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Old 02-09-2014, 11:20 AM   #43
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Back decades ago the white wire was often called the ground—that was before a third wire, the green or bare one, was required as the real ground wire. Then you started to see plugs with three prongs, the round one, usually on the bottom, being the ground. So the white wire was to be called the "common" and not the ground.

Then came a different way to ground tools and they came with plugs without a ground. I think they were called "doubled grounded" for a while. How that works is beyond me, perhaps it is the way they are insulated internally. Some things have the independent ground now in the plug and others don't. Regardless, the 120 v. circuit needs a ground and as J. Morgan explained, it usually goes to the pedestal at a campground where it is grounded to the earth at some point.

12 v. works differently in practice. The body of the trailer or a car or a truck acts as the ground since it is hard to drive very far with a copper rod in the ground. In a motor vehicle one terminal of the battery is wired to the motor and the body for example. It can also act as the common and this works since it is hard to kill anyone with 12 v. Auto (and trailer) wiring is quirky and confuses many mechanics so you have to be careful who you ask to fix 12 v. wiring. The color codes used in 120 v. wiring are often violated in auto wiring causing more confusion.

I am often confused by auto wiring, but usually blunder my way through it. I understand 120 v. much better, but sometimes screw that up too (yes, you can melt the end of screwdriver blade in a breaker box—always use electrician's tool with insulated handles). I've been doing most of my own wiring for 50 years and most people can learn how, but it can be dangerous and you can blow out things that are expensive. I learned some of it from real electricians and then read up on residential and industrial wiring, but there's plenty to learn. There are some basic books like Richter (in its millionth edition by now) that are good for residential and farm wiring, but it is only a start.

Some people take to wiring quickly, others do not. In my opinion it is a lot simpler than plumbing. Getting a shock can be more likely than drowning, but getting wet and holding a hot wire is the ultimate you don't want to experience.


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Old 02-09-2014, 12:15 PM   #44
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It is 240 volt

Larger rigs with 50 amp service in a 4 wire configuration are wired with a 240/120 volt 4 wire supply. While the unit may not have any 240 volt devices such as a dryer. When reading between the 2 hot legs. One will see 240 volts.

The white wire in an AC system is referred to as "neutral" not "common". The term "common" is most widely used in DC circuits.

The automotive industry made a mistake years ago by calling one side of the DC circuit "ground". It was never tied to "earth ground". But we are stuck with the terminology today.

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Old 02-09-2014, 03:50 PM   #45
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TG, I believe "common" is now used in AC as well. I haven't seen "neutral" in quite a while. All these terms can be misleading. I think "common" comes from the fact all the white wires are common to one bar in the breaker box. Thus all white wires are linked together in every circuit. The black wires go to individual breakers in the box.

Further complicating things are that most electrical equipment comes from China and the connections are a little weird and the instructions can be even stranger.

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Old 02-09-2014, 04:09 PM   #46
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Not wanting to beat a dead horse, but modern DC wiring is referred to as "positive and negative" to avoid any confusion with AC wiring. As far as accepted color codes, the US and EU have very different colors for the positive and negative DC leads, and I'll just stick with US color coding for this discussion. And yes, the negative is also the ground in modern DC electrical systems.

Red is generally referred to as the positive lead in DC, with black as the negative lead. This goes awry rather quickly when red is not present in a 12VDC circuit. Without the presence of red, BLACK is generally considered the positive with WHITE as the negative. Other colors (other than white) can also be used for a negative lead when paired with black (but NOT RED!)

Confused yet, don't worry...........If you have a boat, the accepted color coding is red = positive and yellow=negative. it degrades rapidly from there!!!

When working on DC circuits, it's always a good idea to keep a volt meter handy so you can physically check the polarity of your wiring. This is especially true if some unwashed tech has touched the wiring and had no idea what they were doing (I've seen it more times than I care to enumerate!).

Lew Farber...RVIA Certified Master Tech...ABYC Certified Master Marine Electrician
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