Wire for 100 AMP Detached Garage Subpanel. 50 feet direct buried 50 feet in crawl space
Overflowing...

Your existing panel is listed and labeled as a Circuit Total Limiting panelboard, which means that it can only use double-stuff/tandem breakers in spaces that are marked and designed to accept such.  However, the label on your panel designates no spaces as usable with tandem breakers, only standard breaker poles, rendering your panel-stuffing plans a violation of the panel's listing, and thus of NEC 110.3.  Furthermore, space 34 already has a tandem breaker in it, which means that your panel is overflowing right now!

This means that you'll need to change out your main panel for a 42-space, 225A, main breaker loadcenter, at minimum, in order to do this; a 54- or 60-space panel such as the Siemens P5470B1225CU would be my recommendation, even, given that you already are up to 43 spaces' worth of breakers and how useless double-stuff breakers are in this day and age.  The reason for the 225A panel (vs. 200A) is explained below, by the way.

and Overloaded

Applying NEC 220.83 to your current situation, consisting of:


3500 ft2 at 3VA/ft2
5 kitchen SABCs and a laundry SABC at 1500VA apiece
3 more 1500VA allowances for fixed kitchen appliances
a 12kVA range allowance
a 5kVA dryer allowance
and a 4.5kVA water heater
and your HVAC system consisting of 38.3A of heat pump outdoor unit as well as 111.9A of strip heat


puts you at 45500VA of non-HVAC load, taken at a 40% demand factor over 8kVA for 23000VA of factored non-HVAC load.  To this, we add the 38.3A (16.9 + 21.4) of heat pump outdoor unit draw at full rating and the 111.9A (26.3A + 29.2A + 2.5A from one unit, and 53.9A from the other) of heat pump indoor unit draw at a 65% demand factor to get 8809+16729 = 25538VA of HVAC load at a 230V utilization voltage.  Adding those two together and dividing by the 240V service voltage gets us a total load of 48538VA, or 202A, which is actually slightly more load than what your 200A service can handle already!

This forces us to bump the service to 225A to have enough headroom for the new garage. This requires a pigtail of 250kcmil Al to be added at each end of the existing service-entrance wiring (connected using Al9Cu Polaris connectors) to allow the existing wiring to run at 90&deg;C and thus have sufficient ampacity for a 225A service, as well as a 225A panel, such as the one proposed above.

Wedging the garage in there

Now we can move onto the garage, which consists of:


a 28'*40', or 1120ft2, space at 3VA/ft2
2 1500VA allowances for power tools
and a 9.6A/230VAC (representative 20kBTU, 10.4EER air conditioner from here)
OR a 30A/120V TT-30 receptacle supplying a RV, treated as a single 3600VA RV site as per 551.73(A) and the associated demand factor table


By itself, the garage draws 8568VA, or 35.7A at 240V.  However, we can apply the 40% demand factor from 220.83 to the receptacle loads in the garage, meaning that the garage only adds 4752VA, or 19.8A at 240V, to the service load.  (The RV receptacle, which would otherwise put you over even with the bump to 225A, is being treated as a non-coincident load with the rest of the garage loads as permitted by NEC 220.60 given that I doubt you'll be working on stuff in the garage while you have guests over at your house :)

...and getting it hooked up

Once the panel is replaced, getting the garage hooked up is the easy half of the enterprise.  I would use 1.5" schedule 80 PVC for the entire run; in the basement/crawlspace, you can strap it to the bottom of the joists with an expansion joint in-line with the run to keep it from doing the worm on you, and then just transition it directly to the outside run with a pair of 45&deg; sweeps to bring it down to burial depth (24" is fine) and 90&deg; sweeps at each end to bring it into the panels.  Inside this conduit, you'll be running 3 1AWG Al XHHW-2 conductors along with an 8AWG bare copper ground; this takes up nowhere near the fill space availabe in the conduit at 270mm2 used vs. 442mm2 available while providing a fully rated 100A feeder to the garage.

At the house end, a Q2100 is indeed the correct breaker choice for the job provided that the replacement house panel is Siemens as well, while I would in fact take the old 200A panel that got obsoleted by the house panel upgrade and put it in the garage; the main breaker is needed to serve as a local shutoff for power to the garage (and nothing more), while 40 spaces should be more than enough for even the most extravagant garage plans.  If you wish to buy a new panel for the garage, the minimum I would get is a 100 or 125A, main breaker, 24-space panel, with a larger panel than the minimum highly recommended.

In it, you'll want 2 15A breakers for lights (and only lights), 2 20A GFCI breakers for receptacles (possibly combined as a 20A multi-wire branch circuit using a 20A 2-pole GFCI breaker to allow for 240V power tools), a 15A or 20A 2-pole breaker for the AC (depending on your AC) using a single NEMA 6-15R or 6-20R at the load, and a 30A breaker for the TT-30 RV receptacle.

Of course, when making all these connections, you'll need to use an inch-pound torque wrench and/or torque screwdriver in order to meet the tightening torque requirements of 2017 NEC 110.14(D).  Even if your AHJ has not adopted the 2017 NEC, you'll want to do this anyway, lest your electrical system lose you the race!