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I just bought an older house. It looks like it was upgraded at some point to 200-amp service, with a new load center, but most of the old 2-conductor (no ground) wiring remains.

The 200-amp main panel is in the garage, on the same wall as the meter. We anticipate a number of projects on this house over the next couple of years, including converting the garage to a master suite.

It's not ideal to have all the breakers in the master bedroom, we plan to replace the older wiring over time, and I'd love to upgrade to a plug-on neutral load center, so I'd like to install a new load center in the basement as a subpanel.

Current Plan:

  1. Install 200-amp breaker in main panel.
  2. Install 200-amp main breaker load center in the basement.
  3. Run 4/0-4/0-4/0-2/0 Aluminum SE-R from main to sub. It's about 20' as a straight shot, but 40' if I go up into the attic and back down, because the main panel is on the opposite side of the garage as the basement wall.

Some Questions:

  1. Can the 200-amp main feed the 200-amp subpanel and stay up to code? An electrician told me it can't.
  2. Is 4/0-4/0-4/0-2/0 Aluminum SE-R sufficient? I've seen answers for similar scenarios suggesting 250 kcmil, but I don't know why.
  3. Am I doing anything crazy?

Some Details:

  • Main Load Center is a Homeline HOMC30UC.
  • Feeder is Homeline HOM2200BB.
  • Subpanel is Eaton BRP40B200V25.
  • This is in Cincinnati, Ohio.
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Is feeding a 200A sub from a 200A main legit?

Yes, and you don't even need another 200A breaker. The 200A breaker alread present in the panel will suffice, and so you can use a subfeed lug kit as DrSparks advises.

For that matter, if the main breaker were out at the meter (e.g. a meter-main), you could simply "tee" off it with dual 4/0 to two main-lug panels. Panels don't need main breakers, though if they're in an outbuilding they need a disconnect switch, and choosing a main breaker panel is a cheap way to get a disconnect.

Is 4/0 correct?

Yes, 4/0 Al is sufficient. The reason for this is 310.15(B)(7)(1), which allows 4/0 for 200A service wiring due to a favorable derate. And then, 310.15(B)(7)(3), which says feeder never needs to be bigger than the service wires themselves, because that would be stupid. NEC is not about stupid.

Since your service wires are allowed 4/0, feeder off that service need not be bigger than that, even if it supplies less than the entire dwelling.

Is this crazy?

No, but it's unnecessary. You don't need to bring over the full 200A. The fact is, in any random home, if you subtract the large 240V loads - range, dryer, water heater, heat pump, hot tub and EV charger -- everything else will fit very comfortably on a 65A panel. If that makes no sense, it's because of the common belief that circuits each draw what the breaker handle says. Not at all, they're hardly ever loaded anywhere near that. In fact, they're rarely loaded at all. Power companies' rule of thumb is the average house draws 1KW on average - really. 4 amps @ 240V. (8 amps @ 120V).

But don't take my word on it, do a Load Calculation on the non-240V loads.

So, this suggests a simpler and cheaper plan: Leave the large 240V loads in the original 200A panel. There aren't very many of them. Have the new panel be for "anything/everything else". 65A would be plenty; 100A would be overkill; 120A would be gross overkill. All those breaker sizes are affordable compared to 200A. We don't get the 310.15(B)(7) derate*, so 120A takes #1/0 wire, 100A takes #1 wire and 65A takes #4 wire. (we're talking aluminum here). 65A and 120A breakers aren't made, so round up to the next available size (70A and 125A).

The gory details on 310.15(B)(7)

310.15(B)(7) 120/240-Volt, Single-Phase Dwelling Services and Feeders. For one-family dwellings and the individual dwe11ing units of two-family and multifamily dwellings, service and feeder conductors supplied by a single-phase, 120/240-volt system shall be permitted be sized in accordance with 31 0.15(B)(7)(l) through (4).

(l) For a service rated 100 through 400 A, the service conductors supplying the entire load associated with a one-family dwelling. or the service conductors supplying the entire load associated with an individual dwelling unit in a two-family or multifamily dwelling, shall be permitted to have an ampacity not less than 83 percent of the service rating.

(3) In no case shall a feeder for an individual dwelling unit be required to have an ampacity greater than that specified in 310.15(8)(7)(1) or (2).

83% of 200A is 166A. The smallest wire that will cover that is 4/0Al at 180A. That's why we pick it.

Since the service wire from (B)(7)(1) only needs 166a ampacity, (B)(7)(3) says the same is true of the feeder, so it can also be 4/0.



* Well, we do get the derate from 310.15(B)(7)(3), but it only gives us the ability to use 4/0 for that 100A feeder, not #2.

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  • Yeah, I think the disagreement between your answer and DrSparks' is in the interpretation of "feeder for an individual dwelling unit" Sep 24 at 11:33
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    I'm guessing your 310.15(B)(7) quote is a direct copy/paste. Who knew the NEC was written in leet speak: dwe11ing???
    – FreeMan
    Sep 24 at 12:18
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    I really appreciate all the detail! It slipped my mind while writing the question, but part of my initial motivation behind the 200-amp main beaker sub panel was the ability to ultimately make that the main panel in the future without adding a main breaker, running new wire, etc. I'll have to give that some more thought, probably after some inspection of the existing wiring for the current 240V loads.
    – Travis
    Sep 24 at 14:53
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Yes, you can certainly do that. However, you don't really need a 200 amp breaker on the bus bars as the bus is already protected by a 200 amp main breaker. You can get the HOML2225 sub feed lug kit. It looks like a circuit breaker but it has no handle. It simply allows you to tap the bus bars to add a sub feed, and comes with an additional neutral lug as well.

As for the 4/0 SER, that won't do. Since all of the loads associated with your home do not pass through the proposed feeder, you can't use the special ampacity allowances prescribed by the NEC for dwelling unit service entrance and feeders. You'll have to up it to 250 MCM aluminum.

Perhaps another option would be to do a 150 amp sub panel. You would need at least 3/0 aluminum for that. The 4/0 SER is more readily available, so you could use that as well.

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    Come to think of it, the 4/0 SER will do -- I did some more research into the language into 310.15(B)(7)(3) (now 310.12(C)) and it seems that Harper's correct on this one; while I don't have any Handbook text handy to point at, this NCDoI/SFM interpretation does point in the direction of Harper's take being more common/correct Sep 25 at 15:35
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    Also, as long as the feeder is limited to 180A of computed load, 4/0 can be used even in the absence of 310.15(B)(7)(3) as the 240.4(B) "round up rule" permits it to be protected by a 200A breaker Sep 25 at 15:45
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That is not correct, pertaining to the use of 4/0 SER. You ABSOLUTELY CAN use 4/0 in the situation described. While correct that the entire load for the dwelling unit will net be carried thru the new sub feed, that does not change the ability to use 4/0. As 4/0 IS permitted for use, by the sections quoted above, (and most likely is being used) in the segments of you system that do carry the full load. Such as the over head or underground service entrance conductors. You are not required to use larger conductors for feeders or branch circuits than the service entrance conductors. (Only exception being with very long wire runs where voltage drop issues need to be addressed. ) It would defy logic to require a conductor feeding < 100% of the dwelling's load to be sized larger than the conductors that actually DO carry the full load. So stick with the 200a sub if that is your desire. There are no code issues with the 4/0Al.

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