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Old 08-01-2010, 06:42 PM   #1
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TRUE 50a vs BOGUS 50a

We are in a park where I have discovered that the 120v/240v 50a outlets are not wired according to code. I discussed this with the campground manager and she finally admitted that she doesn't really know what it means, but that "they told me we don't have the normal third wire." What I find with a multi-meter is that both power legs have 120v when read to ground or neutral. BUT when metered across the two power legs, my meter reads 1v. I tested several outlets in the campground and got identical results, so am reasonably sure the entire campground is the same. My assumption is that both power legs are hooked up to the same circuit rather than two circuits of opposite phase. As I understand the requirements of a 120v/240v 50a outlet, a 240v appliance would NOT work on this circuit. However ....

Here's my question
... am I right to assume that when a 50a RV is plugged into this circuit, the RV WILL be receiving TWO legs of 120v/50a power? (still receiving the normal total 100a that a corectly wired 120v/240v 50a circuit would provide?).
... and am I right in believing that there is at least a possiblity that the neutral on this circuit could in some circumstances be carrying up to 100a???
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Old 08-01-2010, 07:08 PM   #2
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Paul,

I'm almost sure that when you read voltage across 2 legs of a panel, it will read '0' volts. Someone will correct me if I'm wrong.

Note - you don't have any 240V appliances in your motorhome that I know of. The key to a 50A 240V service is that, yes, it does provide 100A of single phase 120V service into your motorhome, and what makes it work is that the 2 legs of the service cancel out on the neutral wire, so you don't need larger gauge neutral wire going back to the pedestal. In fact, if the 2 legs are balanced relative to the current draw in the coach, there should be little to no amperage draw on the neutral wire at all.

If you were pulling 100A of single phase, one leg service, you would need about a 2 gauge wire for the neutral (I've been out of the electrical business for a long time, so not sure of the exact gauge). That's why the pedestals actually deliver 240V on 6 gauge wire and effectively get 100A service from it.

I think that what you're reading is correct.
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Old 08-01-2010, 07:23 PM   #3
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If you take a reading across both legs it should read 240V. There should be a voltage dial possibly on your meter depending on the model. Make sure you have it set at the proper voltage you are testing for.

Should also state as to what the manager said regarding the third wire. If this is true that they didn't pull the second power leg I would assume the voltage reading would be what you said or less. You still would get the 120V to gound reading though on both sides. What one will do to try and save a buck.
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Old 08-01-2010, 07:39 PM   #4
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What you found is not terribly unusual and your reading of it is 100% correct. Your coach will work fine unless it is one of the rare Prevost or Newell with some 240V appliances. Rest easy - not a problem. However, it is also possible that you have less than 100A available. Often when they wire this way, it is to reduce the overall current load and they provide only 50A total, i.e.wire both legs of the outlet to the same 50 Amp feeder. That way they can supply two 50A sites from one 50A/240v feed and you use one less wire to each site. If that is the case, you have 50A total available but no problem on the neutral.

While the theoretical hot-hot reading should be zero on this wiring, a 1-2v difference is not uncommon due to internal resistance and other anomalies.
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Old 08-01-2010, 07:45 PM   #5
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a 30 amp system has 3 wires and has a neutral, ground and one hot leg. On a 50 amp system, you have a neutral, ground and 2 hot legs.

On the 50 amp system you should read the following.

ground (green wire) or neutral (white wire) to hot leg one (red wire), 120 volts
ground (green wire) or neutral (white wire) to hot leg two (black wire), 120 volts
hot leg one (red wire) to hot leg two (black wire), 240 volts
neutral (white wire) to ground (green wire) you should read very little here, maybe a few millivolts. If you read any appreciable voltage here, you have a floating neutral which needs to be fixed.

Some people will try to cheat and use a 3 wire system and combine the grond and neutral.

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Old 08-01-2010, 07:47 PM   #6
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Detailed answer in another forum but here is the basic:

Normally, with a proper outlet on a 50 amp RV, the neutral wire can carry as little as NO current. ZERO amps. Since the current flows from L-1 to L-2 (The Neutral only carries the difference between the two)

But as this park is wired the neutral, which like the L-1 L-2 wires is rated 50 amps, will have to pass both L-1 and L-2's combined current.

