Restoring a lost ground to an AC compressor

[Jerry Masters]

I'm having a friendly discussion with my local building inspector as to how to resolve this problem:
An apartment complex constructed in to 70s uses the EMT conduit as a path for ground. This is fine inside, but the contractor also ran the EMT out from the foundation, at ground level, and into each unit's AC disconnect. As you can imagine, 40 years later that EMT has rusted away, interrupting the path to ground.
As each unit only used a 20 amp circuit, and the interior circuits are also 20 amps, my proposal was to make a penetration through an interior receptacle box t othe outside, bond a #12 ground wire to the interior box, and pipe with PVC on the exterior to each disconnect and land the ground. The ground would not be in the same conduit as the phase conductors, but it seems to me that it would ultimately end up at the source if there was a ground fault. Am I over-simplifying this? Any thoughts would be appreciated.

 

[Dennis Alwon]

Can you just pull an egc in the emt? The proposal you are making is a violation of 300.3(B)

 

[augie47]

Unfortunately what you propose would not be Code compliant per 300.3
This post gives you some of the reasoning why:
http://forums.mikeholt.com/showthread.php?t=126280&highlight=spider

 

[Cow]

I'd just drive a ground rod.

 

[stars13bars2]

I'd just drive a ground rod.

You would need to drive two!

 

[Cow]

You would need to drive two!

I'll drive two, as long as I can connect that same ground wire to ALL the AC disconnects!:grin:

 

[Dennis Alwon]

I'd just drive a ground rod.

Okay, it's important that the OP knows you a joking-- I hope you are.

A ground rod is not a fix for this situation at all.

 

[kwired]

replace the rusted EMT with RMC or IMC or pull an equipment ground. Make sure the raceway is run in a way that does not allow water to accumulate within. If you are trapping water within you may have rusted from inside out instead of the other way around.

If replacing with EMT do not use the cheapest EMT you can find - you may be suprised how much longer it may last.

 

[hurk27]

replace the rusted EMT with RMC or IMC or pull an equipment ground. Make sure the raceway is run in a way that does not allow water to accumulate within. If you are trapping water within you may have rusted from inside out instead of the other way around.

If replacing with EMT do not use the cheapest EMT you can find - you may be surprised how much longer it may last.

Me thinks you didn't read the original post to throughly:grin:

you missed the part that the original conduit is embedded in concrete slab.

to be code compliant, a new compleat circuit for each unit would have to be ran to each unit, but in my opinion, 250.130(C) (2002) should be allowed for a case like this I would just over size the new EGC for the extra impedance.

 

[Cavie]

Can you access the rusted EMT at the edge of the foundation? If so, chip out the concrete enough to change over to galv conduit to the disconnects.

 

[kwired]

Me thinks you didn't read the original post to throughly:grin:

you missed the part that the original conduit is embedded in concrete slab.

to be code compliant, a new compleat circuit for each unit would have to be ran to each unit, but in my opinion, 250.130(C) (2002) should be allowed for a case like this I would just over size the new EGC for the extra impedance.

OP needs to clarify that I see nothing that says it is embedded in concrete.

from the OP

the contractor also ran the EMT out from the foundation, at ground level, and into each unit's AC disconnect.

I took that as meaning penetrating the foundation wall and running along the wall to its destination close to the ground.

 

[Jerry Masters]

Thanks for all who have replied. The conduit is embedded in concrete and is rusted completely away, as well as collapsed in the slab.
I am aware of NEC 300.3, but I was reading 300.3(B)(2) which allows the EGC to be run outside of the raceway in certain conditions (specifically, 250.130(C), which seemed to apply). My friendly inspector could only offer the possibility of impedance issues as a concern. I had considered that, but felt that since I would be tapping into a conduit that was in the same locale as the AC, the overall path would likely be of a similar length.
With all due respect, it always surprises me when someone offers to drive a ground rod as a solution to a grounding issue.....these folks should take one of Mike Holt's grounding classes!

 

[hurk27]

OP needs to clarify that I see nothing that says it is embedded in concrete.

from the OP


I took that as meaning penetrating the foundation wall and running along the wall to its destination close to the ground.

yep me thinks you could be right, I had a slab foundation in my head for some reason, guess I need a better ufer to bond my brain LOL:grin:

 

[Cow]

With all due respect, it always surprises me when someone offers to drive a ground rod as a solution to a grounding issue.....these folks should take one of Mike Holt's grounding classes!

With all due respect, it always surprises me when someone doesn't realize it may be a joke.