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Old 09-18-2017, 06:13 AM   #1
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240v or 120*2v?

I was browsing here and found a couple of references stating that 50 amp RV power is really 2 120v feeds. This is not necessarily true. Since these were old threads, I thought I would start a new one and provide some clarifying information here. Please excuse me if I get a bit technical and over-simplify a bit for the benefit of those less familiar with AC power.

When discussing electric power we normally talk about three factors: voltage, amperage, and wattage. The simplest analogy is that voltage is the "pressure" of the electricity and amperage is the "volume of flow". For water, this would be pounds per square inch and gallons per minute. Wattage (power) is the combination of the two: voltage times amperage.

Alternating current gets its name because it alternates going back and forth, with electrons first flowing one way and then the reverse. This can also be viewed as the charge on a wire switching from positive to negative and back. A graph of this is a "sine wave" which constantly rises and falls. (I won't go into why that is called "sine wave" here.) There are great advantages to using alternating current over direct current (like battery power) for transmission over distance. (I won't go into that here either.) Suffice to say, AC is the norm.

AC current normally comes to us in a 240v circuit. If we look at each side of this circuit, they both show that up-and-down sine wave. But they are exactly opposite: when one is up, the other is down and vice versa. The usable voltage is the difference between these two "legs" of the circuit. If we measure the voltage from one to the other (with an AC voltage meter, of course), it (essentially) averages the difference in instantaneous voltages and indicates 240 volts.

So where does the 120v come in? It is convenient to add a "neutral" wire which is functionally in the middle of these two values. Think of a teeter totter with each leg of the 240 being one of the seats and the neutral as the pivot point. If we measure voltage between this neutral and either of the "hot" legs, we "see" 120 volts. This is how every house in the country is wired. Half of the 120v circuits are on one leg and half are on the other. We normally don't need to worry which is which. 240v circuits use both hot legs (and probably a neutral as well, depending on the needs of the appliance). As a consequence, if we were to measure the voltage between the hot side of two plugs on different circuits, we would measure either 0 volts (on the same leg or "phase") or 240 volt (on different legs).

Now to our RV power circuits... The 30 amp RV power is 120 volt power and capable of providing a current flow of 30 amps. Obviously, plugging a 30 amp (120v) RV plug into a 240v power source would generate some very-much-not-nice results. Sort of like hooking your 50psi-max water system into a 200psi water source. BAD JUJU!

The 50 amp RV power is... Well, here it gets a bit complicated. Remember that each half (leg) of the 240v circuit is 120 volts? Each is measured from hot leg to neutral. Well, all RVs (that I know of) actually use only these one-side-or-the-other 120v circuits. They don't have any circuits which tap onto both legs of the 240v power. So, what is the 50 amp power supplied? 120v or 240v? Actually, it can be either. Since the legs are used independently, there is no problem with supplying the same phase 120v power to each leg of the 50 amp RV connection. Most RVs will never notice the difference. However, the proper power for the type of connector on 50 amp RV's is, in fact, 240 volts. Note that whichever is supplied, each leg should be capable of handling a 50 amp current flow. BUT!!! If we plug that same 50+50 amp load into an outlet which has each hot leg connected to the same phase of the service power, then the neutral has to handle the return current from both legs and this is no longer 0 amps but is rather now 100 amps. And no, your wiring (and most likely the campground's) is NOT RATED to handle this current. So long as the combined load on each leg does not exceed 50 amps, you're probably just fine. But realize that feeding a 50 amp RV plug with in-phase 120v power can actually cause an overload that the 240v power will not. And no, the circuit breakers (yours or theirs) will not detect this problem because circuit breakers only watch the current flow on the hot legs, not the neutral.

Hence, a circuit tester may be a wise investment!!!!
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Old 09-18-2017, 07:14 AM   #2
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This is great information. Thanks for posting. I'll have to bookmark this one for future reference.

