Electrical Wiring in the Home/bad wiring at air handler

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Furnace
Furnace  
QUESTION: Bob,
I do a lot of construction side jobs and most of my experience lies in minor electrical and carpentry. Recently at a job I came upon a recently installed gas furnace/air handler that had some very odd wiring. According to the property manager, all of the air handlers had been replaced in July 2015 by a licensed contractor. What I observed was a piece of 10-2 leaving the furnace, spliced in-line to a piece of 14-2, with wire nuts and electrical tape. Aside from the obvious ill-contained junction, why is this wrong on a technical level? Obviously, the 10-2 max amperage rating is far higher than the 14-2 it is tied to, but for my own edification I'd prefer to know specifically. Also of note, see image, there is 14-2 feed coming down from the ceiling of the furnace closet that has been cut and taped and two feeds coming up from the bottom of the furnace closet that are cut (one of which is spliced into the 10-2). I appreciate any feedback.

ANSWER: It is OK to use 10-2 on a 15 amp circuit.  HVAC people use 10-2 for the compressor and 14-2 for the furnace, so I guess he had 10-2 left over.

---------- FOLLOW-UP ----------

QUESTION: I suppose I'm still a bit confused. I am aware that using 10-2 on a whip from the a/c compressor to a service disconnect outdoors is standard practice. However, I have never seen a gas furnace have a piece of 10-2 spliced with a run of 14-2.

Answer
It was sloppy, and the splice itself was illegal.  But it is OK to use, from a technical standpoint, 10-2 on 15 amp circuit. The person probably ran out of 14-2, and had only that piece of 10-2 on his truck.  It was not a good idea, but in theory, you can use 10-2 on a 15 amp circuit, as long as it is spliced correctly.
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Bob Sponaugle

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Just about any home wiring question. (not appliance repair) I have done all kinds of home wiring for myself, including adding a new breaker box, etc. Please, questions from North America only. Please be specific with details.

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