How I'd do this
Article 220 is where I would start as well. Since I don't have your utility bills in front of me, I used the standard Part 3 procedure for ampacity calculation as that allows me to use assumed loads for the "three sisters" (water heater, dryer, range), vs the alternate procedure (220.82/83) which requires nameplate loadings for these appliances. Furthermore, I assumed that:
- Your servers will not need dedicated cooling, and all server loads are on the 3000VA UPS.
- The house has 4 kitchen branch circuits, while the apartment has 2.
- 3 1500VA load allowances are sufficient for the garage shop atop the standard lighting load and the EV/kiln/welder receptacle, which is allotted a full 12KVA.
- A pool heater is not needed (it's Houston, you probably want your pool water cool more than you want it warm) and a 1HP pump suffices for the pool.
- A heat pump with a low ambient kit suffices for HVAC, and emergency heat is not needed (I used Trane XL16i 's as representative units, with 3tons for the house and 1.5tons for the apt. as "rule of thumb" sizes. A full manual J sizing would be needed for a precise load calculation here.)
- Both the house and guest suite have loads alloted for them for electric hot water heating, an electric dryer, and an electric range not exceeding 12kW. (This treats the guest suite as a complete dwelling unit, which may be a bit generous in its facilities.)
With these assumptions in hand, we come out with 148A@240VAC for the house, 87A@240VAC for the garage/shop, and 84A@240VAC for the apartment. This allows us to do a fairly neatly balanced split, with a 200A feeder from the new service equipment to the house, and 100A feeders each for the garage and the apartment.
Running the feeders
While running 2" conduit for the feeders is a good idea, the 400kcmil wires are grossly oversize for a 200A feeder. Between 75°C terminations and the adjustment provisions in 310.15(B)(7), we can use 4/0 Al instead. Furthermore, XHHW-2 is the insulation of choice for such large wires (vs. THHN), and large aluminum conductors are compact stranded as a rule, which lowers conduit fill significantly. A single 4/0 Al compact stranded XHHW-2 only takes up just under 180mm2 of space, meaning that without a neutral downsize, the over 800mm2 a 2" conduit can provide in usable fill is more than plenty for the 540mm2 of fill the wires take up.
Likewise, we can use 2AWG Al XHHW-2 for the 100A feeders, although you might as well use 2" conduit there as well so that you only have to buy one size of conduit for your feeder runs.
400A service is hard...
As to the service equipment itself, this is where things get tricky. A typical solution at the 400A level splits the service equipment across two or three panels; however, this is probably not the greatest solution insofar as it requires all the panels to be in close proximity and also has no single point of shutoff. Another possibility would be to use a 400A meter main type device, but 400A meter mains are typically not used with overhead service conductors. While it'd be possible to use a couple of elbows (and a drainage hole) to fix that, your AHJ and/or utility may not approve of such.
A third option is to use a 400A loadcenter with a standard meter socket, but those are only available in large slot counts, which is actually somewhat inappropriate for the application at hand, and not everyone makes them. The fourth and final option, and one I'd be partial to if not for the high price tag, would be to use a 400A enclosed breaker as the service entrance device, with a distribution block in the enclosure (or associated gutter) for tapping the feeders to the various subpanels. This provides a single point of shutoff and minimizes the length of unprotected conductor, but is costly as 400A enclosed circuit breakers are commercial parts, basically. It also invokes the feeder tap rules, which limits the length of the runs from the main to the subs to 25' unless they are run on the outside of the building.
...but not impossible
Thankfully, it turns out, Square D's meter-mains can be used with an overhead gutter accessory (the OCK400) -- this allows an underground-only meter-main/CSED to be converted to work with overhead service. Furthermore, the configuration of the Square D QU12L400CL is ideal for this application -- the 200A factory main can be used to feed the house subpanel, while the other two feeders can be provisioned using QO2100 breakers in the side-panel section. Of course, this meter-main becomes your service equipment, with 4-wire feeders to all subpanels (or feeders in metal conduit).
