Has anyone ever derated a neutral conducter on a 3-phase circuit feeding a sub-panel? I have a 4 inch EMT conduit with 4 wires in it. I would like to use a three phase panel to gain as much capacity as possible. The phase conductors will be 350mcm and the neutral will be 3/0. The sub-panel will be used strictly for jobsite temporary power and if this is not legal I will just make it single phase and fully rate the neutral and use the 3/0 for the EGC.
neutral derating
- Thread starter basscat76
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As long as the possible maximum neutral imbalance current don't exceed the ampacity of the 3/0, I don't see any violation. The problem I do see is there is no way to do a typical load calculation for the feeders of a temporary power panel, as loads are subject to change at any given instant.
In this case you should probably limit the maximum imbalance by way of either a main ocpd or the total-per-phase single-phase amperage of branch circuit OCPDs. For instance, if terminal-limited ampactiy of the 3/0 is 165A, then either use a 150A m-ocpd or utilize no more than (165 ? 20 =) 8 20A-1P ocpds max per phase. That would give you a max of 24 120V 20A circuits. If your terminal-limited ampacity of the 3/0 is 200A, a 200A m-ocpd would mean you could put any amount of 1P breakers the panel will hold. Only L-N loads contribute to neutral current so line-to-line loads are of no consideration in this regard.
In this case you should probably limit the maximum imbalance by way of either a main ocpd or the total-per-phase single-phase amperage of branch circuit OCPDs. For instance, if terminal-limited ampactiy of the 3/0 is 165A, then either use a 150A m-ocpd or utilize no more than (165 ? 20 =) 8 20A-1P ocpds max per phase. That would give you a max of 24 120V 20A circuits. If your terminal-limited ampacity of the 3/0 is 200A, a 200A m-ocpd would mean you could put any amount of 1P breakers the panel will hold. Only L-N loads contribute to neutral current so line-to-line loads are of no consideration in this regard.
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