Electrical design for a house with an ADU, solar, and backup generator
Accessory dwelling units are complicated

While outbuilding feeders and standby generators are both common things to have -- your combination of having an accessory dwelling unit in the outbuilding with standby generator support for that accessory dwelling unit, alongside having shared, or "house", loads, such as the well pump, that will also need standby power, makes this a significantly more complex problem.

Why? Because accessory dwelling units throw a kink into the way NEC 210.25 is interpreted:


  210.25 Branch Circuits in Buildings with More Than One
  Occupancy.
  
  (A) Dwelling Unit Branch Circuits. Branch circuits in each
  dwelling unit shall supply only loads within that dwelling unit
  or loads associated only with that dwelling unit.
  
  (B) Common Area Branch Circuits. Branch circuits installed
  for the purpose of lighting, central alarm, signal, communications, or other purposes for public or common areas of a two-family dwelling, a multifamily dwelling, or a multi-occupancy
  building shall not be supplied from equipment that supplies an
  individual dwelling unit or tenant space.


This code section, more commonly seen in the context of full-fledged multi-family buildings, basically requires one of two power configurations for any property that has more than a single dwelling unit in it:


Master metered, where all dwelling units and the shared/common loads are fed off of a single meter
Individually metered, where each dwelling unit gets its own meter and the shared/common loads are metered separately from all dwelling units/tenant spaces.


Furthermore, the master-metered approach is discouraged (albeit not prohibited!) by PSE in their General Rules and Provisions:


  The Company may refuse to connect service to a master meter in any new building with
  permanent occupants when: there is more than one dwelling unit in the building or property;
  the occupant of each unit has control over a significant portion of electric energy consumed
  in each unit; and the long-run benefits of a separate meter for each customer exceed the
  cost of providing separate meters.


Given all this, and that the general Code requirements for accessory dwelling units are a complex and evolving affair, with local code amendments often written by folks who mean well, but do not always understand the NEC impacts that result from their writings, we need to design our electrical system with the individually metered approach in mind, even if we can get by with a master metered approach for now.

As a result of this, we need to rigorously separate the equipment for the three systems (main, accessory, and shared/"house") everywhere beyond the meter-main.  This means we need 3 main-unit subpanels, 3 transfer switches, and 3 standby subpanels, in addition to the existing main panel in the house, which can be reused as a subpanel, or replaced as needed.  We also need to make sure that we don't paint ourselves into a corner that prevents the setup from being upgraded to 400A.

How can we size this?

Service and feeder sizing is determined by a set of relatively complex rules located in the NEC's somewhat-infamous Article 220.  These rules cover just about every sort of building under the sun, but have several special cases that apply to dwelling units (aka houses and apartments), as well as a simplified set of rules for that dwelling-unit case (220.82), which we will not be using here as they are difficult to apply to multiple-unit situations like yours.

The current situation

Our starting point, of course, is the house you have now, with:


3500 ft2 of floor area (times 3VA/ft2 for a total of 10.5kVA of general load allowance)
3 kitchen branch circuits &amp; a laundry branch circuit (6kVA at 1.5kVA apiece)
A standard, tank-type electric hot water heater (4.5kVA)
An electric clothes dryer (assumed at 5kVA)
A separate cooktop and oven, with the cooktop at it's 7kW nameplate rating and the oven consuming as much power as a 30A branch circuit will let it (aka 7.2kVA)
The existing 3/4HP, 115V drainage pump (1587VA using the 13.8A@115V figure from Table 430.248)
And the existing HVAC system, consisting of a 5 ton heat pump with a 32.3A outdoor unit and a 57A air handler/auxiliary heater. (We are assuming the basement heater is shut off for the purposes of this load calculation to simplify things.)


Applying the Part III (standard) method from Article 220, we start by applying the 35% demand factor for receptacle loads (general lighting and small appliances) over the baseline 3kVA, giving us 7.725kVA of factored general load.  Atop that, we add the water heater, dryer, and cooking appliances, giving us 31.425kVA of non-motor/HVAC load overall as no demand factors apply here.  Finally, we apply the motor and HVAC loads, with the outdoor unit at its branch-circuit selection current as the largest motor as per 440.7 and the Exception thereto, giving us 9286VA for our largest motor (the outdoor unit), another 1587VA for the drainage pump, and 13.68kVA for the indoor unit, for a total load of 55.978kVA, or 233A at 240VAC.  (The alternate calculation in 220.82 yields around 150A, so you probably are not overloading your service at the moment, but this is tight.)

