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I would like to add this light fixture to my ceiling in an area that only has a thermostat 5-wire coming out (something like this). (The other end of the thermostat wire travels about 15 feet, ending in a closet where I can connect the 5 wires to whatever I want.)

The fixture is probably expecting 2 wires with the regular full 120V power, like any light fixture usually gets, (and maybe a third wire for ground). Can I use 2 of my 5 wires to send HOT, 2 wires to send NEUTRAL and 1 wire to send GROUND, from a 3-prong outlet to the fixture via this thermostat's 5 wires?

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    \$\begingroup\$ Note: This Stack Exchange is likely to give you information about whether it's electrically possible, but not whether it's a good idea in a house or whether it conforms to the building code. Questions involving building code are better suited to diy.stackexchange.com. \$\endgroup\$ – user253751 Apr 5 at 19:03
  • \$\begingroup\$ Blaze, you probably should state in your question the wattage of the light. If it is an LED light or two, the current may be quite modest. Also, you may also write a separate question, if you want, asking if there are any suggestions others may have about using that existing 5-wire cable as a way to "pull" other wiring through that does meet code, without having to tear out walls and ceilings. Or if other, better ideas can be had, too. Just a thought. \$\endgroup\$ – jonk Apr 5 at 19:47
  • \$\begingroup\$ "Can I use 2 of my 5 wires to send HOT, 2 wires to send NEUTRAL and 1 wire to send GROUND" - All other problems aside - I find it interesting that you would only use half the wires for safety ground that you'd use for power ;) I'd err on the side of using more capacity for safety ground. (Assuming I'd use a wire like that, which I absolutely wouldn't) \$\endgroup\$ – marcelm Apr 5 at 20:34
  • \$\begingroup\$ Another thought; not only is running mains over that wire unsafe, but the device is probably straight out of china to your doorstep, with no safety certification. It's a combination I'd stay far, far away from. \$\endgroup\$ – marcelm Apr 5 at 20:42
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    \$\begingroup\$ That wire is rated for 150V, but you usually can't have an 18 AWG 120V distribution wire. What you could do is use a low voltage DC LED fixture instead and use the Tstat wire for LVDC. \$\endgroup\$ – K H Apr 6 at 4:53
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Can you do it? Yes. Is is safe to do so? No! Thermostat wire is not rated for 120VAC but rather for 24VAC and also "wire sharing" is not allowed. The wire used to power something must have sufficient capacity to handle the power needs of the circuit by itself.

Generally the smallest wire used is 14 AWG on a 15 A circuit.

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  • \$\begingroup\$ "The wire used to power something must have sufficient capacity to handle the power needs of the circuit by itself." - More importantly, the wire must have sufficient capacity to handle the current in the case of a fault (either sustained below the breaker limit, or over the limit until the breaker trips). \$\endgroup\$ – marcelm Apr 5 at 20:44
  • \$\begingroup\$ That part of my answer was to address the suggestion that doubling up wires in the cable would be suitable in lieu of adequate wire gauge. That is NOT acceptable. \$\endgroup\$ – jwh20 Apr 5 at 21:20
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No, using mains voltage with a cable that is not rated for mains voltage is not only against code but simply dangerous and must never be done.

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