4 Pole Flat Connector

POSITION	WIRE COLOR	CONNECT	FROM TRUCK	TO TRAILER
1	White	Ground	Uncovered Pin	Covered Pin
2	Brown	Tail/Running
3	Yellow	LT/Stop
4	Green	RT/Stop

7 Pole Round Connector

POSITION (see back of plug)	WIRE COLOR (Typical)	CONNECTS TO
1	White	Ground
2	Blue	Brakes
3	Green or Brown	Tail/Running Lamps
4	Black or Orange	12V Power
5	Red or Yellow	LT/Stop
6	Brown or Green	RT/Stop
7	Varies	Extra

125 Volt, 30 Amp AC Wiring

It is worthwhile noting that many modern electrical devices (switches, duplex outlets, receptacles, etc.) now include color coded screws.  The normal color coding convention is green screw = bare/green/ground, silver screw = white/neutral, and brass screw = black/hot.
 

There is a lot of misunderstanding about how MOST 50 Amp RV's are wired.  Some of the larger bus type diesel pushers are all electric and in fact, do have 250 volt appliances in them.  In that case, both sides of the 50 amp circuit are used to power those devices (just like your home) and those coaches MUST have 50 amp, 250V service in order to run things like the 250V stove, AC and the electric water heater.  In the case of the rest of the 50 amp RV's, 250 volts is still supplied to the coach just like your home electrical service but typically one side of the line is used for all the 125 volt appliances except the rear AC and the other side is used for the rear AC only.  The service coming into the RV is still 50 amp, 250 volts.  Please note that all of the 50 amp RV's (at least in the US) use the SAME receptacle wired in the SAME way.  If they didn't, they couldn't all plug into the same outlets at the RV park.

On the back of the 250V receptacle, there will be four terminals labeled W (or white), G (or green), and possibly X and Y.  The wires going to terminals X & Y on the receptacle are the hot wires and are interchangeable, the hot wires from the panel are typically red and black.  The neutral W, (white) and the bare (or green) ground wire G, MUST be on their designated connections. The bare/green wire should also be grounded to the electrical box the receptacle is mounted in (if metallic).  Most 14-50R receptacles provide a strap from the ground terminal to the mounting plate of the receptacle itself. Be sure to use the properly rated electrical box designed to be used with your receptacle.  Make sure you run large enough wire to handle the load over the distance you are running.  Here is a page from the Leviton catalog that shows the wiring schematically.

NOTE: The two 125V circuits are additive when you connect across the two hot legs.  Therefore the neutral wire must carry the difference current between the two circuits.  That is to say, if both circuits are delivering 30 amps, the neutral wire is carrying zero amps.  On the other hand, if one circuit is delivering 30 amps and the other is delivering nothing, the neutral wire is carrying 30 amps.  It is important to make sure the neutral wire is large enough for its load as well.  Under worst case circumstances, it could be carrying up to 50 amps just like either of the hot legs.  Make sure your electrician knows the outlet will be used with an RV and the load might well be unbalanced.  Thus the reason for the caution on the neutral wire size.  Please remember that this applies to ALL wire associated with RV wiring whether it is the wiring from the breaker(s) to the socket or receptacle, the umbilical from the RV to the socket or the wiring inside the RV.  Please make sure that all wiring is sized to handle the required current.  Again, if you are not sure of what you are doing, consult a licensed electrician for help.

The breaker for this service must be a dual (double) 50 amp 250V breaker.  A dual 250V breaker means that the breaker must be of the type that actually connects to both bus bars in the panel, every other bus lug in the panel is on the opposite leg of the incoming power line.  If you were to measure the voltage between opposite lugs in the panel, you would measure 250V, if you were to measure between either bus lug and a ground or neutral bus, you would measure 125V.  If you ask your local building or electrical supply depot for a dual 50 amp breaker for use in a 250V dryer or range circuit for your model/make breaker panel, they will be able to pick out the correct breaker for you.  Please note that electrical panels are very dangerous to work in. Never touch any of the main bus lugs in the panel, they can kill!  Turn off all power to the panel while connecting the new breaker.  Also note that a 250V dual breaker has both breakers bridged or ganged together so that if one of the dual breakers trips it will also trip the other side.  Although the most common dual breakers have one toggle for each side of the line, some 250V breakers have only one toggle tab, and are bridged internally.

The ground bus in the breaker panel is a terminal block with many small holes where the bare ground (or green) wire can be connected and tightened into place with a screw.  This bus is electrically connected directly to the metal enclosure of the panel.

The neutral bus in the breaker panel is also a terminal block with many small holes where the white neutral wire can be connected and tightened into place with a screw.  However, this bus is mounted so that it is NOT electrically connected to the metal enclosure.  It is very important that the neutral (white) NOT be connected to the breaker box.  For safety, the neutral and ground are only tied together at the meter panel on the house.

After completing all the wiring and closing up the boxes, it is a good idea to test the outlet for proper wiring with a voltmeter or appropriate circuit tester.  You should measure 125V from either vertical blade (check them both) on the side of the socket to either the neutral slot on the bottom or the ground contact on the top as well as to the box the socket is mounted in if it is a metal box.  You should measure 250V from one side blade to the other side blade.