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Grounding Parallel Installations

Question asked by grady graham on Nov 30, 2017
Latest reply on Dec 6, 2017 by grady graham

   So this is a very specific question that requires additional thought beyond what you might first think. 250.122(F) Conductors in Parallel. We all know you size your grounding conductors based off of table 250.122. My question is this. If I have a 4000A OCP and I'm using (12) sets of 600kcmil AL then what size AL ground do I need to pull in each raceway?. If you look at 250.122(A) it states that "equipment grounding conductors of the wire type shall not be smaller than shown in Table 260.122, but in no case shall they be required to be larger than the circuit conductors supplying the equipment." The second line creates the confusion. What size are my circuit conductors. I have 12 sets of 600 does that mean my circuit conductors are 12 X 600kcmil which equals 7200kcmil meaning if I select the 750kcmil AL ground in table 250.122 then in reality my ground conductor is not larger than my circuit conductors. Or would my circuit conductors be 600kcmil meaning I don't have to use the 750kcmil AL wire listed in table 250.122 for the 4000A OCP?

   We know when we parallel ungrounded conductors we are adding separate conductors together to make a larger conductor that can handle the load. But with grounds each conductor should be sized according to its individual ability to open the OCP.

   So which size would I be required to use in this scenario?  A 750kcmil or a 600kcmil?

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