Best way to install Emporia Vue Gen2 energy monitor OUTSIDE load center?
The machine and all wiring MUST be inside a Class I wiring methods!
That machine must be inside a NEMA enclosure listed for housing AC mains equipment.  Every part of the system must be inside Class I wiring methods.  You cannot have loosy-goosy wires &quot;merely because you can't figure a better way to install it&quot;. No &quot;science projects&quot; allowed in AC mains electrical!  Code or go home.
Given that I'm basically cheap, my view is that the best listed NEMA enclosure is the one you already own - one of the service panels. Power is also conveniently found right there.  Now we must tackle the &quot;problem&quot; of the interconnects.
I say &quot;problem&quot; because it ain't one in my world. 99% of what I do is inside EMT metal conduit, and boy, oh boy! Does that make life easier.
I have plenty of panels in various buildings.  All of them are interconnected with EMT metal conduit.  When they are right next to each other, there are at least 3: One fattie for potential feeder and at least 2 smalls (near top and near bottom) for convenient routing of anything that needs to cross over.
When the panels are right next to each other, this is simply &quot;nipples&quot; (short pipes and fittings) of EMT conduit.  However, the panels need to  be pulled apart by about 1/2&quot; to install these, so it's best done when the 2nd panel is initially installed.  Harder to do after the fact, you can partially detach and  shift one panel if you really want to, but I would just establish a different route.
So my advice is to route EMT conduit between all the panels, if it's not there already.  3/4&quot; EMT is a good compromise between &quot;biggest stuff every box will accept&quot; and &quot;still easy to work with&quot; once you get the hang of it. If you are more comfortable with PVC conduit, that's not a deal-breaker, but I just can't imagine gluing a connection when I could bolt it and easily disassemble it and adjust if I needed to.
Run the CT wires through the conduit. (along with certain other things)
Locate your device in the panel that will result in the least or shortest routing of CT sensors, then run your CT wires through that conduit.
All conduit must be assembled and anchored totally complete before any wires are placed in the conduit. You cannot assemble the conduit around the wires. That means the conduit must be pullable - you must put appropriate sweeps or &quot;conduit bodies&quot; (corners or Tees with access covers) where they'll be needed.  I recommend no more than 90 degrees of sweep between pulling points for ease of installation (honestly 90% of the time I just push my wires through).
The CT wires do not count for 310.15(B)(3)(a) conduit thermal derate. However a power cable to the equipment does count as 1 - you can have 4 if they're all 15-28 amps. 3 if none are over 40A.  Otherwise you need to break out the sharp pencil and do derate calcs.  The CT cables do count for conduit fill calculations, but #18 TFFN is small.
For extending the CT sensors I would use #18 TFFN wire (the stuff used inside fluorescent light fixtures) as it is 600V rated.  However heed manufacturer advice as far as length of the CT cables.
You cannot go smaller than #18 per NEC.
Normally you cannot put data cables inside a power conduit. However these wires are a special case since the entire system, soup to nuts, is contained inside Class I wiring methods. What you cannot do is bring an Ethernet cable out to a wall plate, for instance.
If you want to use cables instead of conduit to extend the CTs, you still have to use Class I rated cables - you can't use thermostat cable or loose individual wires.  The smallest readily available cable will be 14/2 NM-B cable. One cable per CT unless you use 14/2/2 and then you could get 2 CTs per cable. What a bulky, awkward and wasteful way to do that - and no way to twist it either.
An EMT conduit route is simpler, far neater, and will be useful in the future if you want to extend a power circuit.  Again, nothing wrong with CTs in the same conduit as circuits or feeder even, so if you had a mind to a future feeder upgrade, maybe lay conduit big enough for that!
The factory recommends multiple monitors for multiple panels
Honestly, I recommend the same, however I don't see any problems with the conduit method when the panels are near each other.
They report extending 30 feet has been done with no decrease in accuracy. They simply have no info on extending farther than that.