The AC failed at a neighbor's house and the problem was quickly diagnosed as electrical.

I saw that a few of the wires had corroded and one had actually burnt through. The unit is only 5-6 years old. I had some connectors and 14ga Romex wire on me and I was able to repair the unit so that it will keep her cool for the day. But I saw that the wiring was 18ga and thickly insulated.

I am assuming I need to pick up the appropriate wiring and do I need to do anything special on the connection from AC wiring to replacement beyond applying a nut and tape?

  • Is this wiring in the furnace/air handler, or the condensing unit? Since you say the wire is 18 gauge, can we assume it's the control wiring that energizes the coil in the contactor? Photos are always helpful. Did you measure the voltage and current to make sure everything is within tolerances? – Tester101 Jul 1 '14 at 17:09
  • @Tester101 - It is the part of the unit that the main line hits the transistor and capacitor. Have house coming in one side and unit wiring coming in from other direction. One wire was fried so just replaced that and its tabs- went from transistor to capacitor. Another coming from unit I had to cut off 6 inches (I think that wire was fine but got heat from fried wire). I pigtailed that and threw a new tab on it. There was a nick in another wire (from fried wire). I just wanted to get her up in running on a 95 degree day. No photos sorry - at work now. – DMoore Jul 1 '14 at 17:14
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    Are you working on the line voltage side (120/240V), or the control side (24V)? – Tester101 Jul 1 '14 at 17:23
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    Is there a circuit board in the unit? I'm confused as to why there would be a transistor in the condensing unit. – Tester101 Jul 2 '14 at 12:44
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    Thumps!? What thumps? Has it always thumped? Sounds like she might want to call an HVAC technician to come take a look, before the system thumps its last thump. Burnt wires are almost never a cause, they're a symptom of a larger problem. – Tester101 Jul 9 '14 at 12:42

Generally, it should never hurt anything to use thicker wire, assuming that you can make solid connections. The wire should never be the limiting factor! The system should already be fused so that #18AWG won't melt, so #14 will only help.

Wire nuts and tape is a tried and true method, although some would argue that soldering and heatshrink is better.

You mention "thickly insulated". Is there any rating information (voltage, temperature, etc) stamped on the insulation? You want to make sure the romex is at least as good.

Specifically, though, it sounds like something else is going on. The existing wire certainly shouldn't have burnt through. The system shouldn't have drawn more current than the wire could handle, and if it did, an upstream fuse or circuit breaker should have caught it. I suppose it's possible that it rusted first, became less conductive, and then burned through...

If there is some sort of unprotected over-current problem, then using #14 might cause the failure point to change. The wire became the fuse this time; who knows what wold have melted if the wire hadn't?

Anyway, just some things to think about...

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