Electrical service 50 amp

[AJB]

I heard something for the first time today that left me scratching my head and wondering if I have been wrong. If you have a 240 volt 50 amp breaker supplying power to your RV how many total amps do I have to use. I always thought it was 50 amps but someone told me today that you have a total of 100 amps (50 amps per leg). You have two hot legs (120 volts 50 amps x two legs). So let's here the correct answer please.

[Steve Ownby]

Originally Posted by AJB

I heard something for the first time today that left me scratching my head and wondering if I have been wrong. If you have a 240 volt 50 amp breaker supplying power to your RV how many total amps do I have to use. I always thought it was 50 amps but someone told me today that you have a total of 100 amps (50 amps per leg). You have two hot legs (120 volts 50 amps x two legs). So let's here the correct answer please.

The 240 VAC 50 amp RV service utilizes a double pole 50 amp main breaker. Since most RVs don't have any 240 VAC circuits, that means that there is a total of 100 amps available to use at 120 VAC. This is exactly like a residential electric service. A normal residential service is more likely to be 200 amp but the wiring is the same.

This basically gives a 50 amp RV 12,000 watts to use. If this is all used as 120 circuits then that yields 100 amps. If the 12,000 watts were utilized as 240 circuits then that yields 50 amps.

[MRUSA14]

That is correct. Which means a 50 Amp hookup has more than 3 times the power than a 30 amp hookup.

[elundquist]

Technically you can only use 80% of the circuit/ breaker rating. So on a 50A you can use 40A on each leg. On the 30A you can use 24A of the one leg.

So a typical RV 50A you have 120VAC * 40A = 4,800 watts on each leg or 9,600 watts with both at 120VAC. On the 30A you have 24A * 120VAC = 2,880 watts on the 30A single leg.

[70ChevelleSS]

Originally Posted by elundquist

Technically you can only use 80% of the circuit/ breaker rating. So on a 50A you can use 40A on each leg. On the 30A you can use 24A of the one leg.

So a typical RV 50A you have 120VAC * 40A = 4,800 watts on each leg or 9,600 watts with both at 120VAC. On the 30A you have 24A * 120VAC = 2,880 watts on the 30A single leg.

I think you've interpreted the NEC specs incorrectly. The 80% rule applies to sizing a circuit breaker. An OCPD (circuit breaker) can only be loaded to 80% of it's rating for continuous loads. A continuous load being a load running non interrupted at it's max current draw for a minimum of 3 hours.

For non-continuous or interrupted loads the circuit breaker can be sized for 100% of it's rating.

What this means is you can absolutely draw 50 amps per leg, or for 30 amp breaker, 30amps. As you would expect the loads in an RV are not considered continuous loads.

You're the 2nd person to post this misinterpretation of the specs. Do some research and post facts, not hearsay.

[wa8yxm]

Dual voltage (Center tapped) systems like the 120/240 volt 50 amp RV system or your 150 or 200 or 400 amp home supply (all work the same way) are a bit confusing to folks who do not work with them or who are not trained in them.

It is 50 amp service, but you have the ability to use each amp twice so it is UP TO 100 amp EFFECTIVE, but it is still proper to call it 50 amp.


Might be easier if we converted to watts. 12000 watts. (V/s 3600 for 30 amp service)..

But wait. there's more.. In order to get all the power possible your system MUST BE BALANCED, Exactly BALANCED.. and that.. is all but impossible.. (It can be done... but the odds are like one in several million).

[docj]

You've now stumbled onto a topic that causes endless confusion even among supposedly knowledgeable types. What you've been told in this thread is correct, but don't be surprised when you hear someone in the future vociferously argue that it's really only 50A not 100A. Believe me, I've had to walk away even from some so-called RV technicians who don't properly understand the issue.

It's a shame that the industry uses the "50A terminology" rather than calling them 100A. Since the industry started with 30A RVs it would have made a lot more sense and would have been a lot less confusing to have done that instead of calling them 50A. Maybe there's some historical perspective I'm unaware of.

[Steve Ownby]

Originally Posted by docj

You've now stumbled onto a topic that causes endless confusion even among supposedly knowledgeable types. What you've been told in this thread is correct, but don't be surprised when you hear someone in the future vociferously argue that it's really only 50A not 100A. Believe me, I've had to walk away even from some so-called RV technicians who don't properly understand the issue.

