My house was constructed in 1980 in the City of New York. About twenty-five years ago, I installed a new 29,200 BTU Carrier A/C through the wall of the house. The unit was rated at 208v, drawing 15.1 Amps.
The house was built with a “standard-sized” A/C sleeve through the wall, serviced by a solid-copper-wire, steel-armored, 12/2-BX cable, with a single-pole 120v, 20-Amp dedicated circuit breaker receptacle next to the sleeve.
I ripped-out the “standard-sized” sleeve, replacing it with an oversized sleeve designed to house the new A/C. However, as to the original solid-copper-wire, steel-armored, 12/2-BX cabled, 120v, single-pole 20-Amp dedicated circuit breaker receptacle, I replaced it with a new 240v receptacle. Then I ran a second solid-copper-wire, steel-armored, 12/2-BX cable from the steel main-panel box back to the new 240v receptacle (A distance of about 35-feet!), and then I installed a new 20-Amp double-pole dedicated circuit breaker in the steel main-panel box.
I then “jury-rigged” the two 12/2-BX cables by twisting and capping the two black wires together and running a short “pigtail” to one of the “Hot” 240v receptacle leads, and then twisting and capping the two white wires together and again running another short “pigtail” to the other “Hot” 240v receptacle lead. I then ran another short “pigtail” from “Neutral/Ground” 240v receptacle lead back to the steel receptacle box, thereby converting the steel receptacle box and the steel-armor of the BX cable itself into the “Neutral/Ground” lead.
At the steel main-panel box, where I installed the 20-Amp double-pole dedicated circuit breaker, I again “jury-rigged” the two black 12/2-BX cables by twisting them together and attaching them to one of the 20-Amp dedicated circuit breaker poles, and I then twisted together the two white 12/2-BX cables and attached them to the other 20-Amp dedicated circuit breaker pole. I then securely affixed the two independent steel-armored 12/2-BX cables to the steel main-panel box, and for “good-measure” ran a “pigtail” from the steel-armor of the BX cables to the “Neutral/Ground” bar of the steel main-panel box.
Having said all that, I need to say that (1) I was shown this “jury-rigging-maneuver” by an “old-time-electrician” who claimed to have “converted hundreds” of 120v circuits to 240v circuits by the aforementioned “method”, and (2) my oversized 29,200 BTU, 208v, 15.1-Amp A/C worked perfectly fine for the better-part of a quarter-of-a-century!
Fast-forward twenty-five years and the old 29,200 BTU A/C is dead! However, I found a newer (About ten-years old and in great condition!) Carrier A/C on eBay for a fraction of the cost of a new one, and while the physical dimensions of the newer A/C are identical to the dimensions of the old one, the power requirements of the newer A/C are more “demanding”.
While this newer unit is rated at the same 208v, it’s putting out 32,200 BTUs, while drawing 19.4-Amps and generating 3,930-Watts! Carrier’s specifications require that this newer A/C be run on a 30-Amp double-pole dedicated circuit breaker, and while it’s simple-enough to replace the 20-Amp dedicated circuit breaker with a new 30-Amp dedicated circuit breaker in the steel main-panel box, I’m concerned about the efficacy/safety of my “jury-rigged” wiring, especially using the converted steel receptacle box and the steel-armor of the BX cable itself as the “Neutral/Ground” lead.
From cruising the Internet, it appears that a 12-guage solid-copper wire can safely handle up to a 20.0-Amp load and up to 3,840-Watts at 240v, and that by twisting together a pair of 12-guage solid-copper wires it should produce the functional equivalent of a 9-guage solid-copper wire, which should boost its capacity to somewhere around 35-Amps and about 6,500-Watts at 240v. That said, I’m anxious of the merit of continuing to use the “jury-rigged” steel-armor of the BX cables as the “Neutral/Ground” means to continue to safely handle the power demands this newer replacement A/C.
Sorry for this “long-winded” depiction, but as my quandary concerns a potential safety-issue versus the time and expense of ripping-out the twin 12/2-BX cables and running a single 10/3 (or possibly even an 8/3) cable, I wanted to be as precise as possible in relating my circumstances.