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Old 12-07-2009, 01:42 PM   #1
Jdrobone
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50A versus 30A Supply

Little discussion on 240V versus 120V supply.

Here’s how the 50A differs from the 30A. The 50 has two “out of phase” hot wires furnished to the outlet, and thus to the RV thru the 50A connector cable. That’s why the cable is heavier, number 6 on most 50A cables versus number 10 for the 30A cable, since the number 6 cable is good for 55(max) amps and the number 10 is good for 30.

Reading (with a voltmeter) between the two hot wires will get you a reading of somewhere between 220V-240V . Reading either of these hot wires to ground (or the neutral (white wire) - which is tied to ground for reference) will get you about 117V – commonly referred to as 115V. Most appliances will tolerate a voltage deficit of about 5 per cent but going lower, (as in summer when demand on AC’s sometimes drops voltage) can cause problems with motorized equipment (pumps, AC’s, etc) thus the value of the aftermarket voltage conditioners – anyway that’s a different topic.

So . . . what does this mean? Basically the more power you have available, the more electrical load you can connect to the system. For the 50A, at 230 volts, power (measured in watts) is equal to 230Vx50A which is 11,500W. At 30A 115 volts you have 115x30=3450W – quite a difference! But, how can this be, you say? Well, at the RV panel the engineers have divided the supply buss into two separate sections or rails if you will. Picture half of the RV’s breakers on one buss and the other half on the other buss. The max current available on either buss will be 50A of course. But wait, . . . if I have, for example, 45 amps of connected load on one buss and 35 amps on the other half won’t that overload my neutral since it will have 80 amps (45 + 35) on it? Well short answer is no. Curious development occurs when these loads are out of phase (has to do with center-tapping transformer windings and other tech aspects beyond the scope of this thread) the actual load on the neutral will be 10 amps – the difference between the two loads thus the neutral will be well within its capacity.

Back to that ‘ol dog bone. If the buss is divided in two at the RV then how does one hot wire (30A plug) supply voltage to both sides? Well, it seems that the manufacturers cleverly put an internal jumper from the single hot wire on the 30A dog bone plug to the otherwise dead second hot wire on the female end of the 50A receptacle of this bone, thus feeding both buss’s (is that spelled right?) in the RV. You can check this out – I did – by taking an ohmmeter and reading from the male hot plug on the dogbone to both female outlets on the 50A female receptacle end.

Similar setup on the dogbones with the “Y” and two male plugs, one of which plugs into the 30A receptacle at the supply pedestal and the other into either a 15A or 20A 110V (whichever is available) . Internally the two hot wires are married together giving a max of 45 or 50 amps @ 115V – thus 5750 watts, not the 11,500 available on the 50A 230V plu but better than the 3450 available on the 30A only.. Downside to this hookup is that the second plug will not work if it is a GFCI (ground fault) receptacle. It “sees” the unbalanced load as a fault.

Well, somehow this degenerated into quite an epistle. Hope it helps, seems I’ve been doing this stuff for 30+ years and sometimes kinda run on. Anyway, it really does make a difference – quite a difference – if you have 50A available and elect to drag out that heavy cable.

Regards, Jerry
 
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Old 12-07-2009, 02:07 PM   #2
Art-n-Marge
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Thanks Jerry! If you leave off the first sentence in the last paragraph this is a great read. The information is thorough, accurate and almost leaves no questions.

I appreciate the time you took on this, but here are my questions, or more like clarifications: As I read it in the fourth paragraph, does each leg of the 110v support 50 amps? Therefore you can actually drive up to 100 amps for the trailer? What is the shore power doing if it tries to supply 100 amps? Am I understanding this correctly?

Translation to all - Use 30 amp if it is the only thing available. If there is a choice between the two use 50amp. Izzat right?

This means I won't even try to get another 30 amp cable because the 50 amp cord with a 30 amp adapter will work just fine (and keep me strong). I will just need to remove the adapter I have more often rather than lazily use the 30 amp shore power.

btw - The plural of bus (not buss) is buses or busses. A "buss" is a kiss and I don't think you meant that.
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Old 12-07-2009, 03:57 PM   #3
NCFischers
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Art,
Jerry was right calling it a buss bar. That is the correct spelling for that piece of equipment. Each leg on a 50 amp power cord supplies 50 amps at 115+- volts. The shore power doesn't push power to the rig, the equipment in the rig draws power and the wiring in the rig is protected from appliances drawing too much power by the circuit breakers. I hope this helps.




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Old 12-07-2009, 10:34 PM   #4
ols1932
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That's why I advocate using the 50-amp cable whenever 50 amps is available. Why deny yourself the power? It's really nice on a cool morning to be able to run a 15 watt heater on one end of the rig and another on the other end of the rig without worrying about tripping a breaker. So what if the 50-amp cable is heavier. Which is better for you, denying yourself the use of the entire available power, or getting out the 50-amp cable? I opt for the 50-amp cable any time 50 amps is available.

