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Old 01-09-2014, 08:22 AM   #1
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Inverter Switch for bumper outlet

I'm installing a waterproof 120V electrical outlet in the bumper compartment. This outlet will be powered by a small inverter to run LED outdoor lights and a radio. The inverter will be powered by a dedicated battery. The battery will be removable and only go with us when we're boon-docking for a few nights. The outlet, switch and inverter will be permanently mounted. When the battery is on board and hooked up, I need to be able to switch on the inverter to power the outlet.

Can I use a regular 120 volt switch?
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Old 01-09-2014, 08:37 AM   #2
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Yes, a 120 volt switch can be used to switch 12 VDC. Of course, you want to switch the 12 volt side of the input to the inverter.
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Old 01-09-2014, 09:19 AM   #3
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More info:

The switch will be used to power on the inverter, so it will be in a 12V circuit: Battery to inline-fuse to switch to inverter.
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Old 01-09-2014, 09:23 AM   #4
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I use a portable Power Box for this. Keep it charged and in the truck. Always ready to emergencies or for just the application you are looking for. Here is one from Harbor Freight for $100.

  • Up to 20 hours of 12 volt power on a single charge
  • Heavy duty 6 gauge booster cables with copper coated steel clamps
  • Maintenance-free 18 Ah sealed lead-acid battery
  • 300 PSI air compressor with gauge
  • Super-bright 4-way LED work lights
  • 200 watt power inverter with dual AC outlets
  • Dual 12VDC cigarette lighter ports
  • USB port
  • Heavy duty rubberized case
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Old 01-09-2014, 09:35 AM   #5
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I would never use a 120 Vac switch on a 12 Vdc circuit, especially with the type of load you're likely to push through it to power an inverter.
Go to Radioshack or your local automotive store and buy a toggle switch with a DC rating at least equal to the power of your inverter.
The problem with using household switches in DC circuits is the arcing that occurs when you break the circuit.
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Old 01-09-2014, 10:18 AM   #6
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Switches like everything else electronic, are rated in volts and amps, essentially they are rated in power or watts. You can use a switch rated for higher voltage in a lower voltage circuit and actually at a higher current than their normal rating if at a lower voltage.
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Old 01-09-2014, 10:57 AM   #7
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Ampacity and arcing are the issues. A 15A residential switch is good for switching 15A, and a 600W inverter would draw about 50A at 12vDC at its rated output, so the 15A light switch doesn't seem like a good choice in this application.

I wouldn't hesitate to use a residential light switch to switch 12V to a string of LEDs because the amperage would be negligible, but that's a different application.
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Old 01-09-2014, 10:59 AM   #8
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Ampacity and arcing are the issues. A 15A residential switch is good for switching 15A, and a 600W inverter would draw about 50A at 12vDC at its rated output, so the 15A light switch doesn't seem like a good choice.
You are right, a 15A residential switch is rated at 15A@120VAC. Check one out, it's written right on the side of it. It will switch much higher current at lower voltage. Don't know if it will live long at 50 amps, but the worst that would happen is the switch would fail.
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Old 01-09-2014, 11:14 AM   #9
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You are right, a 15A residential switch is rated at 15A@120VAC. Check one out, it's written right on the side of it. It will switch much higher current at lower voltage. Don't know if it will live long at 50 amps, but the worst that would happen is the switch would fail.
I am not an electrical engineer, but my impression has always been that if there's a difference, it'll be rated for a lower ampacity at 12v.

Here's an example. Data sheet for an APEM rocker switch. It's rated for off-on switching at 16A at 120V AC, 15A at 12V DC.
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Old 01-09-2014, 12:04 PM   #10
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Switches have a rated current, but also they are DC or AC rated. An AC rated only switch, in theory at least, should never be used for DC applications. The reason is that when the switch is opened (turned off) DC current wants to continue to flow and establishes an arc which can either weld the contacts together, or melt them and other internal parts in the switch. That can cause a fire, or lack of shutting the circuit off.

Think of how you weld. You touch the stick to the metal base and then pull it back, forming an arc which melts the metal base and stick. If you pull it back too far, you will extinguish the arc.

It is much harder to form an arc in an AC circuit, due to the self extinguishing AC feature of the power going through a zero condition twice per cycle.

In an AC rated only switch, the gap in the opening is smaller than in a DC rated switch and an arc is possible to form, when used on DC, which will not go out easily due to the smaller internal gaps within the switch.

The higher the DC voltage, the more probably this problem becomes. Quite frankly it does not seem to be much of an issue at 12 volts DC as the arc extinguishes with a small gap. However, when you get into the 48 volt plus range it is a real problem. I found with my 48 volt DC solar system, disconnecting the panels under load could easily need a 1/4 to 3/8 inch gap to make the arc go away. Special DC rated Circuit Breakers and switches are very important in higher voltage solar systems on the DC panel side (and also code required).

Back to the original question. Although I personally would use a DC rated switch to turn the inverter on and off, you probably could get away with a normal household switch on a small inverter with minimal load. By minimal load, I mean < 10 amps at 12 volts DC. That means a 100 watt inverter maximum. The switch contacts may erode more rapidly than normal, and the product is being used where it is not rated though. Pick up an automotive DC rated switch and do it right.
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Old 01-09-2014, 01:04 PM   #11
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So let me point out something here.... If you want to run LED, than run 12volt LED directly to the battery and not run it through 110 with a transformer. The transformer is simply using up battery, you can simply connect the 12volt LED's to the battery and forgo the transformer! You can also find 12volt radio.... any car radio runs on 12 volt... I guess what I'm saying is no need for 110 when you can do what you want on 12volt and get the same results with much lower battery draw!
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Old 01-09-2014, 01:19 PM   #12
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So let me point out something here.... If you want to run LED, than run 12volt LED directly to the battery and not run it through 110 with a transformer. The transformer is simply using up battery, you can simply connect the 12volt LED's to the battery and forgo the transformer! You can also find 12volt radio.... any car radio runs on 12 volt... I guess what I'm saying is no need for 110 when you can do what you want on 12volt and get the same results with much lower battery draw!
Thanks...I was about to post the same thing!
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Old 01-09-2014, 02:19 PM   #13
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You are right, a 15A residential switch is rated at 15A@120VAC. Check one out, it's written right on the side of it. It will switch much higher current at lower voltage. Don't know if it will live long at 50 amps, but the worst that would happen is the switch would fail.
With all respect, that is patently untrue. Unless a switch is rated for DC, I wouldn't use it. There is no "general" correlation between AC switch rating and DC switch rating, as it depends on several design variables.
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Old 01-09-2014, 02:30 PM   #14
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Indeed a light switch might have a short life. Perhaps try an automotive switch and a Hell's foglight relay. Thus is a cheap and easy solution to your needs.
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