Can I connect a 120V output from a generator to a 240V L14 receptor ?

[alfreds-77]

Greetings. I have generator with only 120 V output. It has both standard 120V outlets and a TT-30. A friend's cabin is wired for alternative generator power with a transfer switch to be used during power failures. There is a L14-30 male receptor on the outside of the cabin which connects to the L14-30 female end of a L14-30 cable. (So, I believe it is designed to take the output from a 120V/240V generator). All the appliances and load that are wired for use on alternative power (i.e. when the transfer switch is thrown to the generator side) are 120V only. (There is a 240V dryer but it is not wired for use, and cannot be used, on alternative power.

My question : is it possible to connect either of the 120V outputs from my generator to the L14-30 male receptor to run my load (which as mentioned, is all 120V) ? If so, how would I go about doing that ? Would I have to wire my own custom cable ? And, if so, I assume I'd connect ground to ground and neutral to neutral. But which of the 2 live wires from the L14-30 would I connect to the live wire from the 120V ? Or would I connect them both ?

Or are their adapter cables available on the market I should consider ?

 

[Megawatt]

Greetings. I have generator with only 120 V output. It has both standard 120V outlets and a TT-30. A friend's cabin is wired for alternative generator power with a transfer switch to be used during power failures. There is a L14-30 male receptor on the outside of the cabin which connects to the L14-30 female end of a L14-30 cable. (So, I believe it is designed to take the output from a 120V/240V generator). All the appliances and load that are wired for use on alternative power (i.e. when the transfer switch is thrown to the generator side) are 120V only. (There is a 240V dryer but it is not wired for use, and cannot be used, on alternative power.

My question : is it possible to connect either of the 120V outputs from my generator to the L14-30 male receptor to run my load (which as mentioned, is all 120V) ? If so, how would I go about doing that ? Would I have to wire my own custom cable ? And, if so, I assume I'd connect ground to ground and neutral to neutral. But which of the 2 live wires from the L14-30 would I connect to the live wire from the 120V ? Or would I connect them both ?

Or are their adapter cables available on the market I should consider ?

@alfreds-77 First off you didn’t mention what KW your generator is and I wouldn’t advise you to start modifying the output of your generator. You don’t know how the cabin is wired and if you try to use both 20 amp Receptacles they will be of different phases and possibly go phase to phase and blow up. I promise you that you will ruin your generator. Your generator can’t be that big or it would have a 30 amp 240vac receptacle for you to plug your cord into so having said that the answer is no please don’t try it

 

[Lucien Nunes]

If the system has been designed so that all the circuits / applliances operational from the generator are connected to one hot in the panel, then only the corresponding hot in the L14-30 would need to be energised. If the circuits are spread over both hots in the panel, then both hots in the L14-30 need to be connected to the TT-30 hot. This would provide 120V hot-to-neutral but zero volts hot-to-hot, therefore 240V loads would not function (your point about the dryer noted).

There is a general caution that applies to linking both hots of a 120/240V system to a single 120V supply if this has not been properly designed for, which is that the neutral wire in any multi-wire branch circuits can be overloaded. When the two hots are on different phases, the neutral current cannot exceed the higher of the two, so (e.g.) a 20A double pole breaker will protect both the hots and the neutral against carrying more than 20A. But when they are on the same phase, the neutral current will be the sum of the two, therefore could have to carry 40A even while each pole of the breaker only sees 20A. In theory there is a similar possibility with the supply cable if the power source exceeds the capability of any one phase upstream of the panel; that is not a problem in your case since the generator output socket is only good for 30A so the cabin input neutral cannot be overloaded.

Check the grounding configuration at the panel and the generator; if there is no neutral-ground link present, GFCIs may not function correctly etc. As per MW above, exercise caution with generator loading and do not interconnect multiple outlets from the generator.

 

[alfreds-77]

If the system has been designed so that all the circuits / applliances operational from the generator are connected to one hot in the panel, then only the corresponding hot in the L14-30 would need to be energised. If the circuits are spread over both hots in the panel, then both hots in the L14-30 need to be connected to the TT-30 hot. This would provide 120V hot-to-neutral but zero volts hot-to-hot, therefore 240V loads would not function (your point about the dryer noted).

There is a general caution that applies to linking both hots of a 120/240V system to a single 120V supply if this has not been properly designed for, which is that the neutral wire in any multi-wire branch circuits can be overloaded. When the two hots are on different phases, the neutral current cannot exceed the higher of the two, so (e.g.) a 20A double pole breaker will protect both the hots and the neutral against carrying more than 20A. But when they are on the same phase, the neutral current will be the sum of the two, therefore could have to carry 40A even while each pole of the breaker only sees 20A. In theory there is a similar possibility with the supply cable if the power source exceeds the capability of any one phase upstream of the panel; that is not a problem in your case since the generator output socket is only good for 30A so the cabin input neutral cannot be overloaded.

Check the grounding configuration at the panel and the generator; if there is no neutral-ground link present, GFCIs may not function correctly etc. As per MW above, exercise caution with generator loading and do not interconnect multiple outlets from the generator.

Thanks !! Re. the neutral current concern : I can see that if I tried to connect both of the 2 separate standard 120 V outputs from the generator to the L14 wall connector, because both are independently rated for 20A. On the other hand, I'm not sure I see that problem if I used the (single) TT-30 output from the generator. The TT-30 output is rated for 30A and has a circuit breaker installed at 30A. Does this mean that if I connect the TT-30 output from the generator to the L14 wall connector on the cabin, that the neutral current concern you mention, would be addressed (i.e. limited to the 30A that the TT-30 can deliver ?).

