Grounding Requirements for same structure subpanel

Question

I understand that unless a 250.32(B) (NEC 2014) applies, a separate ground wire should run from the main to a subpanel where the service is in a separate structure.

Still confused however about NEC grounding requirements for the same structure. I am replacing a subpanel and moving the location from the Kitchen to the garage, about 3 feet. The main is 200 amp (grounded and bonded) service and the subpanel 125 with a 3 wire #2 feeder between the two and EMT the entire way in the slab. All 11 EMT runs to the house, as well as the feeder were buried in the slab until an archeological dig to extricate and extend the EMT to the new subpanel. Each of the EMT runs and the feeder were attached to the wire mesh in the concrete by small wire (14 guage or less) within about 6 feet of the panel. I plan to replace this. Not sure if this was for grounding, or just to hold the EMT in place when the slab was poured.

Does the NEC require a separate grounding wire back to the main in this case and/or a separate ground rod by the subpanel?

If not, would a separate grounding rod at the subpanel be a good idea. Not anxious to try and run a separate ground back to the main.

Thank you in advance for any help.

Answer 1

Your ground path is through the EMT conduit. It needs a proper connection to the main panel ground and to the sub panel ground. You could use grounding bushings, but standard EMT connectors meet code.

The “connections” to the concrete reinforcing are not part of a normal grounding or bonding approach. This kind of bond is only used at the service entrance and uses more substantial wire.

One concern with your installation is that EMT conduit can corrode in buried or embedded installations. I have seen conduit rusted completely away after four or five decades of service. This can result in high ground impedance and non-tripping breakers. If you have this problem you should run a new feeder with a proper ground connection. Do not drive a separate ground rod at the subpanel. The ground connection belongs at the service to avoid ground loops and to ensure over current protective devices work properly.

Answer 2

You always have to run a ground ~. Rods will never do.

You must, must, must, in every case run a ground ~ from the main to the subpanel. Doesn't matter if you're running it 3 feet, to an outbuilding or up a space elevator. You have to run a ground ~ or you are out of Code. PERIOD.

Wait. Shouldn't ~ be the word "wire"? Not quite. With rare exception, metal conduit is a permitted ~. That is why you find no ground wires in your EMT work. I love EMT; it's easy, and it's clean. I run it even when I don't have to.

That said, I was caught by surprise by your anxiety about running a separate ground wire. The whole point of EMT is to make additional wires easy to add. I am thinking "why not just fish a ground?" What worries me is that maybe you erred when extending it. If you rendered the EMT no longer fishable, e.g. burying the cover of an LB elbow, that violates Code and you need to go back and fix it. Sorry. By the way, it's also illegal to assemble conduit around the wires. You must pull the wires, alter the conduit, then reinstall the wires, all of which is not that hard, and I do it all the time. This rule "keeps you honest" re: the other rule. If you did shove conduit over wires, pull the wires out (using them to drag in a rope) and carefully inspect them for physical damage, replacing as needed.

Adding a ground won't hurt. You are allowed to parallel grounds, just not anything else. And EMT in concrete can corrode, and that could break your ground connection.

Ground ^'s are a separate issue

Did you think I was going to say ground rod? Again, there are a couple of allowable substitutes.

• A metal water pipe, though this option is fading fast due to use of plastic pipes.
• An UFER ground, which is a ground rod built into a concrete slab using appropriate methods. Concrete guys won't do it by default, but make sure to stick a few in anything concrete. It uses the entire concrete slab (via the re-rod, and the fact that concrete is inherently moist). It's so good you don't even need to do an ohmic test.

By the way, the thing where they tied the EMT to the re-rod at intervals, is not an Ufer. That's just physically holding it in place for the pour.

The right word for all of these is "Electrode".

That said, you don't need an additional grounding electrode for a subpanel if you're still in the same structure, and breezeways count as "the same structure".

One more note on grounding electrodes. The connection between panel and grounding electrode must be a wire, this is one case where you cannot rely on conduits as ground paths.

Answer 3

Grounding can become somewhat complicated, but it really boils down to establishing a zero reference point at the main and sub panels, and is an effective path of return in the case of a short to ground in order to properly trip out the overcurrent protection. So I have posted this picture for reference, note that Building 2 is existing and Building 1 is new.

As you can see it is somewhat complicated, but you can add a grounding electrode system to an existing building and include a full size ground back to the Main Service. The question would be whether your AHJ thinks you building is new or existing since this is a repair and not an addition. You probably need to take that up with them before proceeding.

Note:

1 I am referring only to the 3 wire #2 feeder not the rest of the 11 EMT's.

2 The #14 wire is just to hold the conduit in place.

3 EMT is not a good idea in slabs and underground you might consider replacing it with PVC.

4 EMT is not to be used as a ground path underground it subject to rust and corrosion at the joints. you need to install a grounding conductor in each conduit.

Good luck.