400 Amp Installation for New Home Use of Meter Main

Question

New custom home to be built in Tennessee. TN still uses 2017 NEC. This post is just to see what others have to say about what I plan on having for electric service. The house will be close to 3200 square feet with a 2-vehicle garage attached via a breezeway there will be a bonus room above the garage with a kitchenette and bathroom approximately 900 square feet of living space. An additional 2 vehicle RV garage will be built adjacent to the House and garage. What I envision having is a 200AMP Load Center for the House, a second 200Amp Load Center for the Garage/Bonus room, and a 100amp load center *sub-panel in the RV Garage.

Therefore, my idea is to have a 320/400Amp 1-Phase 3-Wire 120/240VAC 2-circuit Ringless Meter Main (e.g. Siemens MM0404L1400RLM or perhaps the Siemens MC0816B1400RLTM Combination Meter Load Center to accommodate some exterior lighting and security circuits with the slots in that combo panel). And based on NOT having to comply with the 2020 NEC section 230.85 for an emergency disconnect, I am hoping to use the Meter Main to split into the (2) 200 Amp Loads.

The Meter Main will then supply the (2) 200 Amp Load Centers (one in the home utility room, and the second one mounted in the attached Garage, with a 60 or 100 Amp Breaker feeding the 100Amp Load Center in the RV garage.

The home will also have an Emergency Standby Generator (e.g. Generac Natural Gas Generator) located near the Garage with the transfer switch near to the Meter Main Panel.

I do understand that all of this will need to be acceptable to the AHJ and a licensed Electrical Contractor will need to perform the work based on loads and codes. But what I am looking for here is some comments from this group as to how this sounds.

The house will be several hundred feet from the POC line at the road, and my desire is to have underground conductors in the conduit. Lastly -- One thought I want to explore with the POC is whether they will allow us to have a pad mounted transformer near the road and also have the meter near the Pad Mount Transformer (as in using Transformer/Meter Combo produced by Nordic Fiberglass, Inc. where there is a Meter Pedestal built onto the Pad Box).

Please share your thoughts... I'm not an electrician but I am a retired Construction Project Manager with over 50 years of construction experience - having worked in over 11 states and pretty much done it all to some degree - including building my own home previously and doing my own electrical work for myself and for customers when I was a builder.

This time I am having a General contractor build this Custom Home for me, I am just going to look over their shoulders.

** UPDATE

Just a note: Update from OPs "answer" moved here. OP Note that this site is not a "forum". If you have additional information that pertains to your question it should be added here just like the below insert So here goes: Again thanks for the replies -- I hope I can get a clarification about the comment by ThreePhaseEel who stated that an "ADU/guestsuite needs to be metered separately from the main house (something that's a bit of a gotcha with a lot of ADU codes)" This is in regards to the bonus Room I will have above the garage which will have a "Kitchenette" with Refrigerator, Dishwasher, Electric Countertop Cook-top (220V), a microwave, and a full bathroom with Exhaust Fan. The lower level (garage level) will also have a 1/4 bathroom (urinal and sink) plus a Washer and Electric Dryer. The Bonus Room will be cooled with an Electric Mini Split the hot water will be a Natural Gas Tankless W.H. The Garage floor and Bonus Room Floor will have radiant heating that will be served from a Tankless Type Water Heating Unit located in the Utility Room of the House and fed by insulated tubing under the breezeway to the garage/bonus room system. Thus, with the Kitchenette and the Electric Dyer and the HVAC Cooling Unit -- the 200A Load Center was what my thought was for the 2-car garage and Bonus Room above.

So.. I am a bit confused if it would be considered "ADU" based on the comment by "Harper-Reinstate Monica" when they said "You are accustomed to a rule that detached buildings must have one electrical feeder to a panel that serves only loads in that building. The pleasant surprise here is that due to the breezeway, that rule doesn't apply and they are considered one building by NEC."

Most likely the "AHJ", will have the say so on that? But... Initially I did not foresee having more than one METER for the service entrance. If there needs to be two METERS then that will require some HELP -- as I can't yet get it in my mind how to have the 400A come in and split and pass thru more than one POC recording METER. (Especially If I were to want to have the METER adjacent to the Transformer near the road).

Then with the statement from Harper -- "your AHJ may be OK with unfused wire coming from a meter pan, to main breakers on the side of a building" That kind of is what I was thinking I would discuss with the POC and AHJ?

