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Old 08-02-2013, 08:37 PM   #15
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Originally Posted by BFlinn181 View Post

Alternating current goes from 120 v (+) to 120 (-) 60 times (cycles) a second. In an RV 50 amp outlet, as one leg is decreasing, the other leg is increasing, so the neutral leg never 'sees' more than 50 amps. If the two 30 amp circuits were being supplied in the same phase (common in one pedestal) then they are both supplying 30 amps at the same instant. The neutral would then carry the combined 60 amps.
Ok thanks, between you and Jim, I certainly got it now. I should have understood at the onset as I've done plenty of house wiring, including 240v, which always goes on different legs. Senior moment I suppose, could be the Shiraz too I guess
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Old 08-03-2013, 10:52 AM   #16
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Ok, so it would be correct to say the single neutral in the 50a cord would only carry 50a max (even if 100a is being used) because the 2 phases cancels out one of the 50a legs?

That is correct, mostly. The supply lines are out-of-phase so when it's 120V on one leg, it's zero on the other. Possible 50A drawn on one leg, and 0A on the other. So, max 50A in the neutral. There's no cancelling going on.

In other words, if 60a were being used in this case, the neutral would carry the 60a on the rated 50a neutral?

Yes, if you were plugged into two in-phase 30A outlets, (supplying a 50A RV's two legs), you could potentially have 60A on the neutral. Not likely you'd be running everything in your RV at the same time though.

Where I'm still confused is modern power is 3 phase. And were talking the 2 inbound legs from the power company as being 2 phases. I guess that's something I need to investiage myself. I didn't mean to muddy up the thread.

Modern residential power is 2-phase. Where you'd find 3-phase is in an industrial setting. (But a large mansion may have 3-phase in order to run huge pumps for a pool or a giant A/C - heating system). And there are some farms with 3-phase welders and huge water pumps).

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Old 08-03-2013, 12:06 PM   #17
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If L1 and L2 are 180 degrees out of phase (as they should be), then neutral currents are subtractive. If they are in phase, neutral currents are additive. In a theoretical case where both legs are drawing 50 amps, the out of phase situation would have a neutral conductor current of 50 - 50 = 0 amps. The in phase situation would have a neutral conductor current of 50 + 50 = 100 amps, with a neutral conductor that's sized for 50 amps. The worst case situation for the 180 degrees out of phase scenario would be where one leg is drawing 50 amps and the other drawing 0 amps which would yield a neutral current of 50 - 0 = 50 amps.

Residential power is split phase, not 2 phase. It takes a single phase and splits it with a center tap neutral transformer as shown in the diagram in post #14.

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Old 08-03-2013, 04:24 PM   #18
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If L1 and L2 are 180 degrees out of phase (as they should be), then neutral currents are subtractive. If they are in phase, neutral currents are additive.
I think I am going to disagree with you here. Neutral currents are never subtractive - they are always additive.
The two components from L1 and L2 on the neutral current are always L1+L2 by calculation.

To calculate the neutral current you can precisely add the two vectors for the line currents (L1 and L2) by converting vectors into their horizontal and vertical components by using trigonometry. Add the horizontal components, then add the vertical components, then convert this to polar coordinates to find the sum of the vectors. This is called algebraic vector addition.
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Old 08-03-2013, 05:22 PM   #19
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Let's not overcomplicate this for the laymen reading it. We're discussing 2 hot legs that are exactly 180 degrees out of phase relative to the center-tap neutral leg. To be absolutely correct, yes, the currents are additive to calculate the neutral current, but in the case where 50 amps are being supplied by each hot leg, the addition is 50 amps + (- 50 amps) = 0 amps. The net effect is that the currents are effectively subtractive IN THIS SPECIFIC CASE where we are dealing with a split phase supply.

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Old 08-04-2013, 01:01 AM   #20
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RustyJC is correct of course. The misinformation in my post above (#16) was calling an RV parks 50a shore power setup by the most commonly used term, 2-phase. It is in reality one split phase. Sort of like calling your car's power plant a motor when it's actually an engine. A motor is an electrical device, whereas an engine is an internal combustion device.

I hate being wrong, but...sometimes it happens. The last time for me was...let's see, Thursday. 1995.
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Old 08-04-2013, 06:37 AM   #21
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Boy did this thread evolve , but I'm glad it did as it has screwed my head on a bit tighter. A couple more things for me to get straight or confirm.

When the power hits your house it's split phase (for laymen it works just as 2 phase, so don't worry about it). This is created down the street in a transformer. I assume the power company does this for some load sharing or power saving reason?

