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MtnDon
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# Posted: 7 Dec 2011 08:44pm - Edited by: MtnDon
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Quoting: pondjumpr ....ran the wire to a standard switch like you have on a wall in your house...
Note, that the common household wall switch is meant for AC current, not DC.
What will happen if it is used on DC current?
Sooner or later it will fail. If you are lucky it will fail with the light in the off position. If you are unlucky the internal contacts will weld themselves shut and the light will stay on. If you are really unlucky that could cause some device downstream from the switch to overheat, cause a fire, melt some wiring...... You get the idea.
Why does this happen, or what is the problem?
The metal contacts in an AC switch are lighter (thinner) than what would be used in a switch designed for DC current. They will burn out or weld together.
The problem is magnified if the switch is used for turning on-off the same watts in AC and DC. AC is 120 volts. If the load is 60 watts the switch is handling only 0.5 ampere. If the DC power is 12 volts, switching the same 60 watt load involves 5.0 amperes.
Another difference in switching an AC load and a DC load comes from the nature of AC current. AC current used in north america is 60 Hz. That is it changes direction 60 times every second.
This rapid cycling back and forth through a zero point makes it easier for the switch to break the circuit. That means less arcing at the contacts. Less arcing means thinner metals can be used.
A switch or other electrical device will be marked with some sort of reference to "AC only" if it is not supposed to be used on DC circuits. The same rules apply to power disconnect switches and circuit breakers. Most home style circuit breakers are meant to be used with only AC power. An exception to that is the Square D brand, QO and QOC series of circuit breakers. They can be used in 12 and 24 volt DC power systems. The breakers at the store will most likely NOT have the markings for DC use, but SquareD specs do state they are okay. NEC will accept them in circuits up to 24 VDC at their AC rated amperage. Some inspectors will not accept them for DC when they are not marked as the inspector does not know the facts.
Anyhow, just a heads up.
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Borrego
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# Posted: 7 Dec 2011 09:22pm
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Quoting: pondjumpr I just did a similar set up for a friend of mine at his cabin in Georgia. I used 12.3 wire and wired a standard light fixture to the middle of the cabin roof, just like if I was using 110V
Thanks, Guys!
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Borrego
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# Posted: 7 Dec 2011 09:25pm
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Quoting: MtnDon Anyhow, just a heads up.
Is there anything you don't know ? :-)
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pondjumpr
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# Posted: 7 Dec 2011 09:28pm
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Good point about the AC switch. I used the wall switch for a clean look, like you have at home.
I am using a 50W 12V bulb in the fixture.
I am guessing if your concern is in the wall plate switch, you could use a 12V DC rated toggle wrich to accomplish the same thing.
We only use the light to get dressed in the morning or to get ready for bed at night. Maybe a few minutes at a time. All cooking and "hanging" out is done outside the small cabin.
This is still good info to know as I never thought about a thinner material in an AC wall plate switch.
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MtnDon
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# Posted: 7 Dec 2011 10:19pm
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Borrego; I thoroughly researched the DC switch thing when planning our cabin. Spent a goodly number of hours on that.
Not that it matters to most users here, it is very difficult to find switches that are DC rated and also NEC code approved. I did find some, but they were horrendously priced, something like $20 each. The nice thing was they fit regular household boxes and plates. Don't remember where I saw them though.
I used rocker switches made by Carling switch and hand cut the appropriate opening in standard household blank cover plates for them. Like these switches, except in black
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TomChum
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# Posted: 8 Dec 2011 12:43am - Edited by: TomChum
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I wonder how much DC amps you can safely run thru a 15A AC switch? If you are using LED lighting, the amps running thru the switch might be so small. For example 2 6W LED bulbs would need only 1 DC Amp.
I would consider putting a 1A fuse inline, inside the J-box, just to be sure.
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