As we shuffle things around...

Of course, this will change as we add loads.  We start with the new factored load from the ADU and shop space, consisting of the general lighting load from both spaces at 864 ft2 each * 3VA/ft2 = 2592VA per space as well as the 4500VA small appliance load from the ADU.  This sums up to 9684VA, which all gets demand-factored at 35% for service calculation purposes to yield 3389VA of additional factored load on the service.  However, the ADU feeder needs a separate factored-load calculation, yielding 4432VA of factored load for ADU feeder sizing purposes, and 2592VA of factorable load, with no demand factors applied, for shop feeder sizing purposes.

From there, we then add the HVAC loads for each space.  The representative units I have chosen to provide an appropriately conservative load calculation are 18kBTU (1.5ton), 20 SEER, Mitsubishi inverter units that consume 12A at 230VAC, or 2760VA, apiece -- the real units used will likely be smaller still as modern building methods can produce vastly better envelope load performance than 500sf/ton.  This is the largest motor load in the ADU, as well, which means that we must apply a 1.25 factor to it for ADU feeder sizing purposes, resulting in an additional 3450VA of ADU feeder load, an additional 2760VA of shop feeder load, and an additional 5520VA of service load.

We then add the loads specific to each unit.  For the ADU, we have 4500VA for a separate, tank-type, electric hot water heater, 5000VA for its own dryer, and 8000VA as a range allowance, permitting up to 12kW of range as per Table 220.55: this all adds up to an additional 13.5kVA of ADU feeder load and service load.  For the shop, we add a 28A@230V (5HP) dust collector, our largest motor here, which adds 8050VA of load by itself, as well as a 13A@230V tablesaw which adds another 2990VA of load, and a 1500VA allowance for an additional tool that is in use at the same time, yielding an additional 12.54kVA of shop feeder load, and 10.93kVA of service load (as the dust collector is not the largest motor on the service).

At this point, we can compute the loads on the ADU and shop feeders.  The ADU feeder has 3389VA of factored load, 3450VA of HVAC load, and 13.5kVA of fixed appliance load, adding up to 20339VA of feeder load, or 85A@240V.  This is within what a 100A feeder can handle, and also means that there is very little chance that we will need anything larger than 125A for the ADU.  The shop, then, has 2592VA of factorable load, 2760VA of HVAC load, and 12.54kVA of machinery load, yielding 17892VA of feeder load, or 75A@240V.  Again, this means that a 100A feeder will be sufficient for the shop, with a 125A feeder available as a future upgrade option.

We then move onto the shared-loads feeder.  We start with the factorable load, which consist of 320VA for the 16x20 storage shed, 200VA for the 200W streetlight, and 360VA to account for exterior receptacles (180VA for one on the shop/ADU building, and another 180VA for a premises receptacle, if present) for a total factorable load of 880VA.  We then add on a 17A@230V (3HP) well pump motor, or 3910VA of service load and 4888VA of feeder load, for a total of 4790VA of additional service load and 5768VA, or 24A@240VAC, of feeder load.

Finally, we return to the main house.  The 13.68kVA from the auxiliary heat goes away, but we add a 24A@240VAC car charger for an additional 5.76kVA, yielding a new main house load of 48.058kVA, or exactly 200A@240VAC for that feeder.  The service load, though, is computed by adding all the factored and non-factored loads separately, yielding 80.427kVA of total service load, or the need for a 335A service at 240VAC to accommodate the full scope of this.  In other words, you will need to make sure that the service upgrade to 400A happens shortly, or else you will have issues with the main breaker in the meter-main tripping (if you have a single main breaker, that is).

Starting with the meter main...

