Converting Main Panel to Subpanel

Question

Okay, I have a main panel that is a 1978 2-wire system with combined neutral/ground bar and a 100amp main disconnect breaker. I'm Replacing it with new 200-amp service and all load wiring has been updated and includes ground wiring. The easiest transformation is to add a room and convert the exterior wall where the old panel is to an interior wall and simply put the new panel facing the new room to meet code and avoid a j-box. The old panel is also in a kitchen cabinet. The meter box is on the exterior opposite the original panel box but is being relocated to a new ext. location.

I installed a temporary power pole main disconnect with 2-2-2-4 SER to run 25' in schedule 40 from the disconnect to the old panel until the room is built. To convert it to a subpanel, do I remove one of the existing ground/neutral bars and replace it with an insulated neutral bar and put all load neutrals on the insulated bar plus the #2 neutral wire from the disconnect and all grounds to the ground bar? I'm assuming the 2-2-2-4 SER Al is rated to enter the house to the panel.

Other than the #4 ground wire from the new disconnect to the old panel, does the subpanel also need a #6 bare copper ground from the ground bar to a copper ground rod? I already have 2 ground rods at the main disconnect on the temp pole.

Lastly, there is already a 4-wire subpanel that I had previously wired from the original panel to a workshop and put a ground to a copper rod at that subpanel. If I have my wiring description above correct, is any of this going to affect the wiring arrangement at the existing subpanel of the workshop?

Thanks for any advice/recommendations.

Answer 1

I believe you are very close to being in good shape. Yes, to do it properly, the existing 100 amp panel must have the neutral "floated" (insulated) and not bonded to the ground. All neutrals must go to the neutral busbar and all grounds must be connected to the grounding busbar. Since you have 4 wire from the temporary service, you no longer need the ground rods for the 100 amp service. In fact, I think it's probably not even allowed anymore. Your connection assumptions are correct. Will you be gutting the old panel and basically turn it into a big j-box? That's what I've done in the past and inspectors are fine with it. Being in a cabinet may force the issue going forward.

Regarding the other sub-panel in your workshop, if you have 4 wire feed to it, it's the same drill as your other soon-to-be subpanel...float the neutral and use separate wires for neutral and ground. You shouldn't need a local grounding rod there either anymore. I understand the code, and the rationale for not sharing neutral and grounding in a sub-panel, but not why you can't have local grounding. Maybe someone with more knowledge than me can answer that.

Answer 2

Your plan sounds right. Separating the grounds. And 4-wire from main to any subpanel. Using aluminum wire for service and feeder is also wise.

You actually need two ground rods at each building. Unless the one passes the magic test for 25 ohms. But you can't test it, and the cost of hiring it done is more than a second rod. End of the day ground rods are for protecting you and yours, so they're cheap insurance.

You still need the ground wire; that's for returning human-generated current, which is much lower voltage than lightning and ESD, but far too high current for dirt to carry.