Why can't 15A + 15A = 30A?

[BFlinn181]

Originally Posted by mel s

rkresge
While I agree that adding a permanent 30A or a 50A RV receptacle is the "best" option it is not always possible or practical.

We often stay at the homes of friends and family where adding a receptacle is not an option.

On those occasions I sometimes use a cheater box, (and a 30A to 15A adapter), to connect my 50A coach to 2 different out of phase 15A receptacles with 2 14/3 extension cords.
See: Progressive Industries RV Surge and Electrical Protection industry lea

Mel
'96 Safari

While the Progressive Ind. Cheater Box will combine two different sources for a 50 amp RV, if a reducing adaptor were to be attempted to plug into a 30 amp RV, a dangerous situation could result. The PI box never combines two hot sources to one hot in the RV, it keeps them separated as a 50 amp RV service does.

[mel s]

Originally Posted by BFlinn181

While the Progressive Ind. Cheater Box will combine two different sources for a 50 amp RV, if a reducing adaptor were to be attempted to plug into a 30 amp RV, a dangerous situation could result. The PI box never combines two hot sources to one hot in the RV, it keeps them separated as a 50 amp RV service does.

BFlinn181
I agree

However a "30 to 15A adapter" is necessary on the 30A male plug of the Progressive cheater box if/when you power a 50A coach with 2 15A receptacles using that cheater box.

Mel
'96 Safari

[BFlinn181]

Originally Posted by mel s

BFlinn181
I agree

However a 30 to 15A adapter is necessary on the 30A male plug of the Progressive cheater box if/when you power a 50A coach with 2 15A receptacles.

Mel
'96 Safari

And that's fine, the two hot leads never join in a 50 amp system. My point was that it's fine if only used to combine 2 sources to provide power for a 50 amp RV. Perhaps the reason UL doesn't certify it is that IF a 50 a > 30 a adaptor was to be used, it connects the two hot leads, sends a shocking hazard to the second male plug, and very possibly, if the two sources are out of phase, could trip breakers, etc.

[wa8yxm]

Originally Posted by ladagobago

in reality he will have 30 amps.

Theory has it yes. but unless the lines from the circuit breakers to the outlets and from the outlets to the RV are indentical size and length it will not work, the breaker with the "Shorter" (physical) path will trip very fast as it will supply the majority of the current, followed a fraction second later by the other one.

Resistance can be a witch.

[docj]

Originally Posted by wa8yxm

Theory has it yes. but unless the lines from the circuit breakers to the outlets and from the outlets to the RV are identical size and length it will not work, the breaker with the "Shorter" (physical) path will trip very fast as it will supply the majority of the current, followed a fraction second later by the other one.

Resistance can be a witch.

Yes, the shorter line does carry more current but the current difference in the two lines is proportional to their relative resistance. It's not as if all the current goes down the path of least resistance.

For example, lets assume that you are using high quality 12 gauge extension cords and one is 20 feet long and the other is 40 feet. From standard resistance tables, the resistance of the shorter cord is 0.032 ohms and the longer one is 0.064 ohms

The two circuit paths are in parallel and the effective resistance of a pair of parallel resistors is give by: R = (R1)*(R2)/(R1+R2) = .02133 ohms

To calculate the current splitting between the two circuit "paths" we need to assume there is some load on the circuit. For simplicity I'll assume there is a 2kW heater drawing current (plus the resistance of the wires). With a 120V supply voltage a 2000 watt heater draws 16.667 amps which means that it's resistance is 7.2 ohms (using Ohm's Law). This imaginary heater would be too large to put on a single 15A circuit.

So the "equivalent circuit" is the 7.2 ohms of the heater PLUS the resistance of the pair of extension cords = 7.22133 ohms. Therefore, the "real circuit draws 16.617 A rather than the 16.667 A that the heater alone would draw.

To calculate the current flow in each of the legs we need to first calculate the voltage drop across the heater. By Ohms Law that is given by the current through the heater times its resistance. The current is the 16.617 A and the resistance is 7.2 ohms = 119.642 V. Therefore, there is a voltage drop of 0.3576 V across the pair of extension cords.

The current flow through each cord can then be calculated using Ohm's Law. The longer cord has a resistance of 0.064 ohms so the current through it is given by the voltage drop across the combined cords (0.3576 V) divided by the resistance = 5.5875 A. The resistance of the shorter cord was 0.032 ohms so the current through it is 0.3576 V divided by 0.032 ohms = 11.175 A.

The bottom line is that because of the difference in cord length the current flow is unbalanced by a 2:1 ratio but this is a rather exaggerated case and you could easily add extra extension cord length to the shorter circuit to ensure that the split was more nearly 50/50.

The concept works, you do get more than the 15A of each circuit, but unless the cords are precisely matched the imbalanced current flow will mean that you can't get quite the full 30A desired. But, as you can see from the example, you can run my "imaginary" heater which has a load greater than the 15A circuits could provide on their own.