There are multiple types of meter-main hardware out there -- meter-breakers, meter-loadcenters, and multi-meter-breaker stacks.  Due to their lack of flexibility, full-fledged meter-loadcenters (combination service entrance devices, "all in ones") are a poor choice in many applications, so we will not consider them further here, although many manufacturers do not cleanly distinguish between devices that are closer in nature to meter-mains from devices that are designed solely as all-in-one meter-loadcenters.  We will first consider the meter-breaker approach, as applied to your existing 200A service, and then discuss the 400A upgrade options available to you.  Finally, we will talk about multi-meter hardware, as it has some complications in it that are not present in single family metering.

During all this, we also have to keep some requirements imposed by your utility in mind:


As an EUSERC utility, PSE requires ring-type metering hardware
PSE considers "house panels" for common loads in multi-family dwellings to be commercial loads, requiring a manual (test block) bypass facility in the meter socket for the common loads meter
PSE forbids test block bypasses on 200A meters feeding individual dwelling units, but requires a manual block bypass on 320A residential meters
PSE requires engraved identifying plates on individual meters in a multi-meter setup -- any shop that can engrave trophies can handle this for you
PSE also requires a production meter in-line with the solar feed in some (many?) cases, colocated with the rest of the metering


The simple case -- breaking out what you have now

While I would normally suggest "rule-of-six" meter-mains for an application like this, the heavy oversubscription potential of the existing 200A service with this additional wiring is a legitimate concern, and there are a couple ways to handle this.

If you wish everything in one box, the Milbank U5169-XTL-200 is an EUSERC-compliant, 200A meter-main setup that uses a backfed 200A main onto a 12-space main lug loadcenter, leaving 8 spaces for branch breakers.  For this unit, you would be using a Siemens QN2200RH for the house main, with a pair of Q2100Hs for the common-load and ADU feeders.  However, it would mean that you would have to feed the shop through the house panel until you upgraded to 400A service.

The other alternative here, at the cost of an additional enclosure, is to use a single-breaker, ring-type, EUSERC-compliant, single-breaker 200A meter-main, with a 2" nipple with 3 4/0 Al XHHW-2 wires in it connecting it to a 12-space, 200A, main lug, outdoor (NEMA 3R) panel with matching 200A and 100A breakers, as well as accessory ground bars and lugs for any large ground wires exiting.

Either way, you will need to clean up your existing panels at this point.  In the split-bus panel (a Murray, by the way), all the bare wires need to be moved to the left-hand bar (the ground bar), all the white wires need to be moved to the right-hand bar (the neutral bar), and the bonding screw that electrically connects the right-hand bar to the case needs to be removed.  If the right-hand bar is not sitting on black plastic insulators, you will need to replace it with a Murray ECINSNB33 insulated neutral bar kit at this point, as well.

The small outdoor subpanel will need some more extensive work.  Once you banish the cobwebs and debris from the cabinet, you will need to replace the interior trim (deadfront) with a Square-D 4055802102 (as per here and here), mounting the new trim piece using a Square-D 4020513001K fastener kit.  While you are in there, you will need to fit a Square-D PK7GTA ground bar to the loadcenter, remove the bonding screw from the neutral bar, and move all the bare wires from the neutral bar to the new ground bar.

Alternatively, you could replace the subpanel with a brand-new HOM612L100RB with its bonding screw pulled and a PK7GTA ground bar fitted, reusing the existing breakers.  This does require the extra labor of pulling the wires out of the old enclosure and swapping them over to the new enclosure, though.

Going bigger

When you want to go to 400A service, your hardware selection becomes much more limited.  Fortunately, Square-D does make something suitable in the QO line: the CU12L400CB provides a 200A disconnect with space for a second 200A disconnect, as well as a 200A, 8 space QO interior providing room for up to 4, 2-pole, 125A maximum QO branch breakers.  This allows us to put the main house on the factory-fitted 200A breaker, the ADU on a QO2100VH, and the house loads on another QO2100VH, while leaving the spare 200A space available for either more power to the main house or a larger ADU feeder and permitting a separate QO2100VH for the feeder to the shop space.

Giving everyone their own meter

While the normal "go to" for this would be some sort of meter-pack type setup, since you want (and need) 200A to the house and 100A atop that for the shop, you'll need to go with a split setup.  The main house and shop feeders in this case are handled by a CUM400CB with a BMK2Q400 kit used to fit QDL2200 and QDL2100 feeder breakers for the main house and the shop, respectively.

