View Full Version : Metal conduit grounding

Mark Muehleisen
06-09-2007, 11:35 AM
Is metal conduit considered a ground between a main panel and a sub panel?

My thought is no but am questioning myself.

Jerry Peck
06-09-2007, 02:45 PM
Is metal conduit considered a ground between a main panel and a sub panel?

Well, the metal conduit is considered the grounding path between the service equipment and whatever the metal conduit is connected to.

The same goes for metal conduit connected between two panel.

When connected to a sub, through a marine cord, its panel, that metal conduit is typically rusted out due to the proximity to water.

Now, though, there are conditions with regard to connecting metal conduit to the enclosures, typically this requires a grounding bushing, especially when the metal conduit is in a smaller concentric knock out located within large concentric knock outs. I even like to see grounding bushings used for eccentric knock outs as those only leave a small portion of the eccentric knock out contacting the enclosure. With 'punched' knock outs or full size knock outs, where the conduit lock nuts make contact with the enclosure all the way around the conduit at the lock nuts bent down locking tabs, well, I've even seen those with arc marks at the contact points.

Use grounding type lock nuts and bushings so the ground terminals in the enclosure can be jumped (bonded) to the grounding lock nuts and bushings on the conduit.

Getting back to your question, though, not many inspectors are lucky enough to inspect a house with a sub docked out back, much less get to go inside and see its panel. :D

Eric Barker
06-09-2007, 03:07 PM

Jerry Peck
06-09-2007, 03:28 PM

Excellent photo of why grounding type lock nuts and bushings are required for that.

With a grounding type lock nut or bushing, there would be a bonding jumper connecting it to ground, to it would still be grounded (not that that would solve the mechanical problem, but at least it would still be electrically grounded).