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I have a main service panel with two breakers: the main service breaker and a single large breaker to another subpanel in the same structure.

All other circuits are serviced from the subpanel.

My understanding of NEC is that each panel requires two grounding rods at least 6 feet apart.

My home uses PEX but there is a copper main coming in on the opposite side of the crawlspace.

Do I need two full ground rods per panel? So four, total, rods?

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Are both panels in the same building or structure? –  Tester101 Jun 28 '13 at 19:57
    
@Tester101 Yes. –  Matthew Jun 28 '13 at 21:30
    
You can get away with one grounding rod, if it has a low enough resistance. If you use one ground rod it needs to be tested (with a specialized testing device). If you two ground rods you don't need to test, which is easier, so that is why it is so common. –  Brad Gilbert Jun 29 '13 at 20:29
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2 Answers

up vote 4 down vote accepted

Do not connect the sub panel to grounding electrodes. A grounding conductor must be run back to the main service panel. The main service panel is the only connection to a grounding electrode system in a single building without a separate means such as a genset. The underground metal pipe could be used as an electrode, but it sounds distant. So you should install two ground rods near the service entrance and connect those to the service panel.

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The subpanel has its ground bonded to the ground at the service panel. Is there a problem with running the grounding conductor 30' to a rod? The service panel is over concrete. –  Matthew Jun 30 '13 at 13:19
    
I know of no limitation to length of grounding electrode conductors in the NEC, 30' should not be a problem. The conductor must be continuous with no splices or joints, amongst several other requirements, but nothing about length. –  bcworkz Jul 1 '13 at 1:51
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I find it a better model to think of it this way: electricity wants to 'go home' to where it is made. For our homes, this 'home' is the secondary winding of the transformer. If we want the breaker to trip, we want to provide any 'lost' electricity to be able to find a good, solid, low resistance back to that transformer. Hence the desire for having separated grounds and neutrals at that detached building.

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-1, this doesn't answer the question. –  BMitch Mar 21 at 14:43
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