With that, we use two individual meter-mains for the other two meters.  One of them needs to be a 100/125A, EUSERC-compliant, underground-feed meter main with a test block bypass function and a 22kAIC main breaker, such as a Milbank U214MTB/22, an Eaton CHU214MTBLB (with a BRH2100 main), or a Cooper U214MTBH, for the common-loads meter, while the other is  a 100/125A ring-style meter main, also with a 22kAIC main breaker, for the ADU's meter.

Last but not least, we need a way to connect the various service-entrance conductors from the meter-mains to the utility service lateral.  The simplest approach would be to use the meter sockets to house splices, but that doesn't work as PSE's rules forbid using meter bases as junction boxes.  Double-lugging the 400A meter-main doesn't work either as Square-D does not support that, and we can't use a regular NEMA 3R pull box to house the splices as EUSERC rules require a special two-bolt arrangement for the utility service lateral conductors to land on instead of having them land on a customer-provided lug.

This leaves us looking at the approach used in commercial work, where a termination box is used to land the service lateral from the utility, connecting it through to a bussed gutter that distributes power to the various meters.  However, given the constraints of our setup, a bussed gutter is a costly piece of overkill; it is far simpler to splice the service-entrance conductors directly in the termination box using insulated mechanical splice connectors, connecting them to jumpers to the termination box's customer-side lugs.

For this, we use a Milbank TB-014 box, some 600kcmil Al XHHW-2, some 1/0 Al XHHW-2, and three four-port, Al9Cu insulated mechanical tap connectors capable of accepting wire from 1/0 to 600kcmil, minimum.  The 600kcmil conductors from the tap connectors run onward to the 400A meter main via 2.5" EMT and a set of CMELK4 lugs at the meter-main end, while the 1/0 Al wires run via 1.5" EMT to the 100A meter-mains with prefabricated EMT elbows to turn up into the bottom knockouts on said meter-mains.

As to the feeders...

Once we have the meter-main(s) out of the way, we then run a feeder out to the new panel for the main house, using a 4/0-4/0-4/0-2/0 aluminum type SER cable. (Normally, I'd use XHHW-2 aluminum singles in conduit sized to allow for upgrades for such heavy wiring, but accommodating a 400A feeder requires a larger conduit than will fit within a 2x4 stud wall, making that approach rather impractical.)  We can also use this same type of cable to subfeed the existing 200A panel from the new main house panel location in case it is desired to keep that panel in service.

We do use the conduit approach for the other two feeders, though, as the common loads panel can live on the outside of the house, and using conduit allows for easy upgrades of the shop/ADU feeder(s) as well, saving upgrade costs tremendously over using a direct burial cable.  The common loads/solar production feeder, then, uses 3 1AWG aluminum XHHW-2 wires in a 1.5" EMT run off to a convenient spot on the outside of the house where the associated panel can live.

As to the run to the ADU/shop outbuilding, we will use 2.5" conduit here. This provides sufficient space for whatever you wish to run to there, up to 3 2/0 Al XHHW-2s for 125A to the ADU alongside 3 more 2/0 Al XHHW-2s for 125A to the shop space although 1AWG Al XHHW-2 will be adequate for both feeders for now, 3 4AWG Al XHHW-2s for a 60A generator feed to the ADU, and two 6AWG bare copper equipment grounding conductors, one for the shop space, and the other for the ADU (as this avoids splicing in the T-body).  While you have the trench open, you'll want to lay a second conduit (1.5" suffices) to serve as a communications duct between the main house and the ADU, considering that those wires can't be run in the same conduit as mains wiring.  Furthermore, this smaller conduit can also carry the control wires running to the generator set from the transfer hardware in the ADU, as these run at 12VDC.

Alongside this, we have three other conduits to run, all 1.5" in diameter.  First is the conduit carrying the control wires to the generator from the communications box at the house; next to it, we have another 1.5" conduit for the generator source lines, which consist of 3 1/0 AWG Al XHHW-2 wires and 2 10AWG THHN wires (for the generator's battery charger) alongside a 6AWG bare copper ground (really, a supply-side bonding jumper as per NEC 250.35(B), but it basically serves the same function as an EGC). The remaining conduit here can be another 1.5" conduit carrying 2 10AWG copper THHNs and a 10AWG bare copper ground for the well pump branch circuit.

