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4 prong 240 generator plug to 3 prong 240 AC plug (bonded neutral)

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9.3K views 29 replies 9 participants last post by  Corded  
#1 ·
I am wanting to install a 30 amp inlet box on the side of my house and wire it to a standard 15 amp receptacle. So in a power outage i can run a generator cord to the side of the house. Unplug the AC from the normal receptacle and plug it into the receptacle power by my generator.

I know they sell cables and conversion plugs to change it from a 4 to 3 prong. But i have never wired anything like this. I know my new generator is a bonded neutral and the inlet box is metal and i dont want it becoming energized. So i need to figure out to do with the neutral and ground, thanks.

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#2 ·
You don’t want to do that
That would create a cord with live exposed ends

If you’re not familiar with electrical codes and wiring you really should hire an electrician.
It’s faster and safer.

Speaking as an electrician I am hesitant to offer advice on this.
Too many things can go wrong.
 
#5 ·
First I am not a licensed electrician but here is my stab at answering your question. The four wires from the generator allows us to use the neutral (white for example) to divide the 240V in half making 2 120V circuits example black-white or red-white. If you are doing 240V only as in the case of using the outlet pictured, the neutral is not used so L1 (black for example) and L2 (red for example) and Ground (green or bare) get connected to the proper terminals of that outlet device. Would work but risks catastrophe because there would not be overcurrent protection for that 240v outlet. Done how you describe, what amounts to a through the wall extension cord, and bypassing the house system entirely, leave the generator bounded. Then there is the question of electrical code compliance for what you have planned. Be safe.
 
#7 ·
Part of the problem with the interlock idea is my back yard is on a slope and thats where my electrical panel is. It would be impossible to level the ground. I suppose i could build a wooden platform for it but getting the 210+ pound generator up on it after a storm would be ridiculous and border line impossible. I tried pulling this monster up the hill today and it wasn't fun. I am thinking about building an H frame that bolts to the bottom of the generator frame and having a tongue for a pin hitch so i can hook it on my atv and pull it around the side of my house when i need it.

There is only one side of the house i can put it and that just happened to be where the 2 refrigerators and AC is located. So unless i can run a 50 foot drop cord to a proper interlock box i am SOL. If this ends up not working i will just have to list the generator for sale on craigslist. I am already regretting buying this thing i thought it would be easier.

Would it be better if i had some kind of hole to pass the wires though? I have seen extension cords sold for my application but going though the wall would make the cord about 25 feet shorter. something like this but with caps when not in use.

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#9 ·
Part of the problem with the interlock idea is my back yard is on a slope and thats where my electrical panel is.
I hear your plight. Not a lisenced electrician either, so take it as you will....I'd suggest installing the L14-30 inlet receptacle, (I like the one you posted above with the breaker) to a location of your choosing for quick gen access.
From there, thru the wall with romex 10/4 or 8/4 (solid core as pictured) directly to your panel with interlock installed.
Defeat the bonded neutral on the gen and you're set with just a L14-30 cord from gen to the inlet.

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#10 ·
Checkout the Reliance Control Corporation WKPBN30 Portable Generator Through-The-Wall PWR Transfer Kit. Works for me; just make sure you are using a minimum of 14 gauge, preferably 12 gauge extension cords in the house. Also, balance the load between the top and bottom row of plugs as the power is split between the two rows. This is the next best thing if the interlock isnt an option.
 
#11 ·
So i need to figure out to do with the neutral and ground, thanks.
I am not clear on what you are doing, but if you are disconnecting your current A/C unit (I guess this is a window unit?) and plugging it into a receptacle powered by the generator, then the neutral and ground is okay being bonded at generator. The ground would go to the 240V receptacle (post #1) and usually the metal enclosure, and then the two hot wires will connect to the 240V receptacle for the A/C. The neutral is not connected to anything (just capped off with a wire nut) as this is a 240V receptacle. The box in your post #8 should work well for this.
 
#12 · (Edited)
You should not pass an extension cord through a hole in a wall unless it large enough to easily remove the cord
Otherwise what you doing is permanent and the rules on what is and is not permanent wiring are pretty cut and dry

This is a genre link and it’s probably the best way to add a generator for back up power


What ever you do
Do it safely and to code

if you use extension cords buy some good ones rated for you loads and keep the generator far away from your home as practical
About 10 feet if you can
Your biggest concerns are fire, carbon monoxide and shock hazards
Think about all that could go wrong like someone drives over your power cords or you have a fuel spill….
Your gen needs good airflow around it to cool
So if you build a shack for it it needs to open up

be safe good luck

an observation
these days everyone I talk too about electrical is asking me about generators, and loads ect
Everyone has ac systems they want to run and multiple fridges…..

