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I have a 150A Square D QO panel that is full and won't allow for tandem circuits. I'm going to take out the bottom 2 breakers and replace them with a 100a breaker to feed a subpanel via 4AWG copper wire. When I move the 20 amp circuits from the main to the subpanel, do I need to move the neutral wires as well, or can I leave them on the neutral bus in the main panel? Seems like its all the same if I left them alone, but not sure if there is a reason to move them too.

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  • Does your local AHJ (Code enforcement) prohibit the use of aluminum wiring? Mar 29 at 0:32
  • Need an 85A breaker for that #4. Whoever told you #4 for 100A is confused. If they sold you the wire, take it back. If it's any length, consider #1 aluminum. The lugs are aluminum, why create a dissimilar metal "problem" (even though it isn't one). Mar 29 at 0:37
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I think that is covered in NEC 300.3(B), All Conductors of the same circuit...shall be contained in the same raceway, auxiliary gutter, cable tray...

The small portion of raceway where the neutral or hot nipples between panels would break the rule.

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You'll need to move them to the subpanel

You'll need to reroute the neutrals to the subpanel along with their associated hots (300.3(B) is the closest thing that applies); this has the advantage that the rerouted branch circuits can occupy a different conduit or cable from the feeder, though, easing derating.

And that 4AWG wire is a size too small

Your other problem is that 4AWG copper is one size too small for a 100A feeder in the general case; you'll need to either downsize to a 80 or 90A breaker or upgauge the wires to 3AWG. Note also that if your AHJ permits it, 1AWG aluminum is much more cost effective for a feeder this size.

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Yes, you must move the neutrals with the hots, since they are part of the normal current loop.

The logic which brought you to that idea, however, does apply to safety ground. The grounds can stay put where they are.

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  • I think I made a mistake in typing. Its 2awg copper. That should be okay for 100a run over 2ft I think? Mar 31 at 1:41
  • @DustinGohmert Wow, #2 copper? Okay, yeah, that's good for 115A of actual load and you can breaker it at 125A (rounding up since 115A breakers are not made). We generally assume aluminum for feeder, but I'll use copper myself when it's only going a few feet. Take care to set the torques correctly, the lugs have a different thermal expansion rate, being different metals. Mar 31 at 3:41

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