Finally, we have the feeders to the storage shed and solar panels from the shared-loads panel enclosure.  These feeders can be simpler than the rest, due to the relative paucity of loads present there: 3 4AWG Al XHHW-2s and 3 10AWG Cu THHNs in 1.5" conduit with a 10AWG bare copper ground will handle both feeders, with room to spare in the conduit to go up to 2/0 Al XHHW-2 with a 6AWG bare copper ground, while keeping the existing 10AWG shed feeder wires, if you want to expand the solar panels in the future.

And the panels themselves...

The main problem with using conventional panels for this is that we would need 8 panels -- 2 each for the main house, the common loads, and the ADU, as well as a subpanel for the shop and a breakout panel for the generator feeders.  This, along with the 3 automatic transfer switches, the meter-main/meter-pack, and the existing panel(s) in the main house, would make for a simply unmanageable morass of distribution hardware.

Fortunately, Siemens makes what they call a "GenReady" panel.  This is a convertible (between main lug and main breaker) split bus panel that can be fitted with either a manual generator interlock or a power controlled transfer switch that lives within the panel, allowing us to consolidate the number of new enclosures down to 7 from 15 if we are reusing the existing main house panel as a subpanel.

While the transfer switch hardware that normally goes with this is designed to work with certain Generac generator controllers that also provide transfer "brains" themselves, it is possible to get a third party module, the PSP Products KGC-1, that provides the transfer control and generator start logic itself, allowing the use of any 2-wire start generator. (The documentation for the module only mentions Kohler generators, but the "4" output from the KGC-1 is really a standard 2-wire, 12VDC run signal.)

Starting right at home

As a result, we can accomplish this starting with a 12-space, NEMA 3R panel with a 100A main breaker, such as a Siemens PW1224B1100CU with an ECSBPK04 interlock kit fitted, that is used as a generator distribution board to provide individual standby feeders for each part of the system, along with 3 of the Siemens generator panels, and a 30-space, 125A, main breaker panel for the shop space.

The reason we are choosing a main breaker panel with a separate main breaker space for the generator panel, by the way, is it allows us to fit it with an interlock kit, 50A breaker, and CS6375-type inlet to allow a portable standby generator (with its neutral-to-ground bond pulled) to be plugged in and used in case the hardwired one breaks down during a power outage.  You will need a set of ground bars for this panel if it is not factory shipped with them, and you also will need an ECLK2SC lug for the neutral to the main house.

From there, we move onto the main house, which receives a Siemens G4254L1225GEN generator panel as it's new primary subpanel, fitted with its matching GENTFRSWTCH ATS, a PSP KGC-1 ATS controller, and an ECLK2SC neutral lug if needed for the generator feeder neutral.  The existing was-a-main-panel then gets subfed from this panel using an ECLK2225 subfeed lug kit with an ECLK3 neutral lug and an ECCS2 grounding lug to fit more of that 4/0-4/0-4/0-2/0 aluminum SER cable for the subfeed run.

Now, we can tackle the common loads panel. This panel is a Siemens W3042L1225GEN, again fitted with a GENTFRSWTCH and a KGC-1.  This will feed the streetlight, well pump, and any other loads that are common to the property as a whole, such as receptacles and lights that are not attached to either building or on the outside of the ADU/shop building, since the latter is fed from two different sources.  It also is where the feeder for the storage-shed originates, as well as having a 125A meter base for the production meter attached to it with a 1.5" nipple.  Last but not least, here, the 14/4 TC from the transfer switch to the comms/control pull box can be pulled through a 1/2" EMT to protect it.

Heading for the outbuilding...

From there, we move onto the outbuilding.  The 2.5" power and 1.5" communications conduits go up the outside of the building, with an Arlington 936NM in a TB configuration for the branch off to the shop panel and a pair of LBs routing the rest of the conduits into the ADU on the second floor.  The communications duct, by the way, will simply exit into the wall at each end, terminating in bushings to protect the wire edges.

We then fit a G4254B1200GEN for the ADU's panel; once again, this panel sports a GENTFRSWTCH transfer switch and a KGC-1 transfer controller.  From this transfer controller to the main house, we run a 14/4, wet location rated tray cable in the communications duct; these cables are more commonly seen in industrial work, but work just fine closer to home as well.

