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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.

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closed as not constructive by lqlarry, Tester101, Steven, ChrisF Apr 18 '12 at 8:17

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So instead of running the proper cable, you ran an undersized cable? If it's not a problem to run new cable, why didn't you just run the proper stuff in the first place? –  Tester101 Apr 17 '12 at 11:55
Jury-Rigged electrical = FIRE. It aint worth it. And you can be damned sure that your insurance company will be the first one to find this out and not pay you in the event something occurs. –  Steven Apr 17 '12 at 14:32
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2 Answers

I will echo TomG's sentiment: Yikes!

By using the BX as the "neutral/ground" (really it's just the neutral; a three-prong 220v plug is considered ungrounded regardless of the continuity that should exist between neutral and ground), whenever the A/C is on, the armor of the cable is energized. It will have a lower voltage than the "hot" because the A/C is using some of the available power, but it will still be energized. And that means that anything metal that the armored cable is touching (metal brackets to hold the cable, plumbing, HVAC, and gulp gas lines) is energized, and/or can induce an arc from the armor. Arcing causes fires; the temperature of an electrical arc can easily reach thousands of degrees, and that's happening right next to paper insulation backing and drywall, wooden structural members, etc.

This wiring should be corrected ASAP, by running 10-3 armored cable from the panel to the A/C and properly grounding the armor on both sides (NOT using it as the neutral; you'll have a white wire in the bundle for that). The difference, though subtle, is that the neutral is only bound to the ground at the bus strip all the way back at the panel. So, the "shortest path" to ground during normal operation is through the neutral to the grounded bus strip, NOT through the neutral to the bus strip and then back to you through the ground wire. By contrast, if the armor is the neutral, it's normally energized when something on the circuit is on and may well choose to divide the current between the normal return path and through any other way it can get to ground, including you.

Should the wiring fail and a short circuit occur, the shortest path is STILL into the ground wire and to the bus strip from there. Failures that energize the ground leg are also generally failures that will trip a breaker very quickly, so unless you happen to be messing with the wire at the time of the short it is highly unlikely you'll get shocked. The one situation that can still be dangerous is a neutral to ground short, and in that case both the neutral and ground are running in parallel fro the point of the short to carry current back to the neutral bus, so as long as you aren't dripping wet and standing on a grounded metal plate or a puddle of water while holding the shorted cable, the electricity still finds the combined neutral and ground to be the easiest path. By contrast, if the armor is the neutral, the cable is already in its "failure mode", but since a short isn't drawing massive amounts of current through the hot side, the breaker won't trip. This means you can be shocked very easily as the neutral side uses you as the parallel path to ground.

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Yes, just because he got lucky for 25 years, is no reason to continue to take unnecessary risks with his family and home. –  Brock Adams Apr 17 '12 at 22:31
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I am not an electrician, but I would run new 10/3 BX. Your jury-rigged system might work, but it has the capacity to electrocute someone, or maybe start a fire, if anything goes wrong. Possible issues include the BX armor being hot if a fault somehow occurs where it's connected to the neutral bus or overloading one of the hot wires if one of your "paired" conductors is somehow disconnected.

Also, if you ever want to sell the place, it would surely fail inspection. You might as well enjoy a safer installation now, rather than having to fix it later.

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