This is a same building subpanel. There are a bunch of rules that apply to a subpanel in a different building, which don't apply when the subpanel is in the same building. So we get to ignore ground rods, disconnect and some other things. Here are some key items to consider:
Load Calculation
The first step when adding a subpanel is a load calculation. Actually, you need two load calculations. One is for the current utility service. The second is for the new subpanel. A load calculation is a specific formula that takes into account:
- Square feet of the building - i.e., for lighting and other general loads
- Required kitchen countertop circuits
- Required bathroom circuits
- Oven and/or cooktop (if electric) - there are special formulas, so this is not as simple as "nameplate rating"
- HVAC
- Water heater (if electric)
- Other large fixed loads
It is not: Add up the breaker handle numbers - that would be far higher than an actual load calculation.
It is not: Turn on all your stuff and put a clamp meter on the service wires.
One site with an online load calculation that looks good is this one from Ask the Electrician.
A load calculation is normally done when a panel is installed. But if loads have changed significantly over the years it should be done again. A 200A service that was fine originally (perhaps 100A load calculation) may now be marginal (perhaps 160A load calculation) but have nowhere near enough capacity for new loads that you want to add. You can't guess, you have to do the calculations.
If your load calculations show that your service is not large enough to handle what you want to add then you may need a heavy-up (panel and service upgrade). If you already have 200A service then a heavy-up (320A/400A service) can get rather expensive, but there are often things that can be done to mitigate the problem.
Wire Size and Type
Unless (a) the main panel and subpanel are right next to each other or (b) your local code forbids the use of aluminum wires even to feed a subpanel, the normal choice is aluminum. Aluminum wires need to be larger than copper wires but they are much cheaper than copper at the large sizes needed for a typical subpanel. Safety can still be an issue with aluminum at small sizes, but large breakers and panel lugs are designed for use with aluminum wire.
Breaker size determines the circuit size (some rare exceptions for certain equipment, not applicable for a normal subpanel) which determines the wire size. Voltage drop is a non-issue even in a McMansion. So you normally go to a table such as this one from Southwire and look to the aluminum 75 C column. Typical sizes are 2 AWG for up to 90A, 1 AWG for 100A and 2/0 for up to 135A. You generally won't have larger than that for a subpanel off of a 200A service.
Unless your local code says you must do one or the other, you can generally use either individual wires in conduit or cable. Cable or wires must be hot/hot/neutral/ground for any new 120V/240V subpanel. The neutral and ground wires are normally smaller than the hot wires at large sizes.
Subpanel Size/Type
Go big! There are a few reasons:
- You may only need 8 spaces now, but you will inevitably need more in the future. More spaces are cheap now, expensive (another panel) later.
- Most circuits now require GFCI or AFCI. That is often best done in the panel, and most tandem breakers do not have GFCI or AFCI available.
- A big (e.g., 30 spaces or larger) "main" panel is often priced very aggressively in big-box stores and often includes a bunch of "free" breakers, so that the net cost over a small subpanel is very little.
- Since the breaker to feed this new subpanel will fill your main panel, this new subpanel is the only spot to add new circuits elsewhere in the house. Go big!
- Most panels are sized for standard stud bay width. Open space is required in front of a panel, no matter how small (30" x 36"). The result is that a big panel or a small panel will require the same open space and have the same installation process, so very little is gained by using a small panel.
- You don't generally need a disconnect, because this is in the same building as your main panel. But if you get a "main" panel because of all the other reasons then you do get a disconnect (that 200A main breaker) in the subpanel at no extra charge. That is handy if you want to work in the panel and don't feel like heading inside to shut it off from the main panel.
Any panel has a bus rating. If it is a "main" panel then that bus rating will be at least as high (there are reasons for it to be higher) as the factory-installed main breaker. You can always use a panel with a higher rating than your feed, and you can always install a panel as a subpanel even if it has a "main" breaker in it that is higher than your feed. Why? Because the limiting factor is the feed breaker in your main panel. If that breaker is 90A then your 225A bus subpanel that has a 200A main breaker will still stop at 90A, protecting the panel and the 2 AWG aluminum wire. (There is one exception to this rule, but it is really for a true main panel - the main breaker has to be no higher than the utility feed. So if you have a 100A utility feed and you replace the panel but don't do a heavy-up then you have to use a 100A main breaker. Not applicable to the "add a subpanel" situation.)
Now that I have answered this in a way that, I hope, it can be applied to a variety of similar situations, here are some specifics:
Is a 125amp panel sufficient?
As noted, that will depend on the load calculation. If a load calculation for the subpanel <= 125A then yes. But a 200A "main" panel will probably be a better deal.
If so, wouldn't I then just put a 125a breaker in 2 slots of my main panel to attach the subpanel service wires to?
Correct.
Do I need to buy cable that has 3 conductors and a ground, or could I buy a cable with only two conductors and a ground? (Obviously, I am not yet sure how to attach the conductors to the breaker at the main panel or to the subpanel. Where would I attach that 3rd conductor?).
Always 3 conductors (hot/hot/neutral) + ground. The hots connect to the breakers, the neutral to the neutral bar and the ground to the neutral bar (if in a true main panel) or to the ground bar. In your subpanel, the hots will connect to the lugs (if no main breaker) or the main breaker, neutral to neutral bar and ground to ground bar.
Whether I need 3 or 4 conductors plus a ground, what size would my wires need to be? Size of ground? (My wire run to the subpanel will be 50' long. I was considering buying a 3-3-3-5 Cu cable, but wonder if that would be overkill in size, maybe unnecessary conductor and cost.)
Aluminum, size depending on feed breaker.
I am guessing what I listed above will take up 8 slots in the new subpanel. Do you think I shooting too low to only purchase a 12-space panel? (the shop is only 13'4" by 17'4")
Go big! Minimum 20, but practically speaking a 30 to 42 space "main" panel.
Unless I go around the edge of the foundation inside my basement (which would increase my wiring expense by about 50%), I will have to go through joists to get to where the subpanel will be. I guess that will mean that running individual wires is out, since I would not be able to put them through conduit, right?
Conduit can go through joists too.
