Electrical – Paralleling 8-gauge wires in a 220 volt 66 amp circuit

There's really no substitute for painstakingly mapping your house, going to every outlet (by which I mean "point of use", including hardwired loads) and figuring out which breaker serves it. I have done this A LOT. It helps to own a bunch of dollar store nightlights, strings of Christmas lights, whatever you can plug into a receptacle. If I were king, it would be required to be marked on the outside or inside of every outlet.

If you're exceptionally clever, you can put a card with each breaker number on it, at each outlet. Turn off half the breakers, go through and if it has lost power, cross off the still-on breakers, and vice versa. Turn off a different half and repeat. Soon you narrow it down to 1 possibility.

After you've done the mapping, you can think about the likely route of the wires, and ways to access that wiring to add "home runs" back to the box. For instance if your wiring goes Panel-A-B-C-D-E-F-G-H-I-J-K-L, and you can route from point I to the panel, then you run two 12/2 cables there, feed I-L with one cable, E-H with the other cable, and in box D and E, disconnect the cable that interconnects them. So now your segments are Panel-A-B-C-D E-F-G-H-Panel Panel-I-J-K-L.

But this will only be the result of your particular mapping, and the logistics of pulling cable in your particular home. As such we cannot begin to advise you.

** Using the formal definition of "outlet", anyplace a load connects, including hardwired loads like lamps and fans.

Not even close

Look at how service panels work. There are actually two Poles, L1 and L2. Since you are dealing in 120V loads, you can pull 200A off each of them.

A 240V load like a house air conditioner pulls off both at once. You have none, however.

Aside from the 120V issue, you are supposed to radically oversubscribe panels. A family will use the power a family wants to use, how the circuits are subdivided will not affect demand.

The purpose of breakers is to protect wiring. If you subdivide a circuit, say, into 3 circuits, each set of wiring needs its own circuit protector (breaker). That's all that is. And Codes have required a lot of circuit division of late, which is why each bathroom and bedroom requires its own circuit and you are putting 6 circuits in a kitchen.

Tom rewired his kitchen from three 15A circuits to twenty 20A circuits, even provided 2 separate circuits on each countertop receptacle. From the panel it looks like a leap from 45A to 400A, but Tom's wife doesn't know that. She might raise her usage from 30A to 40A as she learns not to be skittish about breaker trips, but that's all.

A lot of your general-use circuits are dedicated circuits with almost no load on them. Fridge, gas range, gas furnace, gas water heater, lighting circuits... These might draw an amp or two yet you are figuring 15-20A for each. This is just the nature of satisfying modern codes, which do a lot of subdividing circuits that would've been combined before. A lot of European homes have 6 breakers.

One thing we do not oversubscribe is big 240V loads that are likely to be on at the same time (i.e. Not electric heat and A/C).

Another type we do not oversubscribe is loads we plan to run 24x7, e.g. A Bitcoin miner will buy a house and (if he listens to me) will spread the servers around the house so the existing 30A A/C can cool it. If he is putting twenty 10A servers around the house, he has to make sure they are balanced 10 on each pole, and provision a full 100A for them, plus the 30A A/C.

In other words we think about the loads that are likely to be on at once, worst case, and fudge in a safety factor.

Make it legal

This studio subdivision, are these mancave/art studio, or do you even have the sparkle in your eye of renting these units out? If the latter, get to the city permit agency and have a lengthy talk about subdivision - if it is legally possible and what utility divisions are required to do it. For instance you will need separate panels for the studios, because the tenant must be able to reset his own breakers without having to call you. And they may need to be (or be easily rearranged into) main panels with their own meter. That's code saying that, not my opinion.

My opinion is, existing units often have these ganky arrangements where a tenant has to call maintenance to reset a breaker, or get into tense fights with landlords over excessive use. Those are terrible which is why they've been written out of most codes today. Just last week we had a landlord in here who evicted her tenant due to excessive electricity use, then admitted the electricity use did not drop after the tenant left, and said "surely it can't be my ________" which it most certainly was. The landlord was a complete ____ obviously, but the landlording business turns people into that. Good meters make good tenants.

And now is the time to be fitting that stuff, it will be much harder later. Aside from the obvious, once the units become rental units, the work must be done by a licensed professional. The moment of it becoming a rental unit is an important detail that defines how much you can DIY.