Orv
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Old 12-07-2009, 11:53 PM   #5
Tom S.
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Excellent write up and explanation Jerry. I'm not sure about the pluralization of 'buss' either but buss's gets the idea across!
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Old 12-08-2009, 02:21 AM   #6
garyka
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If 50 available I use it unless I'm only staying for one night,If 50 isn't available I use the 30 amp cord.
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Old 12-08-2009, 02:47 AM   #7
richfaa
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Very good.. It is beyond most folks to understand that we really have ,in effect 100 amp service. My PT 50C will display the amp draw on each leg. I have seen as much as 37 amps on one leg.30 amps really restricts you. We will use it if there is no other recourse. Many C.G's will charge extra for a 50 amp hookup to pay for the possible increased power use. If over nighting we will take a 30amp hookup and watch what we use.We have every appliance marked as to amp draw. When we are paying for the eletric we take advantage of the 100amp service as you only pay for what you draw.
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Old 12-08-2009, 03:32 AM   #8
Waynem
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How many people will that buss hold?

It's also a "bus bar."

Irregardless (that is a word, although not standard) your explanation was very good and in layman terms. Even I could understand it.

Question: If the bus stops, and one electron gets off, how many electrons are left on the bus?
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Old 12-08-2009, 05:57 AM   #9
KTManiac
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If more than one cactus = cacti, then logic dictates that more than one bus = bi.

Hmmmm, ....I wonder if the electrons on them go both ways?!




Seriously though, I believe that most manufacturers of the product call them "bus bars".
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Old 12-08-2009, 06:28 AM   #10
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To expand a little on the explantion above, even though you technically have 100 amps available, there is never more than 50 amps actually flowing through the wires.
I'll try to explain.
If you are pulling 50 amps thru one leg and none on the other, then there will be 50 amps flowing in one hot wire and out the neutral wire. Nothing in the other hot wire.
If you are pulling 50 amps on both hot legs, then there is no current flowing thru the neutral wire. Since the two hot legs are 180 degrees out of phase, when one leg is positive the other is negative. When the current flows in the positive hot leg, it flows out the negative hot leg, and vice versa, so nothing goes out the neutral. Since the neutral is common for both hot legs, essentially it is self cancelling as far as the current goes. Think of it as 50 amps flowing out the neutral and 50 amps flowing in the neutral, therefore there is no actual flow in the neutral wire.
Normally there will be some difference in how much current each leg is pulling. So there will be some level of cancelling, and with the difference flowing thru the neutral wire.
So you can see that you will have the power of 100 amps, but actually only pull 50 amps.
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Old 12-08-2009, 07:04 AM   #11
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Thanks BB TX... that answered the 100 amps with a 50 amp power supply. Most of my electrical studies was DC (direct current), versus AC (alternating current). AC was always voodoo electronics to me, but my DC training certainly helped me understand this explanation.

It's so much better to hear someone say "Use 50 amps!" and then follow it with the explanation like this. Do the math, which is better, 30 amp or 100 amps. Duuuhhh! Now the extra $1 to $2 per night charge for 50 amp doesn't seem so bad, in fact it's a down right bargain.

I still contend it's bus bar not buss bar. Our tech writers always corrected our documents because buss means "kiss". But use what you want because some uses of English have become pseudo acceptable. I will always say "lar-YNx" when so many others say "larNYx".

We are going RVing next week and I am gonna connect to 50 amps and set up lots of Christmas lights, yippee. It's gonna look like a Beverly Hills shopping center at Christmas! It will have a 10:00pm timer just to be nice.

Merry Christmas, y'all!
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Old 12-08-2009, 07:45 AM   #12
grampachet
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Fighting with the 50 amp cord reminds me of the old timer who was watching me cut up and split firewood. "That there wood warms you up four times, once while cutting it, then splitting it, then stacking it and then burning it" he told me. So with the power cord it helps give you the exercise needed and thus helps warm you up twice. It's a good thing as Martha would say.
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Old 12-08-2009, 11:21 AM   #13
Jdrobone
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BB,
I would very respectfully have to challenge that explanation to some extent. It is true that a perfectly balanced load of 50A per leg will result in 0 amps on the neutral since, as your explanation suggests, the two circuits are 180 degrees out of phase and the neutral carries only the unbalanced load (the difference in amperage between the two circuits).

I would submit, however, that if you were to look at a watt-meter connected across the load you would find that the circuits are drawing approximately 11,500 watts. If the connected load was only pulling 50A that same meter would read 5750 watts. So . . . . although you could say that the RV is only drawing 50 amps, it is in fact drawing 50 amps per leg (100A) thus instantaneous load of 11,500 watts. If this circuitry were used in a branch circuit application it would be referred to as a multi-wire branch circuit. That same 2-pole breaker if used in a 240V application not be considered a multi-wire branch circuit but a standard 240V load. If you were to break that handle-tie off the breaker and turn on only one single-pole breaker you would have the situation I believe you were describing as to only 50 amps available. We could go back and forth and beat the instantaneous load definition to death but bottom line is that if you could get a 50A 2-pole breaker to hold without tripping at full load you would consume twice as many KWH’s as a single 50A circuit would, and that is what our good friends at your local utility charge you for.

