enclosure that requires it in that part of the conduit run. See the last paragraph of 501.15(A)(1).

Yah, the picture is a little hard to comprehend for me -- can't tell what's on the other side of the enclosure or what's above the T body as rbalex pointed out.

If you don't have a complete sketch to post that shows and names all fittings and enclosures, try this:

Start in each "box" that isn't listed in rbalex's first line above.  Travel down each conduit that leaves such a box. If you travel more than the code-required or manufacturer-required distance OR if you reach another box, you've missed a required seal.

Good on ya,

Goober Dave