sub panel in remote building, bond neutral to grou

1. Run separate ground and neutral from the service disconnect to the building.

2. Install two ground rods at the building.

3. Bond my two yard hydrants at the building to the electrical grounds.

I thought this was overkill, but I checked the code and confirmed that both the separate ground wire and the ground rods are required. I also thought that the two yard hydrants should have been more than enough ground, but I added ground rods to make the inspector happy.

Now I think the requirement is different with a panel and subpanel, but in my case (service disconnect at the meter and panel in the outbuilding) that's what code requires.

250.52(A)(1) "A metal underground water pipe in direct contact with the earth for 3.0 m or more (including any well casing effectively bonded to the pipe) and electrically continuous (or made electrically continuous by bonding around insulating joints or insulating pipe) to the points of connection of the grounding electrode conductor and the bonding conductors. Interior metal water piping located more than 1.52 m from the point of entrance to the building shall not be used as a part of the grounding electrode system or as a conductor to interconnect electrodes that are part of the grounding electrode system."

According to table 250.66 from the 2005 NEC, if your largest ungrounded service entrance conductor or equivalent area for parallel conductors is copper 2 AWG or smaller, then the GEC should be copper 8 AWG. If the ungrounded conductor/s are 1 or 1/0 then the GEC should be 6 AWG.

If the sub panel is located in a detached building from where the main panel is, then local ground rods are to be installed at the sub panel and the neutral to ground bond is to be connected in the sub panel. No ground wire run from main to sub panel. I think in this case the sub panel should also have it's own main breaker in it.

If there are any other metallic connections between buildings, like a water line, then a separate ground wire is supposed to be run and the neutral to ground bond is not to be made at the sub panel. All water lines etc. are supposed to be bonded to the ground system even if you have ground rods installed as the main ground.

I have seen different versions of interpretations on this where there are water lines etc. running to the remote building.

It doesn't make a lot of sense though as you kind of get into the same situation as two houses being fed from the same utility transformer. Only difference is where the main breaker resides.

The reason you normally don't want the neutral bonded at both panels and a ground wire run between them is that neutral currents will then be carried by both the neutral wire and the ground wire between the panels if both bond connections are made.

The ground wire should never carry current except for fault current.

Regards
Gary

These are issues which are largely independent of whether the building is supplied with three wires or four wires.

In my opinion, the "right" way to supply a subpanel in a remote building is with 4 wires. Keep the equipment grounding conductor separate from the neutral. Size the conductor in accordance with the breaker protecting the branch circuit conductors. Do not bond the ground to the neutral in the remote panel. Add a grounding electrode system to support the remote building, and bond it to the equipment grounding conductor feeding the subpanel. The service disconnect can be a main breaker in the subpanel, a back-fed breaker, or you can use the "6 motions of the hand" exception in some cases. An alternative in some cases is to use the breaker protecting the branch circuit coming from the main panel as the service disconnect means for the remote building. There are rules for "readily accessible" and/or "in sight" that apply to this latter option.

The code allows a remote building to be supplied with only 3 wires in some cases. In this case the ground and neutral must be tied in the subpanel. I consider this a substandard installation; it is certainly not required.

The grounding, bonding, and service disconnect arrangements are required either way you do it.