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.
Best Answer
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.