wiring code ? for a detached garage

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That's the part that doesn't do anything for me. What's the real point in any ground rods, etc at the building. Isn't there a ground rod of some sort at the transformer 100' away. How much potential difference could develop between the house and the transformer?
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On Tue, 10 Nov 2009 12:21:26 -0800 (PST), jamesgangnc

A bunch. We typically measured 25-35 volt spikes on our dranitz logger between 2 grounding electrode systems in separate buildings. That is why I always suggest bonding the GES if you are stringing data cables between them.
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jamesgangnc wrote:

If you have a water pipe or concrete encased electrode you have a much better earthing electrode than is at the transformer.
If there is a very near lightning strike, like at a tree, the potential of the earth at the transformer and your house could be many tens of thousands of volts different. Grounding electrodes at your house pull the potential of the wiring in your house nearer the earth potential at your house (which is the potential of concrete basement floors and walls). Even though your garage is very near your house, there could be a difference of many thousands of volts between the earth at the house and garage.
If lightning hits the utility pole there is an even bigger difference between the earth potential at the pole (and utility wires) and your house.
Earthing at the house helps protect against surges coming in on power, cable and phone wires.
Around here I have seen missing copper wire to the earthing electrode at utility poles where it has been stolen.
--
bud--

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That's fine, it doesn't change anything I said.

You need a grounding electrode system, which consists of interconnecting _all_ the electrodes present. Then if there aren't enough present, you need to add more. See NEC article 250 part III, starting at 250.50.
Common electrodes are: qualifying rebar or a 20' length of bare #4 copper wire in the footer of a foundation (required for new construction, otherwise only if accessible), any metallic water pipe that is buried for at least 10', and ground rods. The first one (called a concrete encased electrode, or CEE) is deemed adequate by itself; the other two are not adequate by themselves are require a second electrode (which could be a ground rod).
Hope this helps.
Cheers, Wayne
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