I am building a detached garage that is about 140' from my main service panel. Part of that distance will go through the attic of my existing house from the side of my attached garage to the other side, then about 40' will be through the ground. Could I use THHN SER through the attic to a 2nd service panel before the ground section and then URD between the 2nd panel and the panel on the garage?

Is there a 4/0 wire that can go through a building and then conduit in the ground? I have researched to find that SER should only be used within a building and is not suited for going through conduit in the ground and URD should only be used outside.

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    First question, do you need 200 amps? Second do you have enough excess load to take 200 amps from your main panel/service(have you done a load calculation)?
    – crip659
    Aug 25, 2023 at 13:58
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    I recently upgraded to 400A service. I have 2 200A breakers in my main panel. One goes to the sub panel in my basement and the 2nd one is for my detached garage. Aug 25, 2023 at 18:13
  • You'll need 250 kcmil (next size larger) aluminum to go to a garage. It's not a dwelling and it's not your service size, so the exception you're trying to use doesn't work for 2 reasons. Aug 25, 2023 at 19:08

3 Answers 3


If you install conduit, you can use individual wires (THWN-2, XHHW etc.) in conduit going the whole distance. Pulling cable (bundled wires in an insulation jacket) in conduit is much harder than pulling wires in conduit. In addition, direct burial cable has to be 24" down, PVC conduit only 18" (except under driveways) and only 6" with rigid metal conduit (except under driveways).

Unless you have money to burn, 250 MCM aluminum is going to make a lot more sense than 4/0 copper for 200A (at the 75°C rating) If you were thinking 4/0 aluminum at the 90°C rating, that only applies for derating, since terminals are normally 75°C maximum.

If the copper industry propaganda has got you ready to bleed your bank account dry, note that the terminals on the breakers and panels you'll be connecting to are aluminum, so they are very compatible with aluminum wire. The "aluminum wire is bad" tales have a grain of truth from using a bad alloy, no torque drivers, and copper/brass terminals for aluminum wires on small branch circuits "back in the day (1960's-70's, mostly.) Large aluminum feeder wires have never been a problem, and current rules require using a torque driver on all connections.


TL;DR Figure Out Your Actual Load

Also known as Harper's Law: Buy your wire last.

Another answer already recommends conduit the whole way through. I highly recommend that. But whether you use cable or conduit + wires, figure out first what your actual load will be. That is key for two reasons:

  • Wire Size

Wire size depends on breaker size (can be larger than the breaker but can't be smaller). Whether copper (no, just no) or aluminum, larger wire costs more. 200A requires either 4/0 aluminum (if the entire service feed to the main panel is 200A) or 250 kcmil. But you could power, typically, EV charging, a decent size heat pump, lighting and a whole bunch of power tools with a lot less. Some typical sizes are 60A (4 AWG aluminum) because it is a common inexpensive breaker size and 90A (2 AWG aluminum) because it is a common inexpensive wire size.

  • Load Calculation

You actually need two load calculations. One is for the existing service. That will determine how much power is available for the new subpanel. If your service is 200A and your load calculation shows that you actually need 120A then you have 80A to spare for the subpanel. The actual amount can vary widely, particularly because 200A has been the standard service size for new construction in many places for many years, whether you actually needed 50A or 150A.

The second calculation is for the new subpanel. Figure out everything you want to install. If that works out to less than or equal to what is available, great. If it turns out to be more than what is available, provide specifics and we can help. In particular, tankless electric water heating is pretty much a no thank you because it requires a lot power. But there are other things that can be adjusted as well. For example, EV charging for most people can be done on a 30A or even 20A circuit, which is perfectly fine to do even if the EVSE (a.k.a., the "charger") is rated for 60A, as long as the EVSE is configured properly. It is cheaper/easier for an EVSE manufacturer to have one model that covers 15A - 60A than to have 4 or more separate models, and that also allows (for most of them, but surprisingly not all) changing the configuration later on if your electric supply changes. And if getting rid of tankless water and downsizing your EV doesn't get you under the load calculation limit, there may be other things that can be done to solve the problem.

Update: Based on a comment, the garage has one 200A feed out of a 400A service. Assuming that is the case (Harper's answer discusses some alternatives), there is no need to worry about the main service feed load calculation, at least not in terms of adding the garage, as each building is on its own 200A feed. That being said, a load calculation for the garage is still important as that can seriously affect your wire size. In addition, if you ever add any significant additional circuits at the main building (e.g., tankless electric water heating) then a load calculation will be relevant there as well.

  • Would I need that conduit through the entire interior run, i.e. through my attic? Aug 25, 2023 at 16:16
  • You could use NM cable (a.ka., Romex) through your attic and then switch to conduit + wires for the outdoor part. You could also use metal conduit outside and PVC conduit inside. Lots of options. Aug 25, 2023 at 16:19

I have to say, I'm not lovin' your plan. And the reason is the stuff we're seeing happen with solar and battery, and the stuff California and much of Europe is trying to make happen with V2X. (EV battery feeding house etc.)

The problem is, in one of these 400A services, for cost reasons it's installed as a 2-pack of 200A services. You want to send an entire 200A service to the garage and another 200A service to the house, which is fine enough while the grid is up and stable. However it's going to leave you utterly prepared for grid-down conditions. You'll be able to isolate the garage and run a generator, solar/battery or V2H there... but this will not help your house one bit.

Many people have found their dual 200A panels were set up without thought for this... and thus they are unable to accomplish generator or off-grid power, or are forced to do major rewiring of their panels.

Also garages need absolutely nowhere near 200A service. One particular source of this confusion is EVs - a sparkly new toy that stokes the imagination. Unfortunately part of that imagination is that "EVs need lots of power", which is simply not so - 30A for the first EV and 10-15A for each additional EV is fine. It's a car, not a transit bus.

So the typical modern approach to grid-down power is to have a 200A disconnect that severs the house from the AC line. This has an interlock/auxiliary contact that signals the modern grid-forming inverter (with battery) that the house is isolated off the grind, and it may begin creating a local micro-grid. This in turn allows solar to generate and refill that battery. I can only assume EVs will be used to supplement that system battery.

With the setup you're planning, this will be particularly expensive since you'll need a 400A disconnect, and those tend to be costly exotica. For most of the people we talk to, this is a deal-killer.

So instead I would arrange it so both 200A panels serve the house. One of the 200A panels powers house "critical loads". The critical-loads subpanel also sends 100A-ish to the garage.

Then, the off-grid strategy is to write off the non-critical-loads 200A, open the disconnect feeding the 200A critical-loads+garage, and then have the grid-forming inverter and/or EV in the garage backfeed the entire 200A panel, standing up the critical loads in the house.

I also recommend intact conduit on the garage run, so datacomm wires can be added as warranted to work with disconnect, interlock etc.

  • intact conduit on the garage run Do you mean "a second conduit on the garage run"? Aug 25, 2023 at 20:03
  • The garage will have a 1300sf apartment above and I do a lot of welding and other power intensive tasks. Aug 26, 2023 at 4:26
  • @JaredStoker -- we may need to take a big step back here. Who's your electric utility? (I ask this because they may require the ADU to be on its own electric meter, which'd thoroughly hose you under your current plan) Aug 26, 2023 at 5:03
  • Yeah, they don’t allow a second meter in my area. It has to come from my main. Aug 26, 2023 at 14:08
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    See also the NEC and other laws which require tenants to have access to their own breaker resets, various laws on submetering, and rules that only tenant loads can be on a tenant submeter. And if that isn't interesting enough, Right To Charge (EVs). Aug 26, 2023 at 14:45

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