Grounding distant sub panel to main panel earth ground

[curtball]

first, I'm an electrical engineer and not an electrician and need some advice on a home project. I am consulting with a licensed electrician as well. The project is to power a distant (approximately 400') one bdrm structure (600 sq ft) from a 60 amp 240 circuit from the main house main panel. There will be no dryer, range or water heater as these will be propane. 3 #3 THHN cables have been run through 2" schedule 40 PVC conduit. The neutral bar in the sub-panel is isolated from the panel box and earth ground (except back at the main panel). The green and copper grounds in the sub panel are earth grounded via the structure's copper water pipe into the ground. I had planned to run a #8 green THHN cable between the main box ground and the sub-panel ground but I am having difficulty pulling that wire. My questions are: 1. Is this ground circuit between this distant sub-panel and the main panel necessary and/or required? 2. If the ground circuit is required, can the neutral at the sub-panel be tied to the sub-panel earth ground to serve as the ground. I know you wouldn't do it that way if the main and sub-panels are in the same structure.
Thank you for indulging me.

 

[George Stolz]

I am closing this thread, in accordance with the Forum rules. This Forum is intended to assist professional electricians, inspectors, engineers, and other members of the electrical industry in the performance of their job-related tasks. However, if you are not an electrician or an electrical contractor, then we are not permitted to help you perform your own electrical installation work.

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Based on your post, I advise you to hire this work done, and at the least, to have it inspected.