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First time posting on Home Improvement, please let me know if I am missing anything. I don't do electrical but my building designer needs some info to add to the blueprints (I thought this there are some standards and common install notes, but whatever).

I am wanting to build an ADU (in-law home) in the backyard with its own electrical meter with its own account. SMUD (our electrical company) wants us to install a duplex box and then wire our main house panel. The duplex box will then feed into the main panel for the ADU. The main house currently has 200Amp and would like the ADU main panel to be 200Amp.

Do I need a 200Amp Duplex box or 400Amp box?

What wire do I need from the duplex box to the ADU main panel (wire will be underground)? According to some sites online, wire needs to be aluminum #4/0 AWG in a 1.75" conduit. Is that true and is this recommended? or should we consider something else?

Theres a 125Amp box with description that we can add 125Amp to 225Amp with a 225 Amp branch tenant provisions 100,000 AIC. What does that mean? Can I use this box to give two branches of 200Amp main panels?

Thanks for your time on this!

  • Right off the bat, not copper, unless you own a copper mine. Feeders like that are what aluminum is for. Also the lugs are aluminum, so why create a dissimilar metal problem when you don't have to. – Harper Mar 24 at 22:43
  • Thanks @Harper, didn't know its not copper, so then #4/0 AWG Aluminum equivalent according to the same spec sheet online. Updated question with #4 aluminum. – Paul Makhnovskiy Mar 24 at 22:46
  • Who'll be doing the electrical work? I don't care if your electrician will have a tough pull lol... but I care if you do. – Harper Mar 24 at 22:47
  • @Harper, not sure at the moment but will be looking for some decent electricians when we get closer to permit approval. I was thinking I could do it myself, but looks way too complex overall. Is this something a non-electrician can do? – Paul Makhnovskiy Mar 24 at 22:49
  • Yes, easily if he's willing to do the learning required, which is considerable but certainly not half as hard as learning a programming language. One thing that helps is getting a meter box with main breakers in it, so you can fully de-energize equipment. – Harper Mar 24 at 23:03
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The box you linked is the wrong style

Your first issue is that the "duplex box" (meter-pack) you linked uses the wrong style of metering, so it stands a snowball's chance in hell of being acceptable to SMUD. In particular, that box uses a ringless configuration where the cover holds the meter in place, while SMUD, being an EUSERC utility, requires ring style metering hardware, where the meter is inserted with the cover already installed and a sealing ring is then used to attach the meter to the cover.

Fortunately, Siemens makes a similar box with the correct metering setup, namely the WEP4212. This is a 400A box with 2 200A ring-style meter positions and an EUSERC-style utility wiring compartment, as well as the correct approvals for use in an area served by an EUSERC utility, like SMUD.

Fat aluminum is your friend here

Whether you go with individual wires in conduit or with cables (SER for the main house and direct bury quadplex for the ADU, presuming the meter-pack will live on the outside of the main house), you'll want to use aluminum wire here; 4/0 will suffice for a 200A feeder to a dwelling unit, since I doubt that you'll want any more than that for either the main house or the ADU, given the size of the buildings in question. (The main house load calculations come out at around 120A or so, depending on how much current the air conditioner pulls, so a 200A feeder provides ample room for expansion.)

Go big or go home!

When it comes to putting in the panels for the individual buildings, scrimping on spaces is penny-wise and quite pound foolish, as adding a few spaces now is far cheaper than replacing an entire panel down the road. I would go with a minimum of a 42-space, 200A panel for each unit here, preferably of a type convertible between main breaker and main lug as you'll probably need a main breaker on one panel and main lugs on the other, depending on where the meter-pack is located in this setup. Of course, if you can get a 54-space or 60-space panel for a reasonable price, I would spring for it in a heartbeat, as I have not heard a single complaint from someone about having too many spaces in their electrical panel!

As to that (future) solar setup...

Since your solar power will be entirely net-metered by the ADU's meter, you'll need to have all your solar panels mounted on the ADU and wired back to its panel. This is needed to keep the inverter output branch circuits from violating the intent of NEC 210.25.

TORQUE ALL LUGS TO SPEC

There is one more matter to pay attention to when installing this, and that is that you (or your electrician) will need to use an inch-pound torque wrench or torque screwdriver to correctly make up the lug connections on the various pieces of electrical equipment here. This is required by 2017 NEC 110.14(D), and is a good idea anyway to keep your electrical system from developing a case of the loose lugnuts.

  • Thanks for detailed info and recommendation on duplex box, will add that to my project! What outdoor main panel would you recommend thats 42/54 space for the ADU? – Paul Makhnovskiy May 26 at 16:56
  • Also, we can request up to 600 amps from SMUD. Is it worth bringing in more than 400 amps to the duplex box? or no point since duplex max is 400 amps? – Paul Makhnovskiy May 26 at 16:57
  • Where is a good place to buy aluminum wiring? Platt? We are located in Sacramento. – Paul Makhnovskiy May 26 at 16:58
  • @PaulMakhnovskiy -- I would stick with 400A to keep wire sizing and whatnot within what people expect, and yes, I would check local and regional supply houses (Platt included) for the aluminum wire -- they'll have a much better selection than the big-box stores. As to a unit main panel -- I'm partial to Eaton or Siemens myself (Eaton has some 60space panels available in BR and CH in addition to 42 space units; likewise, Siemens has some 54 space units available, but the latter doesn't seem to include NEMA 3R), but Square-D has stuff too – ThreePhaseEel May 26 at 18:47

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