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I'm wiring my house and would like some feedback on the sizing and routing of conductors between the exterior service entrance and an interior subpanel. On the outside, I have installed a combination service entry device--aka CSED or meter main--by Square D (QC816F200C) with a 200 amp service disconnect. This panel features 8 slots, which will serve a whole-house surge arrestor (SqD HEPD80), a back-up generator power inlet (Eaton EGSPIB30), the well pump, and a branch feeder to power an outbuilding. My plan is to utilize the factory-installed feed-through lugs to then power the house subpanel, located approx. 35' to a central point in the interior. The subpanel is also by Square D (QO130M200PC) and came with a factory-installed 200 amp main breaker. The line between must pass through a sealed and conditioned (but unfinished) crawl space. Can anyone help me size the conductors, and let me know if they must be run in conduit? Many thanks.

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Conduit would be a good idea, even if not strictly required

The main advantage running wires in conduit gives you is that you can put fatter wires, or more wires for something this large even, in if you find that your original run was insufficient. In fact, a 2.5" EMT provides plenty of room for 3 250kcmil aluminum XHHW-2 conductors for now, and even expansion room for 3 more of them should you, or the next bloke who wants this place, want a 400A service later on, in addition to being the obligatory equipment grounding conductor. If EMT is too costly an option, 3" schedule 40 PVC run within the structure could work as well, although you'd then need to run a separate equipment grounding conductor.

If you do go with the cable route, I would go with a 250kcmil-250kcmil-250kcmil aluminum SE-R cable; this gives you the full 200A, but limits you to that all the same.

That's an awfully small subpanel you have there

A 30 space panel is rather on the small side; 42 or even 60 spaces would be a much better use of your money and time here, given that it's far cheaper to buy the spaces now than it is to rip out and replace a too-small panel down the road. Also, you can use a main lug panel on the inside of the house as it's a subpanel in the same structure (so it doesn't need to provide its own disconnecting means) and is not fed by downsized tap conductors (which are protected from overcurrent by the downstream OCPD).

  • Out of curiosity does OP need a main breaker panel on the inside of the house? – Harper Sep 2 '18 at 18:05
  • @Harper -- no, he does not – ThreePhaseEel Sep 2 '18 at 18:39
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Your plan of using the thru-lugs is a good one, but necessitates 200A rated wire. That'll be 250kcmil Aluminum. In these sizes, aluminum wire is fine; heck, the lugs are aluminum.

You mentioned a lot of parts specifically but not a generator interlock, so I trust that's in there. This is mandatory.

Spaces, spaces, spaces

I'm concerned that you're already out of spaces in the CSED. Then you also want a 30-space panel, which seems smallish. Please -- heed ThreePhaseEel's advice. I'd even go further and say wildly oversize your panel -- I feel comfortable when I finish with 60% of spaces unused.

At least once a week someone comes in here and says "Help, my panel is full". And what caused that is the person buying the panel back when it was installed. That is where you are right now. So I am from the forum's future warning you that saving money on panel spaces is false economy.

I notice QO doesn't make 200A panels over 30A - if that's holding you back, you are allowed to use panels with higher amp ratings e.g. 225A or 250A - your 200A main breaker will protect it just fine.

The only thing I'll alter from TPE's advice is there tends to be a painful price jump from 42 to 60, so I'd go for a 42-space panel and reserve space right next to it for another 42. Seriously.

I notice that all the items you are buying are sold by Home Depot. One problem dealing with them is their selection is quite poor and they often "force" you into sad choices. Buy from a for-the-trade electrical supply that specializes in QO, and you'll be able to get anything you want. Most are happy to deal with a knowledgeable public (not a problem in your case) and if you discuss your dissatisfaction with the big box stores (a feeling they share), you are likely to get favorable pricing too.

Conduit

I work 99% in metal conduit, and it is a sheer pleasure. I cannot recommend metal conduit more highly - and TPE got the size right -- it's more than is absolutely required, but the headroom will make the pull easier for you, and that's important when you DIY.

Metal conduit immediately resolves the issue of grounding. The conduit provides a hard grounding path from the CSED metal to the subpanel metal. Done. Make sure your neutral-ground bonding screw is pulled in the subpanel, and think no more of it.

After that, in conduit the wires are just easier to work. Multiple wires in a jacketed cable is like wrestling an alligator. In conduit each wire can flex and slide independently. It helps to have a minimum (or no) elbows to pull around and make all the turns conduit bodies.

I randomly web-price 250-250-250-3/0 SE-R alligator cable at about $4.40/foot and 250kcmil XHHW-2 at $1.15 - so conduit wire is $1/foot cheaper than cable.

  • Generally, 42 space loadcenters don't come with feed-thru lugs, unfortunately (the only >8space loadcenter I've seen with feed-thru lugs is the QO12436L200TFT, which is only a 24 space LC and a bit pricey at that. At least it's convertible to main breaker...) You have to get into full-on industrial panelboards to get those sort of fancy config options...*sigh* – ThreePhaseEel Sep 3 '18 at 3:13
  • That's sad, the panels certainly have the room. I'm getting downright defensive of my 11" wide 30" tall CH main lug boards... – Harper Sep 3 '18 at 3:17
  • Thanks to both of you for your detailed comments and advice! – Brett S. Sep 4 '18 at 16:24
  • Actually one follow-up question: if I were to plumb the run to the subpanel in metal conduit, is 2.5" the minimum by code for three 250kcmil XHHW-2 conductors, or could I go with 2"? Thanks again. – Brett S. Sep 4 '18 at 16:45
  • @BrettS. Sure, Code permits smaller. Code minimums presume you are an experienced professional with the pulling tools and the grease. They are saying "more fill than this, even pros can't do it!" The reason we recommend oversizing conduit for DIY, is that most DIYers don't have an electrician's truck full of pulling tools, or the experience not to wreck cables etc. Oversizing makes the pulling task much easier and more in the DIY skill range. Also future expansion. Pipe is cheap. – Harper Sep 4 '18 at 17:03

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