Finally, we fit a P3030B1125CU for the shop panel.  This panel requires no special treatment, unlike most of the other panels here.  However, the shop cannot share any "utility" loads with the ADU; this not only means it will have its own mini-split, but that it will need its own hot water heater as well, considering that the shop will have a washroom space associated with it.  Fortunately, this is a good application for a small tankless hot water heater in the 3-6kW range.

Dealing with that other shed

Now that we have the rest of this in place, we can fit the panel for the other shed now.  This will be a BR1224NC125R, with a BR260 fitted for the main using the supplied hold-down screw and a BR220WH fed using the shed load feeder for the light and outlet circuits.  This does require using a wirenut in the panel for the shed load neutrals instead of landing them on the neutral bar and using a receptacle or deadfront type GFCI for the shed outlets, but allows us to get everything here into a single box.

If you want to use standard ATSes instead...

If you wish to use standard automatic transfer switches instead of the decidedly non-standard/adapted hardware mentioned above, you can still do this with the Siemens GenReady panels -- each panel, instead of getting a GENTFRSWTCH with its associated Q215 voltage sense breaker and Q2100 generator breaker alongside the KGC-1 controller, will have the factory fitted molded-case switch for the utility power replaced with an ECLK125 subfeed lug kit, reusing the existing hold-down parts.  The factory jumpers are removed, and then the bottom half of the panel becomes your standby subpanel, with the factory utility breaker in the panel feeding the transfer switch utility input and the transfer switch output feeding the bottom half of the panel via subfeed lugs.

Another approach to this that gets you by with the same number of additional enclosures is to use standard 200A or 225A panels for the unit main panels alongside ATSes that have built-in standby subpanels.  This does limit your ATS selection more than the former approach, though, but does provide more panel spaces for non-standby loads.

Of course, both of these approaches mean you have 3 extra enclosures in your system compared to using the full capabilities of the Siemens GenReady panels, but if you need transfer switching functionality that the GENTFRSWTCH/KGC-1 combination cannot provide, these alternatives can get you there.

Other notes

First off, the communications duct to the ADU and the control-wires conduit to the generator should terminate in a NEMA 3R pull box on the outside of the house.  This provides a place to connect the control cables from the transfer switches to the control cable to the generator, as the KGC-1 control contacts can be connected in parallel; it also can house things like the house-side primary protector for any telecoms lines for the ADU.

Next up, the outbuilding will need a grounding electrode system of its own.  If you're building it from scratch, a concrete-encased electrode in the foundation is highly recommended (and required by some local codes).  In case that's not an option, though, there is always the old standby of using two 8' ground rods, driven 8' apart.  You'll need to use a 6AWG copper GEC, by the way, run to both panels in the outbuilding (shop-panel and ADU-panel).  This will also need to be bonded to the water service line coming into the ADU with a 6AWG bonding jumper and split bolt if a metal pipe was used for it, as well as to the communications raceway if it is made of metal, and to the primary protectors on the communications cables run to the ADU, using an intersystem bonding termination block as per NEC 250.94 and some bare 10AWG copper for the communications bonding jumpers.

While we are grounding outbuildings, the shed will also need two 8' ground rods driven 8' apart and connected to each other and to the shed's panel with a 6AWG bare copper GEC.  A split bolt and another 6AWG bare copper wire can be used to tap it at a later time if a second panel is added at the shed.

As to the grounding electrode system at the main building, you will want to take the existing GEC, remove it from the panel, then pull it out and re-route it to the largest meter-main.  In the multi-meter-main configurations, you will also need to tap the GEC with split-bolts rated for 4AWG wire and lengths of 6AWG bare copper wire for the tap conductors to the other two meter-mains.  Finally, you will also need to fit an intersystem bonding termination block to the new GEC and run any communications ground wires present to it, again as per NEC 250.94.

Finally, and importantly, you will need to use an inch-pound torque wrench or torque screwdriver to torque all electrical connection screws and bolts to the manufacturer's specified torque.  This requirement was introduced with 110.14(D) in the 2017 NEC, but is a good idea even if your jurisdiction has not adopted it, lest your electrical system do what Greg Biffle's infamous lugnuts did.