Found the manual here. This is a nominal 265V unit, but should run at 240V based on the manual and the nameplate.
Based on the manual:
- The HEH123 is really what you should have. The HEH124 is for nominal 265V, which effectively covers 277V and barely 240V.
- The HEH124 is supposed to be hardwired:
Cord connection to a wall socket is not permitted for 265-volt units. All 265-volt units must be hard wired using the hard wire kit or make use of the plug-in receptacle in the standard subbase.
I'm sure the subbase should be hardwired. So if you have the subbase that should include a receptacle to match the plug you currently have, and you will hardwire the subbase to your circuit.
If you don't have the subbase then either this was originally installed elsewhere incorrectly or the original installation was with the subbase and somebody gave/sold you the PTAC itself but not the subbase. In either case, you should remove the plug/cord and install this hardwired - installing it plug/cord/receptacle without the subbase is violating the manufacturer's instructions and, by extension, violating the NEC.
Assuming you actually have 120V/240V, there is no transformer or anything else special needed. 238V < 240V < 292V. So a simple 20A double-breaker and two hots + ground is all you need.
There are three potential issues:
This shouldn't be a problem here, as the 100A should already include allowance for some sort of HVAC. But if the apartment originally had, for example, gas heat, and the load calculation was already close to the limit then there could be an issue. But probably OK.
Based on what I can see, this system only needs 240V(ish). Some systems actually require 120V/240V - i.e., they have some parts that run on 120V. If so, then you will need a neutral wire in addition to two hots and a ground. It is important to figure out in advance because you can't simply combine neutral and ground. Combining neutral and ground is an old practice that has been against code (except for grandfathered dryers and ranges) for decades. So make sure in advance. You can always run a /3 (3-wire plus ground - black/red/white/bare) cable and cap the white if it isn't needed this time.
Most residential housing in the US is a simple 120V/240V configuration. But some is actually 120V/208V. Mostly this is in large apartment buildings, but not exclusively. Many appliances are designed to handle this - e.g., 200V - 250V. But this particular device is on the high side - lowest permissible is 238V. If you run it at 208V(ish) then it may work OK, it may not work at all, it may work inefficiently, and it may die a horrible death. So double check your nominal hot-hot voltage. That is easiest at a plug-in 240V device (most common is an electric clothes dryer) but if there are none available then you can check it (carefully) in your breaker panel. If you get a voltage of 238V or higher, you're all set. If it is lower then you can't use this device. Note that a transformer is not a realistic solution. A transformer can get you from 208V to 250V or higher. But it will cost quite a bit for a transformer to handle this size load, and a far better solution is to get a different HVAC system that will work with your available voltage.
