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I'm installing 120v to 12v transformers for low voltage lighting. Inside the transformer are output connectors for the low voltage wire, and those output connectors are rated for 14 awg wire. I'm using 10 awg wire. What is the proper, and code compliant way, to connect the 10 awg wire to the 14 awg output connectors?

Output Connections

Tranformer

10awg Wire

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The correct answer is to use approved wire twist on "wire nuts", or split bolt. The package description will usually say how many and what sizes are allowed.

For example, the common red twist on "wire nut" will hold up to five #12s, officially. There is a diagram and chart on the product which gives examples for mixing different wire sizes.

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    I would use a yellow for that job. While wire nuts have an impressive range, I prefer to use them in the middle of that range where they are less sensitive to perfect technique and compatible wires (noting that his wires are finer than THWN). – Harper Apr 28 '17 at 14:25
  • Even though the load pictured is LED which doesn't suffer the VD as magnetic. – Kris Apr 28 '17 at 14:40
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Goodness, that seems like awfully heavy -- and stiff -- wire for low-voltage lighting. Did you get a lot of 10ga free or something? You could attach short lengths of 14ga to the output screws and use appropriate wire nuts to join to the 10 ga inside the box. Not sure that is at all kosher, but it's workable.

  • 10awg wire is often used for 12v systems to compensate for voltage drop – Kris Apr 28 '17 at 14:08
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    Not at all. Low voltage is much more sensitive to voltage drop, and worse, you must flow much more current to do the same work. Try punching a 100 foot run of 120 watt load (120Vx1A or 12Vx10A) into a voltage drop calculator at 3% permissible drop... you'll see how important it is to upsize low voltage DC cables. Also the pictured cable must have 100 strands, much more flexible than the usual 19-strand THWN-2 wire (to say nothing of the solid wire used in Romex!) – Harper Apr 28 '17 at 14:30
  • Indeed true about using higher ga to compensate for voltage drop. Just assumed this was a regular yard lighting situation where wattage was low and runs were not exceptionally long. I know I've done 100 foot runs with 14 ga for LED yard lights, but there's certainly value in upping the wire in many situations. – user8356 May 1 '17 at 12:37

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