Is my plan for a 125A subpanel feeder ok?

Question

I'm adding a 125A subpanel immediately below my 200A main. The background is my kitchen has 10, 15A receptacles running on one 15A GFCI receptacle as one 20A circuit. When the over the range microwave died, I decided to go with a countertop MW. This is in addition to a coffee pot, air fryer, skillet, toaster, etc. I'm keeping the old MW circuit for the gas range and a hood.

I'm adding 3 new circuits to handle these existing receptacles. Also changing all kitchen receptacles to 20A TR. Down the road, I'll be adding a few circuits in the basement workshop.

Since my main panel is a Homeline, I bought a Homeline 125A Main Lug 12/24. My plan is to put a 100A breaker in the main panel. The subpanel will be mounted a couple of inches below the main and use a 2" Rigid metal threaded nipple for the conduit.

I'm familiar with branch circuit requirements, but this is my first subpanel. I believe I can use three 2AWG Cu for the two lines and the neutral, then one 4AWG ground. I've estimated this feeder to be less than 18". I understand the subpanel ground is not to be bonded. I also understand torque requirements.

Does anyone see any holes in my design? I live in TN so I'm under the 2017 NEC. Would it be legal to put a band of white tape on each end of the neutral feeder? I want to make sure I can pass inspection when I'm done.

Answer 1

I would go much larger on the subpanel. 24 circuit is a lie. That requires tandems and almost every circuit needs GFCI or AFCI so tandems are not going to happen and you really have a 12-circuit panel. You'll be full before you know it. Breaker spaces are cheap right now before you've wired in the panel. Much more expensive later.

This stack is a revolving door of people asking for help with full panels, and they have a smallish main panel and one or two smallish subpanels they filled up because they "didn't foresee any future need at the time". That's normal if you're not a psychic. NOBODY foresaw "AFCI or GFCI breakers on everything", surge suppressors, solar or EV charging. Really. 30 spaces (60 "circuits" ha ha) is not too large a subpanel.

Now, it will be easiest to simply connect the main and subpanel with as many 3/4" EMT conduit passages as possible. At least a couple high and a couple low on the panel. You'll also need a large passage for the feeder wires which must run together. You can use any feeder wire out of the 75C thermal column of Table 310.16. 100A = #1 aluminum or #3 copper.

Why did I say EMT? Because it's a happy medium between easy to work with and metal. Metal conduit means ground is automagically linked between panels.

Further, circuits currently in the existing panel can be brought over to the new panel simply by extending their hot(s) and neutral wires using THHN wire via the conduits, or if the original wires can simply reach a breaker! (hence putting passages both high and low). Do not extend ground to the other panel.

Why would you do that? Because a new subpanel is a great time to be thinking about generator interlocks. If you wanted one, now's the time. It's a $40-80 upgrade to that subpanel, and you make it the critical loads sub. And if you dislike the new circuits having the option to be on generator (when is that ever bad?), put them in the main using spaces you now freed up.

The idea that "breaker spaces" should be a subject of scarcity is bonkers. They're cheap.

Answer 2

First of all, I don't think you necessarily even need a ground wire if the two panels are connected by a metal nipple. But it can't hurt.

As far as hot and neutral wire size, 3 AWG copper or 1 AWG aluminum will work for 100A. The "just mark the ends with white" rule (NEC 200.6(B)(4)) is for 4 AWG or larger, so that is OK. (Sorry for earlier confusion - my first search found the Canadian rule which is more restrictive.)

I would recommend adding a second nipple so that you have plenty of room to route individual branch circuits between the panels. As long as the panels are less than 24" apart, you only have to worry about conduit fill and not about multi-circuit derate. You could run those wires down the same conduit as you use for the feeder, but it is cleaner if you keep them separate.

You may also, space permitting, want to install a larger panel. 24 sounds like a lot. But the problem is that many new circuits will require AFCI and/or GFCI protection. GFCI protection for ordinary receptacle circuits (which is clearly one of the main points of this project) can be provided at the first receptacle in each circuit. However, 240V GFCI (which is required on many new circuits, though not as much in 2017 code) and AFCI (practically speaking) both need to be done at the breaker, and those breakers are generally not available as half-size (24 vs. 12) breakers.

Note also that you don't have to use 20A receptacles on 20A circuits. Unless there is only a single receptacle (even a standard duplex counts as two) on a 20A circuit, 15A receptacles are fine. Nearly all consumer-grade kitchen appliances use 15A plugs for compatibility.