Just out of curiousity as I have a Generac Standby Generator that I would like to install but I dont have the ATS, So I thought why not build one Ardiuno/ESP hardware to control the Start/Stop functions of the generator and bypass the Generac controller and incorporate this into HA, But I also need to create a service disconnect for the main service coming in from the power company before the generator is started / running to function as a lockout. Does anyone know of a 200 amp switch / breaker that would be capable of this.
Use any smart switch (of well known quality, not some cheap Tuya crap) and let it control a well suited contactor for such power.
Make sure, the mains and the generator power contactors are secured MECHANICALLY to not be able to be ON both at the same time. Or disasters will happen 
Your best bet will actually be an ATSE - automatic source switcher. Pricey, but most secure. Something like this:
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What country, what voltage, what phasesā¦?
Please DO NOT attempt to roll-your-own ATS with pairs of generic relays or contactors (high current relays).
Buy a proper ATS (such as one with interlocks like a motor drive that turns one set of contacts ensuring a back-feed canāt happen) and control the transfer and genset from a micro. BigClive took an AliExpress device apart on YouTube if youāre curious.
Why?
Faulty ATS can KILL.
Your power goes out due to a cable break outside your property. Your switch turns the genset on. A gerry-rigged ATS doesnāt disconnect from the grid after power spikes / brown-outs locked-up the microcontroller.
A power distribution network technician finds the cable break, and is killed by YOUR GENERATOR powering what should be a dead cable.
The technician should āproveā both sides of the cable fault are dead for safe isolation, especially with increasing genset, battery, and solar installs but ATS are designed to break-before-make to avoid back-feeding impossible.
The more likely risk is burning your home down with a fault in a pair of contactors, or trying to power the street from a small genset.
Iāve built all sorts of custom kit using microcontrollers over the years, but for some safety-critical applicators you just canāt beat physical safeguards that software canāt get past.
(Friends of mine manage power distribution.)
I actually watched that video from from Big Clive while searching for an ATS. I have no desire to try to recreate the wheel in that aspect, I very much want to use an analog device like shown in Big Cliveās video, for the same reasonās you pointed out, and I donāt want to rely on a software implementation to act as a lockout (I have a background in handling up to 10K volt power distribution in the mining industry). Problem is, that device is only something like 63 Amp. I was hoping someone knew of one like in Big Clives video that was 200Amp.
USA, 60hz, split phase 120v
Phew - the knowledge level on forums varies so much, sometimes I feel the standard ābe carefulā warnings need some context. Iāve also worked in mining, albeit remote comms, and have also been āat the coalfaceā at a deep undersea pit. Medium voltage 10kV distribution makes sense when a roadway can be several tens of km long.
UK domestic supplies are mostly single-phase 60-100A, so Iām not familiar with the US āphase and a halfā split 120V / 240V system - but the lower voltage does make the higher 200A make sense.
BigClive is a regular shopper from the likes of AliExpress so they might be worth a look for a dedicated part, but I bet youāve already done that.
There are likely ways to stack contactors with auxiliary contacts and DIN-rail timer modules to guarantee break-before-make, but the simplicity of the timed motor switch is nice - the delayed switching might need UPS for computers, but it does guarantee separation.
I suspect the market isnāt that large, and UL-testing is costly, so the kit is specialist.
Buy ATS, setup GenMon, now your generator smart.
ATS cost $900-950 and it UL listed product.
Do you need one?
Can help
Yeah here in the US in a residential setting, we use two legs of 110v using single pole breakers, Where small appliances are balanced in the load center across both legs (leads) to evenly distribute the load, and 220v large appliances, like furnaces, water heaters, clothes dryers, welders, etc use both legs at the same time using a 2 pole breaker, so 110v conductors typically have 3 wires (Common + Neutral) and an earth ground, whereas 220v conductors have 4 wires (L1+L2 commons), a neutral and an earth ground. Itās typically called āSplit Phaseā due to the supply on the utility pole being a single phase 230v that is split into the two 110v legs using a center-tapped transformer located on the utility pole, and 200 amp is the typical residential load center.
Yeah the underground coal mine I used to work at was something like 24KM from portal to the farthest face
Ah - thatās the detail I didnāt understand.
One phase feeding a transformer with a split secondary giving 120-0-120 in-phase, not two phases on a 3-phase supply (which would be 120deg out).
That suggests an additional complication - if youāve got final 120V circuits split across the two 120V feeds, is the genset 120V-0-120V or does the ATS also need to bridge the two 120V supplies?
(would keep all 120V working, but not 240V, obs.)
That UL-certification is looking even more costly! 
Portable gensets sold in the US typically have both 110v15A outlets and split phase 220v outlets (and maybe 5v/12v DC). Most larger genset typically have both 30A and 50A receptacles used for Split Phase applications, so they feed the load center identically the way that grid does. Whereas most standby gensets that Iāve seen only have a split phase 220V breaker rated for the gensets output to direct wire the genset to a transfer switch on the residence.
Thanks - Iāve learned something today (stay curious!).
Long story short - UK domestic generators are uncommon (rural farms), but kit is moving to automatic phase synchronisation and switching for UPS/ EV/ solar inverters/ battery storage. These tend to be Ā£thousands, and have a lot of safeguards as the BS7671 regulations keep changing to cover the latest failure mode found in real life.
The latest update removes DC isolators (solar cable fires) and clarifies GFCI / ELCB donāt work as isolators when back-fed current from the load side (poor solar inverter installs assumed they are dumb switches).
Dave Jones has a recent eevBlog on YouTube showing his latest solar upgrade with battery storage. His inverter has multiple inputs like a FTU, all controllable via HASS.
UK gensets vary - āblueā IEC309 C20 for domestic single-phase 240V and "yellow IEC309 for 110V (used for isolated building site tools).
Open-source festivals like EMF Camp have interesting videos on their power planning tools to feed hundreds of tents and stage kit from off-grid gensets with telemetry. More like SNMP than FTU!
I want to also create a battery bank to handle whole house short term outages. but the last couple of years weāve experienced power outages that lasted nearly a week in both the heat of summer and the dead of winter due to bad storms. And every time it happens I tell myself that Iām going to get that standby gennie hooked up. then life happens.