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Old 02-25-2010, 04:34 PM   #41
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1967 26' Overlander
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As usual, markdoane's right

Quote:
Originally Posted by markdoane View Post
I wouldn't use a 120V AC switch for DC service. The contacts can arc and weld together...
Markdoane is a constant source of irritation to me. But since I know I tire easily of people who give good advice before the consequences dawn on me, I'm going to let him slide. Again.

FWIW, I believe DC switches/relays usually start out life destined for either AC or DC. DC devices have a diode across the contacts to nip arcing in the bud.

If the only switches you can find are rated for AC service, then let the thread know. Surely there's a practicing EE on the forum to advise you on the correct diode to add to the circuit.

My MN buddy might offer a little more insight if I got it wrong.

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Old 02-25-2010, 07:22 PM   #42
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Switches are designed and built for the maximum ARC before break-over and total amps the contacts can take.

A 12VDC switch installed on a 120VAC line stands a good chance to arc over before the switch "makes".
The switch will fail in short order for standard 120VAC applications. The 12VDC switch contacts are closer together and if you notice the 12VDC switches can be much physically smaller than an AC switch for the same current rating.

120VAC switch can be used in 12VDC applications (within reason).
However; most DC applications require stranded wire while household AC switches are designed for solid wire.
Due to wire resistance in 12VDC applications you have a much higher voltage drop over a given distance and need larger wire over a given distance for equal amps at the load.

120VAC 15 amps 100 ft use 12 gage wire.
12VDC 15 amps 25 ft use 10 gage wire.

Best to use AC for AC and DC for DC.

Nope I am not a EE but I have worked with this stuff longer than I care to remember.
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Old 02-27-2010, 12:29 AM   #43
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maniacs on the loose. water heater has its own switch. most of the dc wall switches you find in an rv are rated for 7.5 amps , you would be hard pressed to find an ac switch rated at 7.5 amps as basic is 15 amps. and why would you ?????????????????????????????
for water heater 120 vac service add aux box with 12 gauge 30' cord, would allow you to plug in water heater without using 12 of 30 amp service that way you can run your water heater electric and still use your ac and converter without blowing service breaker. this is a common industry answer to aftermarket 120vac waterheater on 30 amp rv.
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Old 02-27-2010, 12:35 AM   #44
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afterthought; you could add a motor base (male plug in socket,exterior wall mountable)that way the extra cord could be easily stowed when not in use. or just route in with shore cord and coil in trunk.
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Old 02-27-2010, 03:27 PM   #45
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12V vs 120V switch use

thanks all of you for your answers. I will read the back of the switch before I install it. If you ever drop by MN in Jan.(HA-HA), I owe you a bowl of ice cream. We`eat that to warm up when we come in to warm up!!!!!!!!!!! joe q in Minnesnowda
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Old 03-10-2010, 09:06 PM   #46
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Hey Gen,

I just did this with a tankless water heater demanding 40A. Your system requires 120VAC @ 1400W. The Ohm's Law equation, I = P/E confirms that your water heater will need 11.67A, RMS of power for operation: I = 1400W/120VAC.

If I was you, I'd go purchase a single pole <manufacturer specific> 15A breaker to install in the box; this will protect your water heater. Now you will need to add the circuit. Run a new wire (12-2 gauge will be plenty, "mc-lite" cable preferable for outdoor locations) from the water heater to the distribution box, and then connect: white to the white common, copper or green to the ground common; then connect the black cable to the output of the breaker, and plug it in. I see someone has already tied in the second breaker position with that red wire in your photo of your box so that should be all you need to have your power on the other end where your water heater is. Follow their instruction for connection, and you'll be up and running in no time.


Adding an additional load of <12A at the distance you are stretching it should not be an issue. At worst you'll blow the resettable GFI outlet or park breaker if they aren't supplying enough current to meet your demands. You won't trip anything on your end. To manage that feat, you'll need to have everything on: your AC, your water heater, microwave, chargers, a vaccuum, and then probably a other things...

If you are truly worried about it... all you would have to do is run a new beefier cable to your distribution box; a 6-2 nmb should do ya.' Kirchoff's current law reveals the additional current load would only be exuded by your trailer plug in cable and no where else; aka just because you add a 12A device doesn't mean it will affect any other wiring in the trailer. This, of course, won't solve the problem if you are demanding more than 30A of service, but even with your water heater system installed, you'll have to try pretty hard I think...
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