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Old 09-02-2007, 12:36 AM   #1
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If im using 3 #6 stranded wires thhn/thwn from a main to a sub panel in 1 1/2 inch conduit underground. This would be 2 hots and a neutral. I will be grounding from the ground rod at the sub. Will this be all the wires i need for 120 volts. I was told i needed 4 wires in the conduit.

Last edited by apeman76; 09-02-2007 at 12:39 AM. Reason: change wording
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Old 09-02-2007, 07:32 AM   #2
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Your best bet is to run four conductors to a sub-panel; hot, hot, neutral, ground. In many/most cases four would be code required. In only certain applications a 3-wire feeder is allowed.

You are under the VERY common misconception that the ground rod is "grounding" anything. It is NOT. The ground you would get if you ran a 3-wire feeder is the bond between the neutral and the ground at the panel, same as a main panel.
With a 4-wire feeder the safety ground is the grounding conductor run with the feeder conductor.

In ANY case a ground rod(s) is required at ANY detached structure with a sub-panel, regardless of what feeder is run.

Sometimes I feel like if I answer any more questions it is like someone trying to climb over a fence to jump off a bridge and me giving them a boost.
Answers based on the 2008 & 2011 NEC.
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Old 09-02-2007, 07:36 PM   #3
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i noticed you said
In ANY case a ground rod(s) is required at ANY detached structure with a sub-panel, regardless of what feeder is run. how do i know if i need more than one grounding rod. at the sub?? there are no inspectors for our small town thanks
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Old 09-02-2007, 08:22 PM   #4
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Any sub-panel served by a 3 wire or 4 wire feeder to a detached structure requires a ground rod (probably 2) to be driven to make a grounding electrode system for the detached building just as Speedy is saying.

If no inspection then you will need to drive 2 ground rods.

Also note that if you keep the 3 wire feeder you must bond the neutral and ground to the metal of the sub-panel can using the bonding means supplied with the sub-panel. Sometimes this is a green screw but depends on the panel maker. Also you cannot have now or in the future any other metallic paths between structure/dwelling that has the service equipment and the building with the sub-panel. This means no water lines or phone lines etc... If you run a 4 wire feeder then neutral and ground are seperated and you can have other metallic paths.

Go here and scroll through to find your exact situation. It is the 8th pictorial example in the article.



Last edited by Stubbie; 09-02-2007 at 09:14 PM.
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