I think back…..

no one asked me this 20 years ago
They wanted me to wire up hot tubs and pool pumps and saunas…

again just my observation….
But something has changed.
To date no one has asked me for an electric car charger and the only solar stuff is about cottages…

I feel like I’m reliving the sort of low grade fear people had in 1983 about the best rock to hide under if the bombs start to fall.
I’m not saying one or the other isn’t or wasn’t a real concern, but I do note there has been a change in people’s perceptions about what is or is not a risk to their home and family.
Even me worried too, about a brush fire that rips through or tornado or storm of the century ( never worried about this stuff before)

or am I in an echo chamber?
Could that be it and I just make linkages in my mind with all the other things people ask?
Can’t remember the last time I hooked up a hot tub


1983 is pretty clear however
well that’s my deap thought for the day
 
#13 ·
You should not pass an extension cord through a hole in a wall unless it large enough to easily remove the cord
Best bet would be to come from the box in (post #8) thru the wall to a "load center" inside the house. Just add an outlet box and cover that has 3-outlets. Two standard 120V outlets, and one 240V outlet for the A/C. The neutral wire would be used on the 120V receptacles, but not on the 240V receptacle. Everything will be out of the elements, and any extension cords would be inside the house.
Something like this...
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#15 ·
Well the second fridge is in the basement. There is a basement window almost directly under the AC. So i was going to put a 10 or 12 gauge drop cord though the window to power it. This is the easier thing to run a drop cord to.

The kitchen refrigerator will require a longer cord but 50 foot will reach it easy through the back door.

The AC is whats causing me grief, i thought it passing it though the wall would be relatively easy. I had seen the Reliance Control Corporation WKPBN30 kit as mentioned above in post #10. But it wont do 240v and i dont need 6 120v receptacles although it would be nice to power extras like lamps maybe a TV.

After experiencing a power loss for 8-9 hours and some areas around me were out for a week i thought i would get a generator. 2 friends of mine had them and they told me how great it was to have power to keep their fridges cold. And as a type 1 diabetic i need to keep insulin cold and having hundreds of dollars of food go bad is not something i want to happen either.

But if there is a risk if a fire in the wall and insurance not covering it that is not a risk i am willing to take. So i am thinking about cutting my losses and abandoning this idea, or at least the powering the AC part.
 
#16 ·
I have a 100 FT generator cord that runs to an input box near my main panel, connected via an interlock and breaker. I can set up the generator pretty much anywhere on my property. Other than cost and a little bit of voltage drop, there shouldn't be any concern about a longer outdoor cord powering your house.

For short outages, I use an indoor battery/inverter setup instead of the generator. For longer outages, the generator will be set up for a few hours to recharge the batteries. After that, it goes back into the garage.
 
#17 ·
I am not an electrician but part of my apprehension about doing the interlock thing this is amps....

My original calculations came to 3004 watts 20.2 amps. I dont know if this is starting or running amps. The 240v generator outlet is rated for 30 amps. So i have less than 10 amps to go before it overloads the socket. But if i spread the stuff i am trying to power i know i wont trip the breaker.

So even if my yard was flat and i paid an electrician and $4000 later i had everything hooked up and no need for extension cords and cracked windows and doors. If i wanted to plug some lamps or a Milwaukee M18 battery charger in i would still need drop cords so i dont overload the 30a receptacle.

It just dont seem the price is worth the end product in my case for a $660 generator i thought would be good for the occasional power outage that lasted more than a few hours.
 
#20 ·
Not too bad. I bought the parts on Amazon and put it together myself.
About $100 for the box, meters, receptacle and wiring. It was a fun project too!!

Meters:

Box:

Receptacle:
 
#21 ·
It's critical you know your starting and running amps. I'm betting the window A/C doesn't pull a lot of current once it's running. Same for the 2 fridges. You can exceed the sockets current rating for brief surges, however the constant load needs to be 80% (or less) of the rated amperage of the plugs. For example 24 amps would be the max continuous amperage allowed for a "30A" plug.

There's going to be a cost/convenience factor for sure. You could install the wall pass-thru and run a 240V genset cable inside. Once inside, have genset cable connect to an outlet box with 120V outlets and your 240V outlet. Plug window A/C directly into outlet box and extension cords to the 2 fridges and whatever else.

Something like this could be made from cheap parts at Home Depot. You could install whatever outlets you need (within reason).
Note: I believe generator would need to be grounded using this approach (need to verify that).

Pass-thru should angle up slightly as it enters house so any water that gets in there drains to the outside. Have caps for tube when not being used. I'd also stuff some insulation in the tube to keep heat/cold/wind out.
 
#22 ·
This is all to much of a headache than it's worth to try and run AC. I contacted Reliance to ask them if i could pass it though the wall with the breaker switch box this is there response.

"Because of the UL Listing you can not use the PK30 inlet box. Even if you changes the 30 amp breaker to a 15 amp… I would recommend contacting an Electrician for clarification of the Code "


To much for me to even want to deal with im selling the generator on craigs list and abandoning this idea.

Might buy a small inverter generator just to power the 2 refrigerators with extension cords.
 
#27 ·
"Because of the UL Listing you can not use the PK30 inlet box. Even if you changes the 30 amp breaker to a 15 amp… I would recommend contacting an Electrician for clarification of the Code "
That makes no sense. You can always use a plug or receptacle at less than their rating. For example, using a 30A plug for 5A (& fused at 10A if you like) is perfectly fine.