Future proofing sub panel: wire size considerations

Question

Current status

• 125 Amp panel outside the house (Eaton BR type)
• 18 of 20 spaces filled
• Only 2 dedicated kitchen circuits, others piggybacking on rest of the house

Remodel plan

• Install a 200 Amp sub panel with 42 spaces inside the house (Square D QO142M200PQCVP)

• Feed this panel using 100 Amp breaker installed in the Eaton BR main panel

• Add new kitchen circuits, and relocate some circuits from main panel to the sub

• service wire (individual 3AWG copper THHN/THWN or 1/0 Aluminum SE-2)

Future proofing

• Eventually this subpanel will become main panel, which is why I opted to go with 200 Amp rating and 42 spaces.

Should I / Can I future proof wires?

• According to ampacity charts, if I wanted to future proof service wire, I would have to go with 75C rated 3/0 copper or 250 Al to safely conduct 200 Amps downstream from the meter. Aside from substantially bigger expense, is this even technically/physically possible? Do the 100 Amp breakers even take a cable that large?

• Perhaps the thing to do is to stop worrying about future proofing so much. After all, I can always move the main panel to the garage and convert it to a sub panel and that way not have to waste the 100 amp rated feeder cable I will be using now.

• Edit 1: Since I am opening the entire wall to install the sub-panel, is it advisable to install as many conduits as there are knockouts to extend from the box through top and bottom plates for future circuits? I wouldn't want the next homeowner to have to reopen the wall.

Curious to hear your thoughts and rationale.

Answer 1

125A not 100A

I would use wires fit for 125A, and then, bring 125A over to the sub panel instead of 100A. There's no reason not to do that. 125A breakers are readily available as are 125A wires.

You can waste your time thinking about copper if you really want to, for a short run it's not that expensive... but why bother? The lugs are aluminum. On BR, the bus bars are aluminum. Making one component copper when all the rest are aluminum seems... superstitious.

Your next main panel won't be

You're thinking your next main panel will be a 200A main-breaker panel, just like has been standard for 50 years. Nope. They changed Code, and now the main breaker has to be outside at the meter.

So if this 200A panel will become your "panel with all the breakers", it won't be a main panel. It'll be a sub panel. The main panel is the place with the first main disconnect past the meter. Realistically given the products on the market, that will be a "Meter-Main" outdoors. That's where the grounding electrode has to go, also.

And that will work out nicely, since you need to change the meter pan and weatherhead anyway to go to 200A.

You would need 200A wire from the meter-main to well, presumably this sub panel you're installing now. Or not, there are several ways to do it.

Wire size and breaker fit

According to ampacity charts, if I wanted to future proof service wire, I would have to go with 75C rated 3/0 copper or 250 Al to safely conduct 200 Amps downstream from the meter.

You are quoting that correctly out of 310.15(B)(16), but you're failing to apply 310.15(B)(7). That says for service or feeder to an entire domicile, it only needs to have 83% of normal ampacity, or 166A. 4/0 aluminum (180A) will suffice.

Aside from substantially bigger expense, is this even technically/physically possible? Do the 100 Amp breakers even take a cable that large?

Don't count on it, and not in aluminum anyway. Check the documentation, but typically breakers allow for 1 size larger than the minimum allowed aluminum size.

If you will need to retrofit a different wire size later, you know what to do -- conduit.

Future access

Edit 1: Since I am opening the entire wall to install the sub-panel, is it advisable to install as many conduits as there are knockouts to extend from the box through top and bottom plates for future circuits?

OH, HECK YES. Provided there is some useful points where wires could enter those conduits.

The conduits can hold a lot more if there are individual THHN wires going through them, rather than e.g. NM cable (which count as a single wire of the widest dimension). So if you provide a LARGE junction box down there, you can do more with less conduit.

Regardless, there's a 4-circuit limit to small branch circuits if the conduit is longer than 2 feet. (Code doesn't say it directly, it is the combined effect of 240.4(D), 310.15(B)(16) and 310.15(B)(3)(a)).

Answer 2

Size the wires based on 100 Amp, but make sure the conduit is large enough for bigger wires. It may make sense to size based on 125 Amp, so you can make this the main using the full currently available 125 Amp (assuming that is your current main breaker).

Equally important is to install sufficient conduits to be able to move circuits from the old panel to the new panel, even if you don't plan to move them all right now. Remember that in addition to conduit fill (physically stuffing the wires through), there are also limits to the number of circuits which can lead to having to upsize the wire - because of that, several smaller conduits can be much more useful than a few larger conduits.