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Hello. Welcome to Just Answer.
From reading your question, it sounds like you are trying to comply with the Code as much as you can.
I'm curious as to why the feed to your main panel isn't grounded. Do you not have a ground wire to the main panel, from a ground rod or a cold water pipe?
Not sure. I am 200 miles from the property at the moment.
I understand. In order to be Code compliant, you actually need a 4 wire feed from the main panel to the sub. Since the building is not attached to the house, you also need a ground rod at the sub panel. If you don't have room in the conduit for a 4th (ground) wire, you actually could reduce the size of the 3 existing copper wires. They are substantially larger than they need to be for the length of the run, and the load you will be placing on them.
Reducing the size would free up room in the conduit for a 4th wire, for grounding purposes.
Unfortunately that is not an option. Can I still do the hook up safely?
I won't call it unsafe if you have a ground rod at the sub panel. But it just wouldn't be Code compliant.
So. This is consistent with what I have seen and read. Let me go through what I understand it to be.
I would hook up neutral in the new box to an insulated bar and keep it isolated from the ground bar which would ground hook up to two 8 foot ground rods 6 feet apart. Circuits from this box could be wired as conventional circuits with neutral and ground.
Yes, other than we don't typically install 2 ground rods for a 100 amp service. I would only use 2 ground rods on a 200 amp service.
Great saves time and money. Will I have any problems from the copper pipe if I leave it unattached to the electrical sytem on both ends?
I don't think so. It should be fine. I wouldn't use a heating pipe for a ground.
Thanks I think that does it.
You are most welcome. Is there anything else I can do for you?