

03312016, 10:10 PM

#11

Senior Member
Join Date: Apr 2015
Posts: 133

The extra wires from the 50 is simply for the amps. Not volts. Same 110125volts.
As far as connecting two inverters output together....don't try it as it will fry the inverter.
You can get a heavy enough inverter to power your needs. Just upgrade the inverter if you have the battery power for it. They suck power quick depending on your draw or usage.
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04012016, 05:30 AM

#12

Senior Member
Join Date: Jan 2015
Posts: 909

OK, let me try this again. Seems that everybody is right...and everybody is wrong in this discussion. First of all, the power pedestal in an RV park (and your house) is always single phase. However, the hot leads come off either end of a transformer winding that provides 240 volts AC. The neutral is a center tap for that transformer and is connected to ground. This is referred to as "split phase" wiring. Now stay with me here. Since the transformer supplies 240 volts and each "leg" is 120 volts above neutral and ground, when one leg is "high" at 120, the other is "low" at 120 so that between the two, they add to 240. If they were both high at the same time, the voltage between them would be zero!
So, in effect, the voltage on the two hot lines is 180 degrees out of phase. That means that if you load one with 50 amps and the other with 50 amps at the same time, the current in the common neutral will add to ZERO, hence the neutral conductor is generally way underloaded if the loads are balanced.
If someone who wired the pedestal managed to take both 50 amp feeds from the same side of the panel (not a double breaker), you would measure zero volts between the two hots in the incoming power cable. Most EMS's will try to automatically shed the load to 30 amps under this circumstance, since they measure the voltage between the two hot leads and get zero and assume you are using a 30 amp dogbone on a 50 amp rv cord and attempt to protect your system. If not, you technically would run the risk of overloading your neutral bus, cord and connector, since RVs generally have all 120 volt loads. The potential for overload, however, is eliminated by the fact that the pedestal is fused at 30 amps when a dogbone is used, giving the neutral sufficient protection since it is designed to carry 50 amps to begin with.
However, if both sides of the 50 amp rv plug come from the same side of the transformer, you can definitely overload your neutral since if you were to draw 50 from both hot leads, the current in the neutral would be 100 amps...and the breakers in the panel (and the pedestal) will neither see nor protect for this load.
In short, make sure you have 240 volts across the two hot leads in your 50 amp connection!
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04012016, 06:20 AM

#13

Senior Member
Join Date: Aug 2015
Location: South Western PA
Posts: 1,731

Quote:
Originally Posted by ScottBrownstein
OK, let me try this again. Seems that everybody is right...and everybody is wrong in this discussion. First of all, the power pedestal in an RV park (and your house) is always single phase. However, the hot leads come off either end of a transformer winding that provides 240 volts AC. The neutral is a center tap for that transformer and is connected to ground. This is referred to as "split phase" wiring. Now stay with me here. Since the transformer supplies 240 volts and each "leg" is 120 volts above neutral and ground, when one leg is "high" at 120, the other is "low" at 120 so that between the two, they add to 240. If they were both high at the same time, the voltage between them would be zero!
So, in effect, the voltage on the two hot lines is 180 degrees out of phase. That means that if you load one with 50 amps and the other with 50 amps at the same time, the current in the common neutral will add to ZERO, hence the neutral conductor is generally way underloaded if the loads are balanced.
If someone who wired the pedestal managed to take both 50 amp feeds from the same side of the panel (not a double breaker), you would measure zero volts between the two hots in the incoming power cable. Most EMS's will try to automatically shed the load to 30 amps under this circumstance, since they measure the voltage between the two hot leads and get zero and assume you are using a 30 amp dogbone on a 50 amp rv cord and attempt to protect your system. If not, you technically would run the risk of overloading your neutral bus, cord and connector, since RVs generally have all 120 volt loads. The potential for overload, however, is eliminated by the fact that the pedestal is fused at 30 amps when a dogbone is used, giving the neutral sufficient protection since it is designed to carry 50 amps to begin with.
However, if both sides of the 50 amp rv plug come from the same side of the transformer, you can definitely overload your neutral since if you were to draw 50 from both hot leads, the current in the neutral would be 100 amps...and the breakers in the panel (and the pedestal) will neither see nor protect for this load.
In short, make sure you have 240 volts across the two hot leads in your 50 amp connection!

Very good explanation!
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04012016, 09:18 AM

#14

Site Team  Lou
Join Date: Oct 2009
Location: South Eastern PA
Posts: 19,831

Quote:
Originally Posted by ScottBrownstein
So, in effect, the voltage on the two hot lines is 180 degrees out of phase. That means that if you load one with 50 amps and the other with 50 amps at the same time, the current in the common neutral will add to ZERO, hence the neutral conductor is generally way underloaded if the loads are balanced.In short, make sure you have 240 volts across the two hot leads in your 50 amp connection!

