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i have just read in an electrical reference book that states a 400 amp exterior service panel must have two ground rods spaced six feet apart. i have searched and searched in the NEC but cannot find that info anywhere. is this true and if so where in the codebook? the closest thing i can find is article 250.24 (3) and it is referencing dual feed services. it says one will do. this service will have one line side feed pair of 350 mcm or maybe 500. can't remember for sure what power company said, but it will not be two sets of parallel lines coming in. one or two ground rods guys?
 

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Licensed Electrician
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At least one ground rod is required by code. In order to install only one ground rod, you must be able to prove that its resistance to ground is less than 25 ohms. If you cannot prove this, you need to install a second rod. Most people install two rods from the get-go because proving <25 Ohms is more difficult and more expensive than installing the second rod.
 
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thanks k buz. good advice. go packers. i just can't quite comprehend your last sentence unless you meant it's cheaper to do it at time of installation rather than a later trip. ???
 

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The testing equipment is rather expensive and the testing is rather specific. It's much easier to drive the 2nd ground rod and be done with it.
 

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(Even if a contractor has to make a second trip to install the second ground rod, that total job will probably cost less than choosing instead a contractor who possessed the needed test equipment to prove that one rod is sufficient and who installed and tested the rod all in one trip.)
 

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thanks k buz. good advice. go packers. i just can't quite comprehend your last sentence unless you meant it's cheaper to do it at time of installation rather than a later trip. ???
The contractor would also have to wait at the job site for the EI to witness the test. Here in NJ I have three days to complete an inspection ( not that that is the norm) but it could happen.
 
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