Assuming that concern is addressed, do you see any other issue with using a L14-30 cable, plus this adapter which bridges the hots from the TT-30 output? https://www.homedepot.com/p/AC-WORK...le-Connector-Hots-Bridged-ADTTL1430/301828751

 

[alfreds-77]

@alfreds-77 First off you didn’t mention what KW your generator is and I wouldn’t advise you to start modifying the output of your generator. You don’t know how the cabin is wired and if you try to use both 20 amp Receptacles they will be of different phases and possibly go phase to phase and blow up. I promise you that you will ruin your generator. Your generator can’t be that big or it would have a 30 amp 240vac receptacle for you to plug your cord into so having said that the answer is no please don’t try it

Thanks ! It's a 3600 Watt generator, and I absolutely don't want to modify the wiring if I can avoid it ! And, yes, as the other replier mentioned, I can see the issue with trying to use both the 20 am receptacles from the generator. I am wondering though if there would be any issue connecting the 30A TT-30 output from the generator to the L14 wall connector, L14-30 cable, and adapter like this one: https://www.homedepot.com/p/AC-WORK...le-Connector-Hots-Bridged-ADTTL1430/301828751. (Again, this is with the understanding (vehemently confirmed by my friend) that there is no 240V load on the alternate power - only 120V.)

 

[alfreds-77]

Thanks !! Re. the neutral current concern : I can see that if I tried to connect both of the 2 separate standard 120 V outputs from the generator to the L14 wall connector, because both are independently rated for 20A. On the other hand, I'm not sure I see that problem if I used the (single) TT-30 output from the generator. The TT-30 output is rated for 30A and has a circuit breaker installed at 30A. Does this mean that if I connect the TT-30 output from the generator to the L14 wall connector on the cabin, that the neutral current concern you mention, would be addressed (i.e. limited to the 30A that the TT-30 can deliver ?).

Assuming that concern is addressed, do you see any other issue with using a L14-30 cable, plus this adapter which bridges the hots from the TT-30 output? https://www.homedepot.com/p/AC-WORK...le-Connector-Hots-Bridged-ADTTL1430/301828751

OK,... forget the first half of what I wrote I above. I think I now understand the concern more clearly. I will have to check if any of the 120V circuits on the alternate power, share the same neutral/ground.

 

[Lucien Nunes]

if I connect the TT-30 output from the generator to the L14 wall connector on the cabin, that the neutral current concern you mention, would be addressed (i.e. limited to the 30A that the TT-30 can deliver ?).

Correct. If the generator breaker is 30A, the input wiring that is also rated for 30A can't be overloaded.

do you see any other issue with using a L14-30 cable, plus this adapter which bridges the hots from the TT-30 output?

No, that looks OK. FWIW I am not a great fan of adaptors like that, as the bulk and leverage tends to put stress on the receptacle. I prefer a correct plug fitted directly to the cable. But if it is not subject to abuse or damage it's no big deal.

 

[alfreds-77]

Correct. If the generator breaker is 30A, the input wiring that is also rated for 30A can't be overloaded.

Right, but if I understand correctly the issue I think you pointed out may still exist if there are circuits that share the same grounded neutral. The wiring in the cabin for each 120V circuit uses 12 gauge wire - i.e. rated for 20A max. If there are 2 circuits that share the same grounded neutral, and the total load on the 2 circuits exceeds 20A in the same phase (which is possible from the TT-30 input), then that shared neutral wire may start to overheat. Am I seeing the potential issue correctly ?

(And thanks for you help - much appreciated.)

 

[Lucien Nunes]

Am I seeing the potential issue correctly ?

Yes, exactly. I do not think it is a common situation though, I mentioned it mainly for completeness. I would be interested to hear from our Canadian or N. American contributors how common such multi-wire branch circuits are, as I am in the UK where we don't use 120/240V systems at all. (Although, we have exactly the same possibility of overloading the neutral, if 3-phase wiring feeding single-phase loads has the three phases linked to operate from a single-phase power source.)

I did find two such circuits within some large US-built equipment trailers that we imported to the UK. These were fitted with a 50A 4-wire Marinco twist-lock inlet and a 120/240V panel. I wanted to operate them from stepdown transformers with a single 120V output, but an inspection of the panel revealed a couple of #12 4-wire circuits with a total load of 30A, which I had to modify.

 

[alfreds-77]

Yes, exactly. I do not think it is a common situation though, I mentioned it mainly for completeness. I would be interested to hear from our Canadian or N. American contributors how common such multi-wire branch circuits are, as I am in the UK where we don't use 120/240V systems at all. (Although, we have exactly the same possibility of overloading the neutral, if 3-phase wiring feeding single-phase loads has the three phases linked to operate from a single-phase power source.)

I did find two such circuits within some large US-built equipment trailers that we imported to the UK. These were fitted with a 50A 4-wire Marinco twist-lock inlet and a 120/240V panel. I wanted to operate them from stepdown transformers with a single 120V output, but an inspection of the panel revealed a couple of #12 4-wire circuits with a total load of 30A, which I had to modify.

Just to follow up, there IS in fact one case of this in the cabin ! A single outlet box with 2 120V outputs was wired with each on a different leg of the 240V supply, and sharing the same neutral wire. And this outlet is wired for use with alternate power. So I will have to be careful here. Thanks again for pointing out the issue !!