Still trying to wrap my head around this.
"So, in effect, the voltage on the two hot lines is 180 degrees out of phase."
This is exactly what I said earlier.
"That means that if you load one with 50 amps and the other with 50 amps at the same time, the current in the common neutral will add to ZERO,"
This is still an issue for me. How can one side of the "split phase" and the "other side" of the split phase (two phase?) show zero current flow with a load on the circuit unless there is concurrently zero voltage?
In my mind this can only happen at the exact instant when the two wave forms (L1 AC and L2 AC) cross the zero on the voltage axis. Zero current only when there is zero voltage across the transformer. This occurs once every 1/120th of a second as the secondary current changes direction. At all other times, voltage is building on one side (or the other) and current is building as the voltage does till it peaks at 170 volts AC (Vpeak)  or more commonly called 120 volts RMS.
Since the Vpeak alternates between L1 and L2, the neutral current carries between zero and 50 amps assuming both L1 and L2 are "loaded" with resistance required to demand 50 amps of current.
50 Amp RV Service  the common four pin configuration used for larger RV's. The receptacles are ANSI/NEMA 1450R and the plugs are ANSI/NEMA 1450P. The half round pin is ground, the blade directly across from it is Neutral, and the other two blades each have 120 Volts. If wired per the National Electrical Code, the two 120 Volt feeds are of opposite phases so that you get 240 Volts when you read across them and 120 Volts between each of them and neutral or ground.
Source material:
V peak to peak to Vrms conversion calculator
Well, what is 50 amp service?
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04012016, 09:49 AM

#15

Senior Member
Join Date: Jan 2015
Posts: 909

Quote:
Originally Posted by Herk7769
Still trying to wrap my head around this.
"So, in effect, the voltage on the two hot lines is 180 degrees out of phase."
This is exactly what I said earlier.
"That means that if you load one with 50 amps and the other with 50 amps at the same time, the current in the common neutral will add to ZERO,"
This is still an issue for me. ...

Herk,
You have to dig deeper than that. Again both hot legs come from the same phase...not two phase...but split phase! As I said, in a multi phase installation, there are 3 separate phases and they are 120 degrees apart. This is a split single phase installation as are all residential services.
The issue is not the current that is induced in the neutral, the issue also relates to the direction of that induced current flow. One hot leg is trying to induce say 50 amps in one direction, while the other is simultaneously inducing current in exactly the opposite direction since one is at a plus voltage on the waveform and the other is simultaneously at the opposite and negative waveform voltage. As such, if the loads are exactly the same, they will neutralize in the common neutral and the net current will be zero! Now split phase loads are seldom that well balanced but even it one hot has no load and the other has a full 50 amp load, the current in the neutral will only be 50 amps. As they approach a balanced load the neutral current will drop to zero again. That way you can draw 100 amps at 120 volts and only load the hots at 50 and the neutral load will approach zero. The neutral only carries the difference in current from the two legs, not the sum!
The design allows smaller service entrance cabling for higher current levels.
Mind bending...but true.
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04012016, 10:01 AM

#16

Site Team  Lou
Join Date: Oct 2009
Location: South Eastern PA
Posts: 19,831

Sorry, I am just dumb I guess.
If there is 100 amps on the L1 and L2 wires and none on the neutral, aren't they undersized for that current?
Like I said, I am just putting all this in my too hard to fathom pile.
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04012016, 10:09 AM

#17

Senior Member
Join Date: Jan 2015
Posts: 909

Quote:
Originally Posted by Herk7769
Sorry, I am just dumb I guess.
If there is 100 amps on the L1 and L2 wires and none on the neutral, aren't they undersized for that current?
Like I said, I am just putting all this in my too hard to fathom pile.

Nope. EACH of the hot legs are carrying 50 amps, and they are sized for it. The neutral sees 50 amps in one direction from one of them and 50 amps in the other direction from the other, for a net current of...0 amps. Perfectly balanced load.
That is the whole idea.
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04012016, 10:14 AM

#18

Site Team  Lou
Join Date: Oct 2009
Location: South Eastern PA
Posts: 19,831

OK, I just read this Wiki article and I think I am now up to speed.
https://en.wikipedia.org/wiki/Split...electric_power
"He can be taught!"
Herk
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04012016, 10:14 AM

#19

Senior Member
Join Date: Jun 2012
Posts: 3,781

Quote:
Originally Posted by ScottBrownstein
Nope. EACH of the hot legs are carrying 50 amps, and they are sized for it. The neutral sees 50 amps in one direction from one of them and 50 amps in the other direction from the other, for a net current of...0 amps. Perfectly balanced load.
That is the whole idea.

What he said!
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04012016, 10:43 AM

#20

Senior Member
Join Date: Jan 2012
Posts: 332

Ok my question is that if you measure access L1 L2 with volt meter would i get 0,120,240? Im thinking im getting 0 with this set up . I have a 30 amp set up at this time but trying to get my head around 50 service for the future. Sorry for the mental block
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