Joel Weiss (AKA docj)
Ph.D. (physics) Harvard 1975

[BFlinn181]

docj, in a perfect world that would be true. What about the resistance in the connections at the many plugs involved in such a convoluted cheater box? There are so many places to create resistance and imbalance in the set up.

It's much easier to tell the OP, no, for many reasons such a set up would be unsafe and illegal, don't do it.

[docj]

Originally Posted by BFlinn181

docj, in a perfect world that would be true. What about the resistance in the connections at the many plugs involved in such a convoluted cheater box? There are so many places to create resistance and imbalance in the set up.

It's much easier to tell the OP, no, for many reasons such a set up would be unsafe and illegal, don't do it.

I don't have a problem agreeing with you that this isn't a great idea. But I didn't want to have readers get the impression that the current flow would all be sucked up by the shorter cord. Yes, it is unsafe and (probably) illegal but you know someone is going to try it. So they might as well have as good an understanding of the situation as they can have.

[photopilot]

The resistance in the connections is clearly a bigger factor in the balance between circuits. Where have you ever seen a power cord with melted wire insulation? They always burn up at the plug where resistance is highest.

[docj]

Originally Posted by photopilot

The resistance in the connections is clearly a bigger factor in the balance between circuits. Where have you ever seen a power cord with melted wire insulation? They always burn up at the plug where resistance is highest.

I'm sure you are correct that the balance issue will be dominated by the contacts at the plugs, outlets, etc. Therefore, any attempt to actually use this approach as described above could result in current swings brought about by relatively small changes in resistance at points in the circuit.

If one wanted to make the system more stable, it would be better, conceptually, to install identical loads in each of the parallel legs of the circuit, in addition to the main load (the imaginary heater). This would mean that small resistance differences between the wires, the plugs and sockets would be small compared to the resistance of these new loads which could be something like 100W lightbulbs. Of course, these new loads would result in a significant reduction in the voltage drop across the main load (since they would be in series with it) which would completely negate the entire purpose of the arrangement!

Which is why no one really wires anything in this manner!!!

[ladagobago]

Originally Posted by F4Gary

Thank you Captain Obvious. I was just curious as to why 2 15A couldn't be combined. I'm not actually trying to do it. I run a 15A cord to keep my batteries charged and run my fridge when loading up.

Why didn't you say that in the beginning.
Well everyone - I guess that F4Gary just want to see if anyone knew fundamental electricity and Ohms law.

It was fun to see all the ingeniousness exhibited by the many respondents.

[pasdad1]

I think everybody is being too literal and not imaginative.....there is a real market for this device, once somebody builds it. It might take something like two synchronised inverters, similar to how the inverter generators are able to parallel their output.

Think about it.....with this box, ANY two generators can have their output paralleled into one 30 amp RV outlet......or plugging into two shore power outlets on different breakers.....it won't matter the phase because the electronics and inverters in the "magic box" handle all that!

[nothermark]

Originally Posted by pasdad1

I think everybody is being too literal and not imaginative.....there is a real market for this device, once somebody builds it. It might take something like two synchronised inverters, similar to how the inverter generators are able to parallel their output.

Think about it.....with this box, ANY two generators can have their output paralleled into one 30 amp RV outlet......or plugging into two shore power outlets on different breakers.....it won't matter the phase because the electronics and inverters in the "magic box" handle all that!

Not really. The "magic" is in getting the connections right. Inverters would complicate the problem not solve it. They would also add significant cost. Probably more than most people would be willing to pay.

After I wrote that I remembered that some line conditioners actually do something similar to what you are talking about. The basic idea is to convert the line power into DC to feed a small battery then use the output to power an inverter that provides clean "pure sine" (it's not) power to the equipment that it is protecting. The battery is a load leveling device and provides enough run time to gracefully shut down the protected load or allow a motor generator to come on line as emergency power while not disturbing the load power. Off hand you are looking at something in the range of $1500 and up for the setup. The phase of the feeds would not be an issue because of the rectification. They would still need to be different circuits from the panel. All in all it just seems like a lot of money for a small problem with alternatives like just using the dry camping setup in the RV. ;-)

Paralleling generators is a different issue. Rotating armature generators will tend to synchronize themselves if there is enough power available to do it. Otherwise bad things will happen due to the high currents when out of phase. Electronic inverter generators like the Honda need a way to phase lock that I suspect Honda builds into their units. The whole thing is a diversion from the original question.

[TQ60]

Bottom line is yes it can be done many ways from stupid simple dangerous to high tech automatic but the end result is the cost and or risk is too much for any manufacturer to attempt it.

On a 50 amp unit many options but a 30 amp the best "creative" option is a long 10 X 3 cord with a hand full of breakers (to allow use at different places) and for a visiting stop just open the panel punch in a breaker with cord and remove when done.