Generator wiring into Automatic Transfer Switch – wire gauge

Question

I will be wiring a Generac 24kW Generator, connecting into a 200A Generac Automatic Transfer Switch.   This Transfer Switch’s output is then powering a 200Amp service panel.  The Generator’s max output is 100A (24kW @ 240V), and it also has a 100 Amp main line breaker on the Generator itself.
When wiring (3-wire) to the generator, can I to wire for 100A (e.g. 00 aluminum wire)?  Or do I have to wire for 200A (e.g. 0000 aluminum wire)?  There is a tremendous difference in price and effort.  It will be about 80 ft from transfer switch to generator (about 2/3 inside and about 1/3 buried outside in pvc conduit)
The maximum that can actually travel down that wire is 100 amps of current (even though it’s connected to a 200A transfer switch & 200A panel).  If 101 amps are called for, the 100A breaker on the generator will flip to prevent generator overload.  So is 00 aluminum allowed in this situation?

Harper - Reinstate Monica · Accepted Answer

You have to run 4-wire to a generator.  A separate neutral and ground wire is mandatory.  A ground rod will not substitute. The only way to avoid running a 4th wire is to use non-flex metal conduit the whole way.  (or run 120V only, but that's no fun).
But that's OK, I'm about to save you enough money to pay for the ground wire or the conduit.  Minimum ground wire size is #8 copper or insulated #6 aluminum.
Since the generator run will be circuit breaker protected at 100A, you can use 100A rated wire.  Voltage drop will not be a concern at only 80 feet.
That is #1 aluminum wire if a) the wire or cable insulation is a type permitted 75C operating temperature, and b) the terminations (lugs) are rated for 75C operations.  The breaker panel lugs certainly are; whether Generac graces you with 75C lugs is a question for Generac.  you would think...
Do not put cable in conduit if you can possibly avoid it.  It's not illegal; just exceedingly difficult and serves no useful purpose.
If your entire run is in conduit, it behooves you to use THWN-2 or XHHW-2 wires which are indeed allowed 75C in practical use.  (technically they could go 90C but that requires meeting a long list of conditions; the electrical inspector is likely to nix this).   NM is not allowed outdoors and UF is allowed only 60C.
If your entire run is not in conduit, and making it so is impracticable, you can transition from one cable type to another in a reasonably large box using splices. The box must remain accessible without tools or dismantling any part of the house.

Generator wiring into Automatic Transfer Switch – wire gauge

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