Two Building Config ?

[ElectricTundra]

Background:

We are building a new house. Instead of having a large attached garage w/ my shop/studio above we're doing a small attached garage and then a separate building with the other half of the garage and my shop/studio above that. This is better for our neighbors not having to have a large garage next to their yard and has other benefits (construction costs not one of those benefits).

Original plan was Tesla shingles on both but with the continued delays we're now looking at Tesla shingles only on the studio since the main structure needs a dry roof by mid November. From a power standpoint we'd planned to feed 600a to the studio building and then 400a from there to the main house. This would be quite straightforward for integrating solar, power walls, backup generator, etc.

Our power company, Xcel energy, had agreed to this but then changed their mind. Normally they provide power at the meter but are now saying that they'll not do that for over 400a service and that for 600a they'll provide only a 13.8kV tap at the pole and then we'd be responsible for trenching from the pole to our building, installing a transformer, etc.

So, we're back to square one with 400a to our house, a separate 200a to the studio and possibly another 200a TOU drop to our house for chargers.

The studio will frequently not make use of the power generated by the rooftop solar. One option, since we have net metering, is to just feed it back to the grid.

Question 1: I don't know for how long we'll have net metering though so want to at least be prepared for being able to feed solar generation to our main house if not do it now. Thoughts on how best to do that?

Question 2: We may install a backup generator (Powerwall's would be much too expensive at this point I think though holding off until we see what happens with prices) and the plan was for this to be by the studio which would be fine with the single 600a feed + 400a sub but may be complicated w/ separate feeds? Also not sure about code for tying stuff together on our side of two different meters (I assume all meters will be off the same transformer).

Any other advice very welcomed.

Thanks,

 

[ElectricTundra]

Another issue w/ power walls is apparently a question of if they work reliably on greater than 200a service. I'd welcome any clarification on this.

 

[wwhitney]

What is the NEC load calculation for the house and for the shop? Does it really exceed 400A?

At present the Backup Gateway is only rated for 200A. So you can use Powerwalls on a 400A service, but loads need to be divided so that only 200A worth get backed up.

Cheers, Wayne

 

[miimura]

Are you planning two separate drops, each with their own meter? It seems like a good strategy to get around the 400A service limit from the utility, but really, can't you work out serving all your loads from one 400A service? The 200A limit for the Powerwalls is just how much load can pass through the transfer switch. It seems like you really need to tabulate your loads by breaker size. I would work them into the following categories:

Critical House Loads
Non-Critical House Loads
Critical Garage/Studio Loads
Non-Critical Garage/Studio Loads
Solar Generation

 

[Ampster]

Another issue w/ power walls is apparently a question of if they work reliably on greater than 200a service. I'd welcome any clarification on this.

You do not mention the size of any subpanels and whether you are going to separate some loads into a crtical loads panel. It is the size of the panel that is being fed by the Powerwall that is the issue. AFAIK any panel with more than a 30 Amp breaker is not recommended with a single Powerwall. It is not the service panel size that is an issue with a Powerwall, but as noted earlier the transfer switch is limited to 200 Amps.

 

[ElectricTundra]

Thank you all. The amperage requirements are per code based on number and types of devices, having two buildings, etc. Essentially the main house must have a minimum 400a and the studio a minimum 200a. I doubt we'll ever use even half that though.

For those in to architecture and construction some blog posts here: Bamasotan

We will need to do critical loads panels. Thank you guys for clarifying that and and the 200a limitation.

 

[wwhitney]

So, as a general answer to your questions, I would suggest installing 2 spare underground 3" conduits between the vicinity of your main house electrical panel and your studio electrical panel. That will give you the flexibility to run feeders back and forth to reconfigure your electrical distribution in the future.

As far as using a single backup source for loads on two different services, each service would need to have its own transfer switch, and you'd need a feeder for the backup source running from one building to the other.

A more specific answer would require your load calculations for both buildings, specifying the loads to be backed up in each building.

Cheers, Wayne