Regards,
Jerry

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Old 12-08-2009, 01:23 PM   #14
firetrucker
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50 amps in two out-of-phase 120 volt circuits is the same as 50 amps across a 240 volt circuit...same power (11.5 kW), max of 50 amps in any leg (120 volt loads are in series).

50 amps in two in-phase 120 volt circuits (120 volt loads are in parallel), and you've got 100 amps flowing...same power (11.5 kW).

50 amps from a 20 amp and 30 amp 120 volt connection (in phase, loads are in parallel), and you've got 5.75 kW and still only 50 amps flowing.

And you still pay the utility for the power and the heat the current generates in the wiring.

Bob
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Old 12-08-2009, 02:00 PM   #15
BB_TX
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Quote:
quote:Originally posted by Jdrobone

BB,
............ So . . . . although you could say that the RV is only drawing 50 amps, it is in fact drawing 50 amps per leg (100A) thus instantaneous load of 11,500 watts. ..........Regards,
Jerry
I certainly respect your opinion. But I believe I am correct on this one. But then I have been wrong before.
You are correct that it is drawing 50 amps per leg and the total wattage is 11,500. But it is the same 50 amps. Since under a balance load on both legs there is no flow thru the neutral, the 50 amps that flows into hot leg 1 flows out hot leg 2. And as the phase shifts, the current reverses and flows into leg 2 and out leg 1. But it is still only 50 amps.
If you put an amp clamp on each hot wire, you would measure 50 amps on each. If you measured it with a scope, you would see both legs have 50 amps, but 180 out of phase.
The fact that there is 11,500 watts is due to the common neutral reference for both legs (even though there is no current flow thru neutral under that scenario). So in effect, each leg appears to be independent.
But if you were to break the neutral wire, then you would have 50 amps at 230 volts and the same 11,500 watts. At least momentarily.
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Old 12-08-2009, 03:12 PM   #16
Leaseit
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I didn't know that all the RV parks were suppling 180 Degrees out of phase power when using 50 amp power. I have measured a few power poles Hot to Hot and gotten 208 volt which would seem like two legs off a three phase power pole transformer. Thinks i need to look at this again next time i'm in a park and see if 2pi/3 radians not the right calculation.
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Old 12-09-2009, 02:57 AM   #17
richfaa
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All I know is that we can see a bunch of amps on leg one and a bunch of amps on leg two and any leg can exceed 30 amps. We have never drawn close to 50 amps per leg but we have exceeded 30 amps on one leg. We spent 37 years as a Electronic tech and many years of schooling in all that theory.. Don't need no stinking theory anymore..It works and that is a good thing.. And....both you guys are right..nothing wrong with your theory. Sometimes theory is..well..to theoretical...

Oh...I looked it up


BUSS may relate of any of these:

an alternate spelling of bus, used mainly in the case of an electrical bus, also rarely for a computer bus
a word meaning to kiss
BUSS, the Birmingham University Speleological Society
BUSS Ltd, a Bradford-based independent software house which developed the SimplePlot graphing and data visualisation libraries.
Buss (also called a Herring Buss), a small fishing boat with two masts
Buss Island, an imaginary island that was believed to lie in the north Atlantic
Bus (film), a 2004 Lithuanian film

ALSO...

Electrical bus
From Wikipedia, the free encyclopedia
Jump to: navigation, search

Symbolic representation of a bus: The thick line is the bus, which represents three wires. The slash through the bus arrow and the "3" means that the bus represents 3 wires.An electrical bus (alternatively spelled "buss"), derived from busbar, is a common electrical connection between multiple electrical devices.

A bus can either allow signals to be transferred between devices, the summing (mixing) of output signals from the devices or the distribution of input signals or power amongst the devices. A bus often takes the form of a wire or printed circuit conductor that terminate at multiple connectors which allows the devices to be plugged into the bus.

Buses are used for connecting components of a computer: a common example is the PCI bus in PCs. See computer bus.
Buses are used for communicating between computers (often microprocessors). See computer bus.
Buses are used for distribution of electrical power to components of a system. The (usually) thick conductors used are called busbars. In an electrical laboratory, for example, a bare bus-bar will sometimes line the wall, to be used by the engineers and technicians for its high electric current-carrying capacity, which allows a convenient approximation to zero voltage, or ground in the US, and earth in the UK.
In analysis of an electric power network a "bus" is any node of the single-line diagram at which voltage, current, power flow, or other quantities are to be evaluated. These may or may not correspond with heavy electrical conductors at a substation.
In pro audio, "bus" refers to a place in the audio signal chain where one can hear a mix of different audio signals—usually at the output of a mixer, or as a separate sub-mix within the mixer (for example all of the microphones used to capture a drum kit). Sometimes it is used interchangeably with the noun "mix

Again.....everyone is right......
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Old 12-09-2009, 04:06 AM   #18
azleflyer
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Old 12-09-2009, 06:03 PM   #19
OntMont
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At least this explains to me why the neutral wire is only 8 gauge while the other three are 6 gauge, thank you!
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