1)What is the maximum length can be exposed for BX and Romex wires? 2)The code specify a need for receptacle within 25ft of a Heating or Cooling unit. What if the unit is outside of the house and the receptacle is inside of the house, but it's still within 25ft? Would that be OK?
My answer to Q. 1 There is no limit to the length of exposed "Romex" cable or "BX" but there may be some issue with environment or physical protection. 334.15 for NM (Romex) and 320.15 for Armored Cable (BX) Q. #2- 210.63 is silent about the indoor or outdoor issue. It only says that it must be accessible and on the same level as the A/C unit.
The BX cannot be exposed for more than 3ft I believe. When I say expose, I mean it will not be hidden in the wall. As for the attic, I believe this rule do not apply, but for finish wall, this is another case.
Don't know where you are getting the idea the BX, which is type AC now, cannot be run exposed. You can run it as long as you want subject to 320.15.
As to your question about the service receptacle, the code only says that it must be within 25 feet. I would not consider a receptacle in another space or inside the house to meet that requirement however.
In most buildings the inside floor is at a higher level than the outside grade.
Well that may be stretching it a bit but I think the intent is to have the receptacle available for easy access by a serviceman and that's why the code requires one to be installed. If you had to run an extension cord through doors or windows we would be back to the days when the receptacle wasn't required.
1) Many local town are adopting "exposed run" restriction, but that's a local Code issue, not one found in the NEC, as long as support requirements are met. My town started that a few years ago, limiting exposed runs of AC, MC & Greenfield to 24" from penetrations (except for motor and lighting whips). Most of the IL Lake Shore has already outlawed NM, so that's moot for us.
2) I've always installed one within L.O.S. of the HVAC in the basement - usually a lot less than 25'.
I've always heard the "25'" rule applied to roof mounted HVAC / mechanical stuff - so they wouldn't have to run extension cords from inside the building. (or to sign receptacles with adaptors)