Seeing the water analogy took me back to my days in Fluid Dynamics class in college. Nice one!
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Old 09-18-2017, 07:17 AM   #3
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Your description is exactly correct except there are higher end 5th Wheels and Motorhomes on the market with 240vac appliances. And I suspect more as time goes on as people demand more home like appliances.
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Old 09-18-2017, 07:47 AM   #4
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Good explanation, but you may not be 100% correct in all cases. The 50 amp on my RV has 2 hots & 2 neutral wires running from the breaker panel to the transfer switch. Since my generator is 120 volts only the , the 2 neutrals are required to prevent overloading of the wiring with the generator running.
The 50 amp cord has only 1 neutral and is designed to plug into a 120/240 volt shore power as you have stated. If the shore power was wired with both sides of the 50 amp plug connected to the same leg , it should have a single pole 50 amp breaker which will limit the current on the neutral to 50 amps . If it has a double 50 amp breaker you will have 120/240 volts , if not , the receptacle is incorrectly wired. The only way to be sure is measure with a meter.
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Old 09-18-2017, 07:49 AM   #5
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So if you only use one hot leg (or the other), why is the 240 VAC 4-wire wiring carried all the way from the pole, through the cord, and into the RV to the distribution panel.

I've never owned a 50 Amp RV but I assume the distribution panel is setup just like a house with breakers connected to alternating legs?

If only using 1 hot leg, how is the choice made as to which one?
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Old 09-18-2017, 08:18 AM   #6
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So if you only use one hot leg (or the other), why is the 240 VAC 4-wire wiring carried all the way from the pole, through the cord, and into the RV to the distribution panel.

I've never owned a 50 Amp RV but I assume the distribution panel is setup just like a house with breakers connected to alternating legs?

If only using 1 hot leg, how is the choice made as to which one?


He means that since you are only using 120 vac circuits, each circuit is only using one hot leg or the other. You are using both hot legs because your service is 50 amp at 240 volts. You have 100 amps at 120 volts available at your breaker box.
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Old 09-18-2017, 08:23 AM   #7
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I believe that the National Electrical Code requires the 50 amp receptacles to be 125/250 (Figure 551.46(C)(1)). That would rule out a double feed from the same side of the main feed, otherwise you wouldn't get the 250 from the outlet.
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Old 09-18-2017, 11:27 AM   #8
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Hi ! Welcome to IRV2! We're sure glad you joined the gang!

Thanks for the power lesson! Keep her between the ditches!

Good luck, happy trails, and God bless!
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Old 09-18-2017, 11:36 AM   #9
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Some of the higher end MH's do, in fact, have 240 volt appliances. My dryer is 240 volt as are many others in the higher line rigs. My generator (12.5 Onan) also delivers both 120 and 240 volt output.
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Old 09-18-2017, 11:46 AM   #10
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Most surge protectors will warn that the two hot wires( on a 50 amp plug) of the power post are in phase , an indication that the RV park wiring is suspect.
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Old 09-18-2017, 11:50 AM   #11
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And I assume you can plug into the same 50 amp campground outlets the rest of us use?


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Some of the higher end MH's do, in fact, have 240 volt appliances. My dryer is 240 volt as are many others in the higher line rigs. My generator (12.5 Onan) also delivers both 120 and 240 volt output.
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Old 09-18-2017, 11:59 AM   #12
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And I assume you can plug into the same 50 amp campground outlets the rest of us use?
Correct.
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Old 09-18-2017, 01:32 PM   #13
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He means that since you are only using 120 vac circuits, each circuit is only using one hot leg or the other. You are using both hot legs because your service is 50 amp at 240 volts. You have 100 amps at 120 volts available at your breaker box.
OK, that's what I thought.

But the 2 hot legs are only 120 degrees out of phase, not 180 degrees.

And the neutral is the return for both 120 VAC legs. So if you only used 1 hot leg at the 50 Amp rating the neutral would carry that entire 50 Amps.

The fact that the 2 hot legs are 120 degrees out of phase (call it 180 to simplify it) reduces the current on the neutral leg if running loads off both hot legs.
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Old 09-18-2017, 01:38 PM   #14
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OK, that's what I thought.

But the 2 hot legs are only 120 degrees out of phase, not 180 degrees...
That all depends on if the park is serviced by 3 phase or split phase power. 3 phase will make it 208, split phase will make it 240. Doesn't matter for the 120 volt appliances, they all see 120 volts, but 2 120 volt waveforms 120 degrees out of phase will only yield 208. Center tapped transformer like you have at home will give you 180 out 120 volts or 240.
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