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Old 07-11-2008, 09:44 PM   #1
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30 amp plug schematic

I know this has been discussed probably 1000 times, but because of that a search will take forever to wade through.... I need to know what each of the three prongs on the 30 amp shore power plug is for. I am building an adapter so I can draw 30 amp power from a four-prong locking type plug, where the blade with the "dog leg" is ground, the opposite prong is neutral, and each of the two prongs to the left and right carrie 110 volts. I tried wiring my adapter such that ground from the locking plug went to ground, neutral went to one of the thirty amp "blades", and one hot lead went to the other "blade", but that didn't work. What does work is to have the neutral from the source/locking plug go to the ground on the thirty amp plug, and a hot wire to one "blade" and the ground to the other "blade". Is this safe? Is the trailer wired wrong? Is the source wired wrong? Are you as confused as I am?
Thanks in advance...
Greg
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Old 07-11-2008, 10:09 PM   #2
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Looking at the plug from the blade side, with the round blade on the top.

The top (round) blade is green. Ground
The Left (flat) blade is white. Neutral
The Right (flat) blade is black. Hot
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Old 07-12-2008, 01:15 AM   #3
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you will want to connect the wires to the same sets in both ends,
ground to ground (green)
Neutral to neutral (white or silver screw)
hot to one hot leg (black or brass colored screw)
the second hot leg on your four prong outlet is not used (your only using 120 volts out of the plugs 240 volt potential)

connecting the ground to the other hot is a big no no, anything you plug into your motorhome with the third ground wire will make all metal case parts live! and possibly the motor home chassis itself.

remember, electricity is always looking to go to ground, if the shortest path is through you to earth you can be seriously hurt.

-Matt
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Old 07-12-2008, 01:26 AM   #4
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I'll try to post a diagram tomarrow to clear up confusion
-Matt
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Old 07-12-2008, 07:09 AM   #5
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These are the standard connections for both the 30A 3 and 4 pole connectors. The first way that you had your adapter wired sounded correct (however there may have been a hot and neutral swap - I couldn't tell from your text). Try using these diagrams to wire your adapter exactly as shown - G to G, W to W, B to B, etc. and let us know what happened.
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Old 07-12-2008, 07:55 AM   #6
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I tried matching colors-- green to green, white to white and black to black, but got no power that way. What finally worked, at least for delivering power, was white to round (ground), black to one blade and green to another. I guess this means that the plug on the trsiler itself might be wired incorrectly. It is not original.

Here is some more info: my trailer also has an outlet in the front. If I have it powered up, I can take my meter and read off the rear plug, which is the one I am trying to figure out. What I get is one hot blade and the round prong is ground.

This leads me to another question: what does the polarity light tell you? I thought A/C basically goes bacik and forth at 60 cycles, which tells me the polarity, ie pos/neg changes 60 times per sec.

AND, if I have everything wired correctly, if I have seperate electrical sources cominf into the trailer, one in front and one in back, is the amperage additive?

Greg
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Old 07-12-2008, 08:06 AM   #7
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It sounds like your white wire is not connected to anything. Since the original plug was most likely for 220 volts it was not used. The way you have it now connected, you have no ground to the frame of your trailer. Why not trace the wires back to the panel and see what's up with the white wire. It may be as simple as connecting it to the neutral bus.

AC does not change direction, it cycles in a sin wave 60 times a second. The black wire should always be hot and the white wire should always be the return. If these two wires are reversed the polarity light should turn on.
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Old 07-13-2008, 09:45 AM   #8
1977 24' Argosy Motorhome
 
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The way it sounds to me, the four prong outlet that your trying to connect to may be miss wired. the connections should be color for color.
try testing the meter on your source plug, you should read about 110 from hot to ground,
110 from hot to neutral, and 220 from hot to hot.

Thanks crusty for posting that diagram, I didn't have time yesterday.

Is the twistlock plug on your house or a business?
I only ask because there is a thing called a grounded B on some old businesses, but I'm not going to explain it at this time because it will just screw with your head.


Good luck
-Matt
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Old 07-13-2008, 10:41 PM   #9
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The source I am trying to tie into was installed by a professional electrician and ws put in to power the huge Halloween light display my wife's family puts up every year. I will trace it back to the box and make sure it is wired right. Thanks for all of the help.
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