So if L-1 is pushing 25 amps, And L-2 is pushing 25 amps.. Normally Neutral would be hauling nothing.. But in this configuration it would be hauling 50 amps.

If you draw more current, you are now over the wire's rating

There is SOME room for this kind of thing.. But if your peak draw is the full 12000 watts (or rather Volt-Amps) such a circuit can, in theory, take... That wire is going to fail.
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Old 08-01-2010, 07:56 PM   #7
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OK, I had to get out my trusty old Sperry volt meter to get it straight in my own head. Yep, if you test across both legs of the panel, it sure does read 240 Volts.

Sorry for the misinformation on the voltage.
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Old 08-01-2010, 08:56 PM   #8
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Quote:
Often when they wire this way, it is to reduce the overall current load and they provide only 50A total, i.e.wire both legs of the outlet to the same 50 Amp feeder.
Ahaa!! This kind of devious alternative had not occured to me.

What puzzles me a bit about finding the non-standard 50a outlets is that this is a military campground, and they normally don't go in for non-code deviations in things like electrical wiring. I believe these sites are somewhere between 10 and 15 yrs old.
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Old 08-01-2010, 09:12 PM   #9
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Also, coaches with true 240v appliances are not as rare as implied above. Top of the line units from main-line manufacturers, such as Newmar, Tiffin, Monaco can have a true 240v dryer, if nothing else. And a true 240v generator to match. Ours certainly does, and it's an option on the Essex as well.

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Old 08-02-2010, 06:14 AM   #10
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I suspect that only 50A total is available, not 100A which would seriously affect how many appliances you could run.

You're not getting what you paid for.
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Old 08-02-2010, 08:31 AM   #11
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Quote:
Originally Posted by AFChap View Post
Here's my question:
... am I right to assume that when a 50a RV is plugged into this circuit, the RV WILL be receiving TWO legs of 120v/50a power? (still receiving the normal total 100a that a correctly wired 120v/240v 50a circuit would provide?).
No, You can feed a half dozen circuits off of the same bus and still only wind up with 50A total. It's amazing that the main breakers are holding at all if the whole campground is pulling off of a single phase.

Quote:
... and am I right in believing that there is at least a possibility that the neutral on this circuit could in some circumstances be carrying up to 100a???
In my opinion, No since the sum of the power available is only equal to the phase that it's fed from. If you are not metering 240 V between the phases, then the entire circuit can only be on a single phase.

I would be interested in seeing what the sub panel breakers look like that feed the pedestals. If they were tripping at 50A who knows what they might have put in there. Campground loads are very critical and unless you have 3 phase coming in, this place should be in big trouble at some point.

There any number of yahoos that will go into one of these places and wire God knows what.
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Old 08-02-2010, 08:55 AM   #12
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Regarding neutral current, the main factor to remember is that neutral current is subtractive if L1 and L2 are 180 degrees out of phase as they should be (240VAC between L1 and L2) and that neutral current is additive if L1 and L2 are in phase as they appear to be in this miswired installation (0VAC between L1 and L2). Therefore, if one were pulling 50 amps on both hot legs (whether this would be possible depends on the ampacity of the circuit(s) supplying the pedestal 50 amp breakers), neutral current would be either 0 amps (50 amps minus 50 amps) in a correct installation or 100 amps (50 amps plus 50 amps) in an incorrect, in-phase installation. The latter, of course, represents a severely overloaded neutral conductor and could result in a fire!

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Old 08-02-2010, 10:46 AM   #13
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The end result of all this is that the outlets are in violation of NEC. Dangerous or not depends on the breaker feeding the outlet. Is there a local breaker for the outlet (its required). The simple fact that its wired in violation of NEC standard is dangerous as even a competent electrician could assume it to be correct and make connections that would not be safe due to the non conforming outlet.

They really need to bring this into NEC compliance. Hard to believe this is in a US Govenment owned site. But they are protected from liabilty....
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Old 08-02-2010, 11:20 AM   #14
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Not entirely... There are are cases where the government has been sued for neglience.

And this is negligent in my book. dangerous in fact.

HOWEVER.. That said.. I am NOT a lawyer.. and you would need to have the professional advice of one if something bad happened.
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