I understood all modern power was generated as 3 phase (120 degrees vs 180) but was shot down. So when a "mansion" needs 3 phase for large motor use, is it converted on site to 3 phase? Or was it meant we dont get 3 phase at the house but could without transformation? In other words, it is generated as 3 phase?
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Old 08-04-2013, 06:50 AM   #22
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More info on 3-phase power distribution.

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Old 08-04-2013, 07:03 AM   #23
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It has been a long time since basic electricity/electronics so let me ask this. I made an adapter similar to the Camping World model linked here. I already had a lot of the parts so it was about a $10 gadget. Most of the time when trying to use it we where at a GG with a 30A and 20 at the pedestal. I have yet to be at one that the 20 was not a GFI protected circuit so it never worked since it would pop and I understand why. The one time it worked flawlessly was in a state park where the ped supplied two sites (made one on the wrong side) and had two 30A connections. We had tent campers next to us so I asked if they minded and of course they had no need for it.

So my questions is: all I need to do is use my VOM to measure across the two hot sides of the 30A outlets to read 240V and it is OK to use and if it is zero it is not?

Honestly we can get by easily on 30. The EMS will shut things off if we try to draw too much and one of the first is the electric hot water so we just switch to propane.

EDIT: I figured it out Of course if it reads zero it is the same phase. It is kind of like putting both leads in the same plug.
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Old 08-04-2013, 07:04 AM   #24
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Facts: Most of those devices come with a 30 to 15 adapter and they yammer about how you get 40 +20 and that's 50.. Well, they are only tellling a partial truth.

IN most parks, the 15/20 amp outlet (And in a few the 30) is GFCI protected.. If you plug one of these GFCI testers into the GFCI outlet the click you hear will be teh click of darkness (The GFCI TRIPS) if the GFCI does not trip. it's not working properly (Hence the term GFCI tester above)

If you have two NON-GFCI 30 amp outlets on a single site, Or if management will let you take TWO sites. Then you get 60% of 50 amp service, (50 amp service is 2 legs of 50 amps each, you now have 2 legs of 30% each or 60%) However campgrounds that allow this are few and far between.

And if you plug into two sites without permission.. Management may request you leave... No Refund.

As for the phasing of the two 30 amp outlets and measuring the voltage to be sure they are on different legs.. Don't worry about it, The Neutral wire can handle the load. However voltage drop WILL be reduced if you manage to get different legs.

NOTE: I have one of those "Cheater boxes" and do use it.. IN ONE location where they are never full enough to worry when I'm in residence there.. They do fill up later in the summer, but not when I'm there.
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Old 08-04-2013, 09:52 PM   #25
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Originally Posted by steveclv View Post
I think I am going to disagree with you here. Neutral currents are never subtractive - they are always additive.
The two components from L1 and L2 on the neutral current are always L1+L2 by calculation.

To calculate the neutral current you can precisely add the two vectors for the line currents (L1 and L2) by converting vectors into their horizontal and vertical components by using trigonometry. Add the horizontal components, then add the vertical components, then convert this to polar coordinates to find the sum of the vectors. This is called algebraic vector addition.
Are you going to quote Kirchoff's law and explain it ?
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Old 08-04-2013, 09:54 PM   #26
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Voltage or Current?

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Old 08-04-2013, 10:03 PM   #27
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Originally Posted by clyon51 View Post
Boy did this thread evolve , but I'm glad it did as it has screwed my head on a bit tighter. A couple more things for me to get straight or confirm.

When the power hits your house it's split phase (for laymen it works just as 2 phase, so don't worry about it). This is created down the street in a transformer. I assume the power company does this for some load sharing or power saving reason?

I understood all modern power was generated as 3 phase (120 degrees vs 180) but was shot down. So when a "mansion" needs 3 phase for large motor use, is it converted on site to 3 phase? Or was it meant we dont get 3 phase at the house but could without transformation? In other words, it is generated as 3 phase?
There are methods of making 3 phase power out of single phase but those are special devices and usually used on farms or small workshops to take advantage of a piece of industrial equipment someone has purchased. Sometimes you can catch an industrial buisness going out of buisness and get lathes, or milling machines, or plasma cutters of high capacity very reasonably.

Most places that use 3 phase power the power is brought in as 3 phase. Unless you have an intense desire for the knowledge don't worry about 3 phase as the other posters said it is used almost exclusively for heavier industrial usage. You will probably not see it in a campground. Although some farmers may use it for welders/ plasma cutters etc. That is also fairly rare just because of availability.
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