For reference, I'm an electrician and help manage an electrical shop.
At your main service and at subpanels in detached buildings(garages, shops, etc) you are required to install a grounding electrode system.
There are several different grounding electrode options:
ground rods
ufer(rebar)
metal water pipe in contact with the earth for 10 feet or more
metal in ground support structures in contact with the earth for 10 feet or more
etc.
If these buildings are existing and do not have an available rebar ground, than standard procedure is to drive two 5/8" x 8' galv rods at least 6' apart. Rods must be flush with or driven below the surface. The inspector must be able to see the wire-ground rod connection before they are covered though. If you cover them with a concrete patio before they get inspected, then it sounds like you'll be driving another set of rods the inspector can look at. Rods are usually inspected during a ditch(underground conduit) inspection or service inspection.
If you have a metal water pipe that meets the 10 feet in contact with the earth criteria, then you would be required to also use that as a grounding electrode.
Any grounding electrodes that are present must be used, when none are present, is when rods are typically installed.
+1
This sounds pretty darn close to what is required here in Michigan. I am a state licensed Journeyman electrician (11 years in October) and grounding & bonding have always brought about conflicting views and misunderstandings as requirements and the logic behind them have changed more than a couple times over the years. I agree with Cow, just want to add my $0.02 explanation to what he outlined, maybe your area has similar requirements? Sounds like there is some gray area in the understanding of what is required and when it is required. The following is based on the mixture of NEC (
National
Electrical
Code), MEC (
Michigan
Electrical
Code) and MRC (
Michigan
Residential
Code) electrical codes as it applies to residential installations in my area:
If there are no other means of obtaining a service ground, then the (2) 8' x 5/8" ground rods are your primary source of service grounding. If there is a means, such as rebar or a metal water service pipe like Cow said, then
that is your primary source of ground and the ground rods then become a
supplemental source of ground. The rebar and/or water service pipe offer a less resistant path to ground vs the ground rods which is why they are a more "preferred" option. If a metal water pipe is present, the conductor used for grounding and bonding the service is sized in accordance with the size of the service.
Sidenote: It has always been my understanding that ground rods are only rated for #6 awg. Can anyone confirm?
If you are installing a sub panel, how you handle your grounding dictates how you handle your grounding.
What the heck? Here's what I mean: If a ground wire
is supplied from the main service panel, sized in accordance with the feeders, no grounding electrodes are required in the sub panel as the ground conductor supplied is sufficient. Furthermore, adding grounding electrodes to a sub panel with a ground conductor present from the main panel can be dangerous as it creates an additional grounding path for the main service, aka "redundant grounding." If the path to ground has less resistance
through the sub panel, any unbalanced current on the ground system will use that pathway
instead of the prescribed grounding at the main service panel. Another thing to point out with sub panels - be sure to separate your neutrals and grounds for exactly the same reason. The unbalanced load at the sub panel (neutrals from circuits) should utilize the feeder neutral back to the main panel, not the ground conductor or ground system tied to the sub panel;
your grounds are not supposed to be current carrying conductors. Think of the sub panel as an
extension of the main; it feeds off of the main panel,
it does not supplement the main panel.
If no ground conductor is present from the main service panel, then a means of grounding is required in the sub panel, i.e. ground rods.
In practice, I have always run a ground wire with my feeders. It is my preferred method and I have never had to debate with inspectors over it.
If you have questions about specific requirements in your area, I suggest contacting the inspector in your city/township or a local licensed electrician. In my experience, most inspectors are happy to offer up the requirements they look for you to meet when inspecting your work, they might even send you (the homeowner) a copy of the specific code sections that apply to whatever you are working on. Usually (again, in my experience) inspectors are more critical of the electrical contractor because they should know what is expected and why vs the average homeowner just trying to save a couple bucks. I have seen both sides of the fence.
Hope that helps some.
