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Old 10-23-2014, 12:57 AM   #1
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Brand: Thor Motor Coach
Model: 31F
State: Georgia
Posts: 166
THOR #631
Adding 12 volt Vanity lighting

My wife and I think that we need to add some vanity lighting so that we can better see our aging faces. I've seen some three bulb 12 volt light bars that could potentially be mounted either above or on the side of the vanity.

Have any of you done similar additions of lighting? Could it run off the same switch as the overhead light or would I have to put in a separate switch (my guess).

I've worked on auto lights and accessories and understand that I need to find a close-by hot wire and then get the fixture to a suitable ground.

Is there any worry about overloading the transformer? If there are open slots on the access panel, should I run a totally new circuit or just tap into an existing one?

Thanks in advance for any feedback.
John
Athens, GA
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Old 10-23-2014, 02:13 AM   #2
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Brand: Thor Motor Coach
Model: 2011 Four Winds 28Z
State: Michigan
Posts: 1,273
THOR #531
There is a special issue with 12VDC wiring, and that is excessive voltage drop. This goes way back to the argument between Nikoli Tesla and Thomas Edison.

Unlike AC circuits, DC circuits have a voltage drop you may have to contend with. While you won't likely see any issues with a low-current appliance - especially if it is LEDs, it's something to be aware of every time you wire in DC.

The thing is that the DC wire must be sized for voltage drop, not current carrying capability.

And the length of the wire matters as well. The longer the DC wire, the higher the voltage drop.

Fact is, voltage drop is a more significant issue than the ampacity capability of wire, and you will find you will likely need to increase the wire size due to limiting voltage drop far before you will have ampacity issues.

The way to do is to figure out the ampacity of the branch circuit (existing) as well as any added ampacity requirements of the new wiring, then determine wire length and AWG. This will allow you to determine the voltage drop in the wire.

For non-critical circuits, you can usually get by with no more than a 10% loss in voltage along the wire.

Here is a voltage drop calculator, that works well for both AC and DC circuits:

Voltage Drop Calculator

Be sure though that you use the calculator to determine round trip distance (of course if you are using the chassis ground, you can use the one-way distance as the ground is so massive that it eliminates concern about voltage drop).

I am providing this info for you for future use as much as your present project. If you are using LEDs, then the current demand will not likely present many voltage drop issues. But if you used incandescent lighting, it could be an issue as a 6 bulb fixture could draw 10A @ 12VDC.
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Old 10-23-2014, 09:23 AM   #3
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Brand: Thor Motor Coach
Model: 31F
State: Georgia
Posts: 166
THOR #631
Thank you! That information was very helpful. Honestly I had not thought about voltage drop with DC yet, but probably would have as I got deeper into the project. Your helpful reply took me where I needed to go mentally. I appreciate it.
John
Athens, GA
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