using existing romex to run 12v dc power for under cabinet led strip lights

[loboman81]

im doing a kitchen cabinet install and customer requested led accent lighting. i bought the leds off ebay,the chinese made ones that come on a reel.thing is the electricians are saying to use the romex thats already ran in the walls and stubbed out where cabinets are hung to power the strips. originally was one wall switch controlling all on and off. then customer changed it to have upper and lower controlle seperately. now they want dimming added.this is the 4th electrician contractor has hired and not one has ran low voltage wire, done been rocked and painted.so how well will using 14-2 romex work for 12v dc. i know i haave to get the right rated power supplys and dimmers.my thinking is this isnt the way to run it but im a cabinet guy. any info input on how else to run the wiring and power supply with dimmers would be great. thanks

 

[Harvey]

Romex is specified to carry many amperes at 120 volts AC so it won't have any problems handling 12 volts DC. Just be damned sure it's not still connected to the AC mains when you handle it.

BTW - For your next post, please try using capitalization, punctuation and spacing in your text. When it comes to reading all that text blur, my mom warned me, if I didn't stop it, I'd go blind!

 

[jgy]

Don't listen to Harvey, wire can handle less power when running 12 volts than it can when running 120. For example, 14 gage romex is specified to handle 15 amps; at 120v that would be 1800 watts of power. This is a large but acceptible draw for a 15 amp 120v circuit. Now try pushing that same power on a 12 volt circuit. 1800 watts / 12 volts = 150 amps! Whoops, you've just fried your cable.

A good rule of thumb is that a 12v circuit can only handle 1/10 the power of a 120v circuit. So your load should not be more than 180 watts for 14 ga romex.

 

[Evadman]

For wire, voltage doesn't mater, only amps does. (unless you go over the max voltage rating; usually 600 volts, check the insulation). If there is 14 gauge wire, that means it can handle 15 amps. Voltage doesn't matter as long as it is under the maximum voltage specification.

Keep in mind that I am applying the spirit of the national US electrical code, there may be a code that says you can't run low voltage over r/nomex that I don't know about.

 

[Micrornd]

Actually, don't listen to jgy, as he has no concept of DC wiring
Harvey is on the right track, but left out a couple things

DC wire sizes are determined by voltage drop and length of run.
Here's a typical chart -

http://www.offroaders.com/tech/12-volt-wire-gauge-amps.htm

For what you are doing in a typical size (even commercial size) kitchen, your electricians are correct, the 14 gauge Romex for LEDs is overkill.
As you can easily see on the chart, you can almost create a tanning booth in the kitchen with LED strips on a single circuit using 14 gauge wire.

The only thing I would warn about, is to make sure the LED power supply is in a ventilated area so as to not overheat.

 

[Harvey]

Don't listen to Harvey, wire can handle less power when running 12 volts than it can when running 120. For example, 14 gage romex is specified to handle 15 amps; at 120v that would be 1800 watts of power. This is a large but acceptible draw for a 15 amp 120v circuit. Now try pushing that same power on a 12 volt circuit. 1800 watts / 12 volts = 150 amps! Whoops, you've just fried your cable.

A good rule of thumb is that a 12v circuit can only handle 1/10 the power of a 120v circuit. So your load should not be more than 180 watts for 14 ga romex.

I don't know your qualifications to be spouting like that, but you're a bit off the point. His question was whether the Romex he has in place will meet his requirements. An LED array can require as little as 10 -12% of the power required to produce the same luminoscity as a typical incandescent bulb and about half of the power to obtain the same luminoscity as a CFL.

The actual current through the Romex to the LED array will be determined as much by the design of the system, including the power supply (output voltage, and whether it's switching, linear), any dimming system and whether the LED's are connected in series, parallel or a series/parallel configuration.

You're right that the issue is current draw through a given length of wire because even 6 nines pure silver has some finite resistance.

Using plumbing as a metaphor, voltage is the equivalent of water pressure, current is the equivalent of the rate at which water flows through a pipe, and resistance is the equivalent of the diameter of the pipe. When you try to shove too much water through a pipe of a given diameter, you get back pressure. When you try to shove too much current through the resistance of a wire, you get heat. The only difference between a wire and a fuse is that the fuse is designed to melt and open when too much current passes through it.

He has professional electricians helping and advising him. If they're licensed and bonded, I would think they would be aware of the safe range of operation of his Romex and qualified to do a safe installation. If so, he won't have the added expense of running new wire to do a job the current Romex will handle.

 

[Wall Street]

I would also note that LEDs don't dim in a linear fashion with voltage like incandescent bulbs do. There is a threshold voltage below which no light is produced. Therefore, most LED dimmers are PWM based.

Also, the wire will be fine.