I have two scenarios involving sub panels and my son (who is doing the work) and I are at odds, so please tell me what you think:
Scenario #1 (his project):
Commercial work; 200 amp, 120/208Y service and the panel is full. We need to place a 100 amp sub panel to add a couple of circuits. In order to do this, I plan to take three existing 20 amp single pole breakers and move them into the sub panel to make room for the 3-pole 100 amp breaker to feed the sub panel. My son wants to leave the three S/P circuit's neutral on the neutral bar in the main panel. I told him no, that all conductors associated with a multiwire circuit must stay together. This meaning that he needs to bring the neutral over to the neutral bar in the sub panel. Am I right here?
Scenario #2 (my project):
Residential work, 200 amp single-phase service, panel is full. I hate using skinnies or piggyback breakers to make space, even though they are legal in this panel. I want to use a sub-panel to accommodate a hot tub circuit. My plan is to pull the 2P30A breaker out for the air conditioning to make room and replace it with a 2P100A breaker to feed the sub panel (yes, the existing GE panel is rated for 100 amp breakers when using type THQL). The sub panel will be mounted directly below the main panel and just be connected with a PVC nipple. I plan to extend the existing #8AL conductors for the air conditioning unit through the nipple into the sub panel, but leave the ground connected to the neutral bar in the main panel. All four conductors for the hot tub circuit will originate from the sub panel which will have the EGC and neutral isolated.
Since the EGC isn't a current carrying conductor associated with the air conditioning circuit, is it OK to leave it alone and just bring the current-carrying conductors into the sub panel through the nipple by themselves?
As you can see, my son and I are butting heads here because he's still having trouble comprehending the EGC/neutral issue with sub panels. Now that he's pushing me, I guess I am starting to question myself at this point. I feel that an EGC can connect to either panel, but the neutral(s) associated with a circuit must originate from the same panel. Any suggestions that you may have for both scenarios will be greatly appreciated. I hope that I win this debate.
Forgetting the fine print of the code, it's good practice to have all the wires that have anything to do with a circuit wind up in the same panel. It's also nice that nothing go through the feeder conduit but the ... feeders
Keep in mind, regardless of the distance between the panels, or the method of connection ... always run the necessary four wires. Depending on a nipple, a screw, or any other hardware for uniting the grounds is unreliable - and if you try that with the neutral (heaven forbid!), you're setting things up for a major failure down the line. (Visit the chat room if you want to hear how I spent my Thanksgiving!)
Oh, I would never even dream of installing a sub panel without all conductors. I've seen enough sub panels with no neutral to make my head spin. $5.00 worth of wire makes it hardly worth cutting that corner and that's with today's copper prices.
I also agree that it's tacky to have anything but feeders in the nipple (that didn't sound good). I plan on butting the sub panels below the existing panels on both jobs with dual 1.5" nipples. Ouch, that sounds even worse, but you know what I mean from a professional standpoint, right?
Back when there were computer rooms we always spec'ed the computer room panels to NOT have a neutral brought to them. We landed the EGC on the grounding bus and used the neutral bus for the IG that went to the machines. That insulated IG conductor went straight to the ground bus in the service disconnect unattached. All real mainframe equipment was 208 or 240, no 120.
A suggestion I'd makre wrt to the resi sub panel job is to think about the next project (the genset/ats) and isolate the circuits most likely to be fed by the undersized genset the customer will want to install.
As to the comm'l job, all branch circuit wires associated should be only in the panel they originate from.
Design-Build isn't supposed to mean design *as* you build.