I am installing a 400A service panel on the outside wall of a garage. For the time being it will only be supplying power to a 125A panel located on the inside wall of that garage. The 125A panel will be mounted inside the wall directly behind the 400A service, and the two will be connected by means of a nipple through the back of each panel.

My question is this: Do I treat the 125A panel as a subpanel and float the neutral? Also, for the EGC, do I run it from the 400A panel and then use a bonding jumper to the 125A? For the nipple, since I'm using a steel close nipple, do I need to use grounding lockrings?

I appreciate your help, I've looked through my NEC and even broke out my NEC handbook. Thanks again.

What you don't state exactly is the location of the main breaker.

I will assume it is in the 400 amp panel.

For the 125 amp subpanel, does not matter, this panel must be considered as a sub.

When you say "float the neutral", you mean not bond it? Correct.

The neutral must be kept insulated and isolated from the metal can and equipment ground. A separate equipment ground bar gets bolted directly to the metal can with machine screws, for all branch circuit equipment ground wires and the feeder ground from the main panel.

The nipple only needs standard bushings, does not need bonding bushings. Only need to bond raceways containing service entrance conductors, for example, those from a meter to a main panel.

The grounding electrode conductor needs to connect to the neutral bar in the enclosure that contains the main breaker, and that neutral bar needs to have the Main Bonding Jumper to the metal can.

From this point on, all feeders need a separate equipment grounding wire.

Hope this helps.