As to the subpanels themselves
I would go with 42-space panels each for the garage and apartment, and one of two configurations for the house:
- If you value having everything (that isn't on the generator) in a single panel, I would practically require getting a 54 or 60 space, 200A panel for the house, as a 42 slot panel sounds rather...tight, given the amount of electrical stuff roaming around your house.
- If you don't mind having two panels for the house, then instead of a traditional main-panel/subpanel setup, I'd use two 42 slot/200A panels set up in a daisy-chain using a set of subfeed lugs in the first panel to feed the main lugs (or a backfed set of subfeed lugs, which may be preferred for wire-bending reasons) in the second panel. This gives you a minimum of 74 slots spread across the two panels, while allowing to use identical panels for the house, garage, and apartment/guest quarters.
Also, depending on how many outdoor loads you have, you may wish to have an extra panel for the garage that is dedicated to those. In that case, I'd use a 200A, 8/16 space "mobile home" type loadcenter with factory-fitted feed-through lugs that has been coverted to main lugs from a main breaker configuration (or is factory fitted with main lugs), connect it in line with the garage feeder (the main lugs connect to the feeder to the meter-main, while the feed-through lugs connect to the feeder off to the garage), and mount it to the outside of the house in a convenient place. That provides a nice place for circuits to outside loads to go (such as the pool pump, or outside lights and receptacles).
Backup power complications
I would not use a 400A ATS and try to jigger a zillion load relays here -- that sort of complicated load-shedding orchestration is not a particularly good idea from a "number of things that can break" standpoint. Instead, I'd split the standby loads out into a subpanel of their own, with a smaller ATS feeding that subpanel in addition to a feeder from either the meter-main or the house panel. If you desire standby loads in the garage or guest quarters, then you can have an additional ATS tapped from the generator feeder conductors feeding a second standby subpanel.
If you want the apartment separately metered...
Some AHJs or electric utilities may insist on separate metering for the apartment here, which complicates matters as most gang-metering setups are designed for a relatively symmetric split between the units, rather than the 300A for the house+garage+pool and the 100A split meter for the apartment. This means that you will need to have a utility sealable trough that connects to a 400A meter-main for the house/garage and a separate 100A or 125A meter-main for the apartment using conduit nipples; this does obviate the need for the OCK400 to bring the overhead feed down to where the meter-main can get at it, though.
Your choice of meter-mains on the 400A side in this case will depend on if you want to have the house use a 200A panel (or panels) with a parallel 200A panel in case more ampacity is needed, or if you wish to use a single 400A panelboard/loadcenter. In the former case, the same QU12L400CL from above will do the trick -- the 2 200A breaker positions provide parallel feeders to the house, while a QO2100 or QO2125 protects the garage feeder, and the remaining 125A position is left open to provide expansion space.
In the latter case, though, things get interesting. The meter-main becomes a Square-D QUM400CL with a BMK2Q400 mounting kit and a single QDL22100 or QDL22125 branch breaker fitted; however, the jumpers in the BMK2Q400 are not used to feed the branch breakers, but 6-wire, double-entry, Al7Cu mechanical splice connectors rated for 250kcmil wire -- one connector for each leg, with both jumpers going into the connector. From there, 3/0 copper pigtails are led off to the line lugs on the branch breaker, forming a feeder tap under 240.21(B)(1) rules. The feeder wires, which are 250kcmil aluminum with 2 wires in parallel for each leg and the neutral (this is fat enough that 310.10(H) permits parallel wires), also go to the splice connectors, one pair of hots on each connector, with the neutrals landing on the center set of neutral terminations. This fat feeder requires a 1AWG aluminum EGC (or metal conduit instead), and all this wire is wrapped up in a whopping 3" conduit. You'll also need to use a 400A loadcenter/panelboard or paired 200A loadcenters here, with the latter subjecting you to the feeder tap rules again (25' tap rule, 240.21(B)(2)).
Once that is done, the garage feeder can be run in a more normal fashion, landing its hots on the load lugs of the meter-main's branch breaker. The apartment meter-main can be any suitable 125A class unit with a 100A main breaker, and the apartment and garage panels and feeders are as above, or as suited if you decide to bump them to 125A in this case.