It's a shame that the industry uses the "50A terminology" rather than calling them 100A. Since the industry started with 30A RVs it would have made a lot more sense and would have been a lot less confusing to have done that instead of calling them 50A. Maybe there's some historical perspective I'm unaware of.

Very true post. So many owners don't understand this topic & seem to believe that RV electric service is somehow different from standard residential service.

Here is a pic of a decal at the entrance cable of my coach:



It refers to 3 pole 4 wire, which would be 2 hot wires controlled by a 50 amp double pole breaker, a neutral wire and a ground wire. A look at this decal would be all the info that an electrician would need to know what service was required. I don't have access to a 30 amp RV today to see what that decal says.

[mriderLeon]

Originally Posted by docj

It's a shame that the industry uses the "50A terminology" rather than calling them 100A. Since the industry started with 30A RVs it would have made a lot more sense and would have been a lot less confusing to have done that instead of calling them 50A. Maybe there's some historical perspective I'm unaware of.

I suppose the correct way to call it would be "50 amp 2 phase " vs "30 amp single phase" or "20 amp single phase"

Do the generators on RVs supply the two legs of service in different phases? And are they phased such that they could be used for a 220 appliance? I guess I need to get out my voltmeter and hit the hot wires in a few different places.

[Steve Ownby]

Originally Posted by mriderLeon

I suppose the correct way to call it would be "50 amp 2 phase " vs "30 amp single phase" or "20 amp single phase"

Do the generators on RVs supply the two legs of service in different phases? And are they phased such that they could be used for a 220 appliance? I guess I need to get out my voltmeter and hit the hot wires in a few different places.

Speaking of Onan generators, the 10kw and 12.5kw generators produce true 240 power consisting of two out of phase 120VAC legs. The smaller generators produce two legs of 120VAV which are on the same phase so can't be used for 240VAC.

[PushedAround]

Originally Posted by mriderLeon

I suppose the correct way to call it would be "50 amp 2 phase " vs "30 amp single phase" or "20 amp single phase"

No that is incorrect. Both 120 volts (L1 to neutral) or 240 volts (L1 to L2) are single phase power. Commercial power is typically 3 wire where you have L1, L2 and L3, typically plus a neutral. There is such a thing as 2 phase power that was found in older cities like New York and Philadelphia where there are 4 wires that supply power and had to be properly connected, but that is an obsolete technology that is pretty much gone now.

[70ChevelleSS]

Just have to remember that a 50 amp RV outlet is a 240 volt supply. So it can supply 50 amps at 240 volts or 12,000 watts. A 30 amp RV outlet is 120 volts and can supply 30 amps at 120 volts or 3,600 watts.

The amp rating of the breaker is based on the supply voltage. True most loads are 120 volts and that's what we think of but if it's a 240 volt breaker then it's rated amps are at 240 volts not 120.

[mriderLeon]

Originally Posted by PushedAround

No that is incorrect. Both 120 volts (L1 to neutral) or 240 volts (L1 to L2) are single phase power. Commercial power is typically 3 wire where you have L1, L2 and L3, typically plus a neutral. There is such a thing as 2 phase power that was found in older cities like New York and Philadelphia where there are 4 wires that supply power and had to be properly connected, but that is an obsolete technology that is pretty much gone now.

Ok. Are we talking the same thing? I thought that normal house current gets any two of the three phases of commercial power.

My house gets L1 and L2 plus neutral. L1 to L2 is 240 single phase, L1 or L2 to neutral is 120v single phase. The house across town gets L1 and L3 for the same connections. But the commercial building gets L1, L2 and L3 for 3 phase power.

[70ChevelleSS]

Originally Posted by mriderLeon

Ok. Are we talking the same thing? I thought that normal house current gets any two of the three phases of commercial power.

My house gets L1 and L2 plus neutral. L1 to L2 is 240 single phase, L1 or L2 to neutral is 120v single phase. The house across town gets L1 and L3 for the same connections. But the commercial building gets L1, L2 and L3 for 3 phase power.

It can be confusing. 3 phase wiring in residential areas is not common. It's not possible to have 3 hot legs that are all 180* out of phase with each other, do the math. 2 120v hot legs, 180* out of phase is what is used in residential areas to supply 240v power. Upstream, prior to the step down transformers is a different subject and I don't think we want to go there. Let's stay on this side of the transformer supplying 240v.