225A HOML2225 Subfeed Lug Block in 200A main breaker panel at service entrance feeds 200A breaker subpanel w/ 40' 4/0 AL SER cable in buried conduit

Question

From this site, and others, I understand that NEC allows powering a Square D Homeline 200A sub-panel from a HOML2225 225A Subfeed Lug Block installed in a 200A Homeline main service panel, using 4/0 AL SER cable sized for a max 200A current load as limited by the main service panel breaker, and running in PVC conduit under the garage floor. Please note that although the HOML2225 225A Subfeed Lug Block occupies four (4) circuit breaker positions in the main service panel, this is NOT a current limiting circuit breaker, but rather provides only a subfeed lug capability from the two (2) bus bars which, in my case, is limited to 200A by the main service panel breaker. Square D manufactures the HOML2225 subfeed lug block (rated @ 225A max.) for use in their Homeline series fuse panels only, which are otherwise not provided with a conventional feed-through lug capability. My main service panel is located inside a newly constructed detached garage w/ ADU above, and is connected to an exterior service meter box / disconnect, completely separate from my residential service. The sub-panel's 200A circuit breaker will function primarily as a conveniently located ADU sub-panel disconnect, offering only redundant conductor thermal protection while feeding the ADU branch circuits to an 800 sq-ft living space located over a 2-car garage adjoining an RV bay, where the 200A main service panel is located. I understand that using a 200A sub-panel to separately power this ADU is "overkill", and that the ADU could have been adequately served with a smaller capacity subpanel and a slightly smaller feed cable cable. But I was able to purchase the two identical 200A Homeline panels at a significant cost savings, and EACH panel provides 40 spaces/80 circuits. I would NEVER expect the ADU, having only a "normal" electrical kitchen layout, small sitting room, small bedroom, and small bathroom, to ever even come remotely close to maxxing out the structure's entire 200A service capacity, thereby allowing ample current capacity for the remainder of the garage lighting, heating, and power outlet requirements to be fed from the main service panel in the RV bay.

Any comments regarding my proposed electrical configuration would be greatly appreciated. Thank you.

Answer 1

You may not use 125A feed-through lugs (and corresponding wire) in a panel fed by 200A service. That would be prone to dangerous overloading. You can do some things sort of like that with conductor taps, but not with 40 feet of wire and a 200A breaker at the destination.

You need either a breaker (instead of lugs) sized for the feeder wire (if sending less than the whole service), or else lugs and wire sized for the whole service.

Answer 2

Your service is only 200A. So first of all, it is pointless to send more than 200A to a subpanel because even if everything in the main panel was totally off and never used, you still couldn't use more than 200A.

But aside from that obvious point, you have to actually consider Load Calculation. Any time you add large loads, you need to figure out:

• How much power is available?
• How much power do I need?

Available is determined by taking the 200A utility feed/main panel size (which is a very standard size) and subtracting a formal Load Calculation of your existing loads. That is a non-trivial computation that takes into account:

• Size of your house
• All hardwired electric appliances (water heater, HVAC, etc.)
• All other major known loads (e.g., clothes dryer)
• Cooking equipment (there are special formulas so if you have a cooktop and an oven that are separate the amount you actually include is often less than the sum of the two loads separately)

and a bunch of other things. If that is 100A then you have 200A - 100A = 100A available. If that is 150A then you have 200A - 150A = 50A available.

Then you do a similar calculation for the ADU. That is what you need. If what you need is less than what is available, great! If what you need is more than what is available then you have to look at specific ways (there are many) of lowering either the existing load or the ADU load.

I really see only three ways that it is remotely likely that the subpanel will end up over 100A:

• Tankless Electric Hot Water - No, just NO. Don't do it.
• EV charging - depending on setup, that could be in your main panel or could be in the subpanel. You may not have any now, but there is a good chance you will want to add it in the future. That can be anywhere from 20A @ 240V to 60A @ 240V depending on many factors. But 20A is actually enough for most people, most of the time, so that is often a piece of the equation with some wiggle room.
• Electric resistance heat - Don't get it. Use a modern heat pump (electric but much lower load) or, if available, natural gas.

Once you've figure out how much you can safely (according to code) supply, then you figure out what kind of wire to run. Even then, you don't need to max. out. For example, if your existing house uses 80A and your ADU needs 60A, it probably doesn't make sense to put in a 120A feed. If instead you put in a 90A feed you can save a little more on the cables and still have 50% expansion room.

But a 200A or larger feed? Almost never makes sense. The only way you could make use of more than 200A total is with a utility upgrade, typically to 320/400 service. The 320 is based on 80% of 400, and 400 happens to be exactly 2 x 200. In fact, in most residential situations if you have that large service you end up with 2 200A panels, often next to each other. But just not necessary here unless you are playing the tankless electric hot water game (please don't do that).