Twin 30 to one 50 amp RV cord

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goldstar

goldstar

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New Jersey
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Electrical Contractor
A friend asked me if he was able to parallel two 30 amp RV outputs on two separate generators to arrive at one 50 amp output using this cord and if it was possible to run both the AC units in his RV at the same time:

http://www.campingworld.com/shoppin...le-plugs-y-adapter-with-powersmart-leds/71205

In my mind I'm thinking that once you reach the capacity of either generator, that generator's 30 amp breaker will trip. Am I missing something ? The link doesn't give you too much regarding specs.

Thanks.
 
goldstar said:
A friend asked me if he was able to parallel two 30 amp RV outputs on two separate generators to arrive at one 50 amp output using this cord and if it was possible to run both the AC units in his RV at the same time:

http://www.campingworld.com/shoppin...le-plugs-y-adapter-with-powersmart-leds/71205

In my mind I'm thinking that once you reach the capacity of either generator, that generator's 30 amp breaker will trip. Am I missing something ? The link doesn't give you too much regarding specs.

Thanks.
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Generators can be run in parallel to increase capacity, that's what utilities do. But they must be synchronized and I'd wager that 99.99999% of the inexpensive portable gensets someone would have for an RV are not capable of that. The ones that can be run in parallel are the more expensive inverter type, i.e. the generators are DC and each one has an inverter that makes AC for you. Some of those have a feature where two generators of the exact same make and model will synchronize their inverter outputs when tied together. You must specifically look for that feature. Honda offers it I know for sure, using your exact scenario as the reason.
 
goldstar said:
A friend asked me if he was able to parallel two 30 amp RV outputs on two separate generators to arrive at one 50 amp output using this cord and if it was possible to run both the AC units in his RV at the same time:

http://www.campingworld.com/shoppin...le-plugs-y-adapter-with-powersmart-leds/71205

In my mind I'm thinking that once you reach the capacity of either generator, that generator's 30 amp breaker will trip. Am I missing something ? The link doesn't give you too much regarding specs.

Thanks.
Click to expand...
AFAIK the AC units are on opposite lines of the 50 amp supply cord. If you ran them on a single adapter from 30 amp receptacle the adapter just bridges the two sides together, you can run either AC because of this but both at same time is too much load for the 30 amp supply circuit.

I would think that twin 30 adapter would just combine the EGC's and the two neutrals but each "hot" would remain separate. You should be able to run both AC units. I would have concern about possibly having too much current on the neutral though if both sides were drawing near 30 amps then the neutral is carrying double that.
 
If the air conditioners are the only issue, a workaround to syncing the generators might be to install a 30-amp DPDT switch on the feed to one of the A/C units. Tie the A/C to the common poles, the normal supply buss to one side, and the pigtail to the generator on the other side. That way, flipping the switch divorces one A/C from the RV panel buss and supplies it from it's own generator. Be sure to protect the feeder from over current.
 
OldSparks said:
If the air conditioners are the only issue, a workaround to syncing the generators might be to install a 30-amp DPDT switch on the feed to one of the A/C units. Tie the A/C to the common poles, the normal supply buss to one side, and the pigtail to the generator on the other side. That way, flipping the switch divorces one A/C from the RV panel buss and supplies it from it's own generator. Be sure to protect the feeder from over current.
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That could work too, but the ONE TIME you forget to flip that switch and connect the two generators out of synch, the fireball will be substantial, and expensive.
 
Jraef said:
That could work too, but the ONE TIME you forget to flip that switch and connect the two generators out of synch, the fireball will be substantial, and expensive.
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That's not possible. If you forget to flip the switch, you will overload the ONE generator that is supplying the RV buss. And the second generator will be out there running with zero load.
 
If all the loads in the RV run on 120v, you can hook up two unsynchronized 120v, 30a generators.

Each generator hot leg will feed one hot leg in the panel; they won't be connected to, or interfere with, each other.

