Installing a new 100 amp service panel for a new basement suite in a house serviced by 200amp

Question

I have a house serviced by 200amp panel. I want to develop a basement (separate) suite, it will have a stove, Laundry, dishwasher, and 2 2000W electric heaters & few lights. I plan to put a 100amp(60amp?) service panel in the suite with required breakers which comes with one 100amp main breaker. Can you please share your knowledge on how I feed this panel from the house (200amp) panel? Do I split the main feed to the house before it reaches the 200amp panel or do I put another 100amp breaker in the 200amp panel and connect to it? Alternatively, should I put a subpanel that comes without a 100amp main breaker and put the 100amp(or 60amp) breaker in the 200amp panel and connect to it?

Answer 1

If you have a small panel at the meter that feeds the "main" panel and has space for additional breakers then you can put a 100A breaker there and feed from that directly to your new subpanel. But most older homes (and probably a lot of newer homes too) don't have that, in which case you put a 100A breaker in your main panel to feed the new subpanel. That breaker (wherever it is located) protects the wire going to your subpanel, so a main breaker in the subpanel does not substitute for it. In fact, a main breaker in the subpanel can be any size greater than or equal to the feed breaker size. So you could actually put in a 200A panel if the price is right, and often the price is right because of bundled packages.

You will need a big panel. Not huge, but definitely big. Assuming there is a bathroom, you will need, at a minimum:

• Stove = 2
• General lighting/receptacles = 1 (but I recommend at least 2)
• Kitchen receptacles = 2
• Dishwasher = 1
• Heaters = 2 (2,000W continuous could be a 20A 120V circuit, though that is right at the limit)
• Bathroom = 1
• Laundry = 3 (1 for the washing machine, 2 for the dryer)

So that's a minimum 12, but you'll likely be close to 20 by the time you're done. A "12/24" panel won't work - depending on NEC version, most, if not all, of these will require AFCI or GFCI. GFCI can be easily provided at point of use (bathroom, kitchen, laundry) but not (usually) AFCI, and AFCI and GFCI breakers are not available in half-size.

You can use aluminum wiring to connect the panels, and that will save quiet a bit over copper.

I don't think the subpanel requires a disconnect at the subpanel because it is in the same building. But it can be useful, and a big panel that comes with a main breaker can use that as a disconnect for when you work on the panel.

While upfront cost will be higher, if you have any need for air conditioning then rather than putting in separate units or using the air conditioning from the rest of the house (if you were doing that, you wouldn't likely need the new heaters), consider putting in a heat pump, as that should be much less expensive to run than giant toasters.

And most important of all, load calculation. You must do a proper load calculation when making changes like this. Your existing service may be close to fully utilized already, or you may have plenty of room to spare. Can't tell without a lot more details. If you need a utility service upgrade, better to do that now than find out the hard way (main breaker trips!) later on. If you do need an upgrade, that may affect how you feed the new subpanel as well.

Answer 2

To complement manassehkatz' answer:

You would start by selecting a subpanel with plenty of breaker spaces, because breaker spaces are dirt cheap, and running out of breaker spaces in the future is very expensive, and needlessly so, as the problem could have been avoided by selecting a large panel in the first place (i.e. today). So tell that "thrifty" part of your brain to take a holiday when selecting the subpanel. Don't worry, it will get plenty of exercise later.

The subpanel does not need a main breaker of its own, assuming it's installed in the same building.

You don't run wire anywhere that isn't protected by a circuit breaker. Nothing prevents as much as 10,000 amps from flowing on your service wires, which is why they are guarded and kept as short as possible.

So yes, the feeder to this subpanel will come out of your existing main panel, off a 90A to 125A breaker (depending on the size of feeder you want). You would not use a 60A breaker since 60A feeder does not exist, and 55-65A feeders are prohibitively expensive. The 4 cheapest feeders are 90A, 30A, 100A and 120A in that order. Needless to say I recommend 90A, 100A or 120A.

That is because aluminum heavy feeder has always been proven reliable at the 90A+ sizes (despite what you may have heard about 15-20A small branch circuit wires). The lugs in the subpanel will be made of aluminum, so there's no point creating a dissimilar metal splice.

If you don't need the full 90A, you can pinch pennies by using an inexpensive 60A breaker, at least for now.

Manassehkatz is correct that you will need a new Load Calculation on both the house and the subpanel. If the house cannot support that much service, you're a few years early for the demand-side management kit that's coming in the pipeline, so your choice would be a 400A service. This is done as a 400A meter-main with two 200A main breakers in it, which then feed two 200A panels. Your planned subpanel would simply become panel #2.