As for disconnecting MEANS... I did not envision having a "MAIN BREAKER" at the meter (?) But that the Underground feeders from the transformer (ran in PVC Conduit) would enter a Service Type Panel with a Main Breaker 320/400, then be split into TWO runs to the (2) 200A Load Centers, one mounted inside the 2-car garage and the second 200A LC located in the basement of the house in the utility room.

And as to the Standby Generator it will be a Generac Natural Gas and my plan was to just have it take over the 200A load of the main house.

Again, Harper's comment regarding the Garage Bonus Room being connected by the Breezeway and they are considered one building by NEC. That means any panel can power any number of circuits or subpanels anywhere in this "one building". And that makes it easy to designate one 200A panel as the "critical loads panel", and simply let the other panel "die" when utility power fails This does seem to be the answer to remove the requirement for a second meter under the ADU Code, where ThreePhaseEel stated: a ADU/guestsuite needs to be metered separately from the main house

And yes... I know it ALL depends on the AHJ and the POC as to how ANY of this will be permitted. After all --- I was the AHJ as a Federal Construction Inspector for a large part of my career!

I also realize I am here just kicking ideas around --- But as I am still in the pre-planning stage... and because I like to be able to see it in my mind and know what is possible ahead of time I like to get other's input who may have experience doing what I am hoping to do.

The Breezeway will be 12 foot connection between the House and the Garage/Bonus room -- The RV Garage will be about 50 to 75 feet from the House/Garage. The RV Garage will not initially be used for a workshop but the idea was to at least have the capacity there if there was a need for the amperage. Feeding a 100A Load Center with a 50-60A breaker was the thought, perhaps to power the RV for guests? The only other high Amperage power need at the RV Garage is for an Electric Mini-Split to heat and cool the building - maintaining a 60 degree range temp.

OK -- this is long enough for now.... Please feel free to add whatever you like as suggestions or concerns and comments! Thanks again Richard

21 hours ago

Answer 1

Dual 200A panels

That way of using "400A" panels with dual 200A breakers is standard and reliable. You pretty much put the meter where the utility tells you to. You may have heard about a "meter-main" requirement; that's not really a thing in the way it sounds - what they want is a single place where all disconnects can be found and it's on the outside of the building.

I'm not getting a warm fuzzy feeling about mains several hundred feet away from the house. Tripping a main breaker is extremely improbable, but still, it's a long walk.

There are strict rules about totally unfused service-entrance wiring between meter and main inside buildings; Code requires "as short as possible" e.g. maximum 5 feet. However, underground is a different matter, and your AHJ may be OK with unfused wire coming from a meter pan, to main breakers on the side of a building.

Regardless don't be bashful about using aluminum wire for this long haul. Aluminum is proven reliable at large sizes.

and a licensed Electrical Contractor will need to perform the work based on loads and codes

Only if you plan to flip it or rent it out. Homeowner-occupied single family homes, the homeowner is allowed to DIY (with permits and inspections, of course). I'm assuming from "3200sf" and "2-RV garage" that you're not in New York City :)

Which means functionally you can have handymen do the work as long as you are meaningfully supervising, since you'll be on the hook for mistakes.

Which panel is served by the generator?

400A panels and breakers are prohibitively expensive. Using dual 200A main breakers and dual 200A panels makes them much more affordable. 230.85 is not being widely enforced because 400A main breakers are not affordable yet - I think NEC authors are hoping to create "economies of scale" to bring the price down.

The trade-off is that only one panel can be fed from a given generator, unless at least one of the transfer switches is 3-pole (switching neutral). Note that the automatic generator must be large enough for "the load to be served", so putting everything on a 400A house will require a potentially 96kW generator, and methane supply for that would be very, very challenging.

You are accustomed to a rule that detached buildings must have one electrical feeder to a panel that serves only loads in that building. The pleasant surprise here is that due to the breezeway, that rule doesn't apply and they are considered one building by NEC. That means any panel can power any number of circuits or subpanels anywhere in this "one building". And that makes it easy to designate one 200A panel as the "critical loads panel", and simply let the other panel "die" when utility power fails. That makes implementation a lot cheaper.

So for instance you might have the garage panel be the backed-up one, and then have it feed a subpanel in the house that is the "critical loads subpanel" for the house. Then it could also feed the RV garage, in case someone (say extended family) needs a place to sleep during a crisis. It's really up to you.

Since it is one building in the eyes of NEC, the AHJ may want to see both 200A panels near each other. No limit to the number of subpanels, and in a 3500sf house I'd encourage use of subpanels simply to avoid financial fiascos like having 10 Romex runs running parallel to each other for 100 feet. 120V circuits hit 3% voltage drop at about 80 feet, and 2-2-2-4 SER feeder is cheap.