If they happen to be synchronized and in phase with each other, the white wire will carry the return current from both hot legs, but that will be a maximum of 60 amps on a wire with 50 amps ampacity. Not quite kosher, but not a catastrophe.

If they happen to be synchronized and 180° out of phase with each other, the white wire will carry the difference of return currents, same as in a residential 120/240-volt single-phase split-phase Edison circuit.

If they're unsynchronized, chances are they will also be running at slightly different frequencies and they'll drift from nearly-synchronized to nearly-anti-synchronized and back. The average current in the white wire will be equal to the current in either hot leg.

If there are any 240-volt loads, they aren't going to work. The voltage will drift from 0 (when they're synchronized) to 208v (when they're 120° out of phase) to 240v. (when they're 180° out of phase)
 
OldSparks said:
That's not possible. If you forget to flip the switch, you will overload the ONE generator that is supplying the RV buss. And the second generator will be out there running with zero load.
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The flash-bang would take place immediately, long before any protective device could operate.
 
Jraef said:
The flash-bang would take place immediately, long before any protective device could operate.
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You are not thinking of the switch that was proposed.

The main panel is driven by one generator and has branches to both A/Cs. All that the proposed switch does is transfer on A/C from the panel branch circuit to a circuit fed directly from the second A/C. No flash-bang at all since, as with any transfer switch, there would be no way that the two generators could end up connected.
The failure (overload) situation described was if both A/Cs were turned on without first switching one to the second generator. A simple overload on the generator which would either stop the generator or blow the first generator breaker if the first generator had enough capacity to do that.
 
So use a T-sat to control a spdt switch or a dpdt switch. One a/c will be you normally closed and when it opens it allows the other a/c to run. Same as a spdt switch for a water heater
 
Dennis Alwon said:
So use a T-sat to control a spdt switch or a dpdt switch. One a/c will be you normally closed and when it opens it allows the other a/c to run. Same as a spdt switch for a water heater
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I think he was trying to run both units at the same time.
 
drcampbell said:
If all the loads in the RV run on 120v, you can hook up two unsynchronized 120v, 30a generators.

Each generator hot leg will feed one hot leg in the panel; they won't be connected to, or interfere with, each other.

If they happen to be synchronized and in phase with each other, the white wire will carry the return current from both hot legs, but that will be a maximum of 60 amps on a wire with 50 amps ampacity. Not quite kosher, but not a catastrophe.

If they happen to be synchronized and 180° out of phase with each other, the white wire will carry the difference of return currents, same as in a residential 120/240-volt single-phase split-phase Edison circuit.

If they're unsynchronized, chances are they will also be running at slightly different frequencies and they'll drift from nearly-synchronized to nearly-anti-synchronized and back. The average current in the white wire will be equal to the current in either hot leg.

If there are any 240-volt loads, they aren't going to work. The voltage will drift from 0 (when they're synchronized) to 208v (when they're 120° out of phase) to 240v. (when they're 180° out of phase)
Click to expand...
This makes perfect sense now. My mind wasn't in RV mode.
 
Jraef said:
The flash-bang would take place immediately, long before any protective device could operate.
Click to expand...
One side of the DPDT switch has a pigtail to a generator. The other side is wired to the RV buss. The A/C unit is wired to the common contacts. There is ZERO possibility of tying two sources together. This is a very simple switching circuit. Please explain in detail where I am incorrect.
 
OldSparks said:
One side of the DPDT switch has a pigtail to a generator. The other side is wired to the RV buss. The A/C unit is wired to the common contacts. There is ZERO possibility of tying two sources together. This is a very simple switching circuit. Please explain in detail where I am incorrect.
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You would only be incorrect in the very rare situation of a make before break switch. I have never seen a snap switch that was built that way, but it is more common for rotary A-B switches.
 
GoldDigger said:
You would only be incorrect in the very rare situation of a make before break switch. I have never seen a snap switch that was built that way, but it is more common for rotary A-B switches.
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Agreed. So, just to clarify, it is necessary to use a conventional Break-before-Make toggle switch.
 
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