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Subpanel in the garage is currently fed by 10-2 w/ground romex and is grounded both at the main panel via the ground wire and to the plumbing next to the subpanel (underground iron pipe). I want to relocate the subpanel (still in the garage) and upgrade the feed to 100 amps. I would like to just run a 4-wire feed back to the main panel and bond/ground it there. The original construction was a detached garage but now there is structure connecting the house and garage. Electrically speaking, I don't understand what difference it makes to safety ground attached vs detached. That is, if I consider this to be an attached garage am I missing something since the original design was detached?

  • Does detached structure versus attached make a difference? I do not recall ever running across that, except in industrial settings. AFAIK, all subpanels have the same requirements. – wallyk Dec 26 '14 at 21:01
  • Yes @wallyk, there definitely is a difference. – Speedy Petey Dec 26 '14 at 22:09
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There's no longer any difference, aside from the fact that a detached structure needs to be bonded to a separate grounding electrode. In both situations you're now required to run 4 wire cable, and keep the grounded and grounding conductors separate in the second panel.

A detached structure has to be bonded to a grounding electrode, to reduce voltage transients between the structures. This is because the ground "earth" can be at slightly different electrical potentials, even over small distances. Bonding the electrode to the grounding system of the main structure, insures the entire grounding system is at the same potential.

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    Well @Tester101, you do need a grounding electrode at a detached structure, but I disagree that it is to keep the buildings at the same potential. The earth is simply not a good enough conductor for this. The reason for a grounding electrode is the same as with any other structure. This: "Electrical systems that are grounded shall be connected to earth in a manner that will limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operation." – Speedy Petey Dec 26 '14 at 22:14
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    @SpeedyPetey "and that will stabilize the voltage to earth during normal operation." – Tester101 Dec 26 '14 at 23:13
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Making a long ground is a bad practice, not only for the reasons Tester101 mentions, but because there is more opportunity for it to corrode and disconnect.

Generally, you want any ground to be near as possible to the equipment and as thick and deep as possible.

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