Shore Power adapter, 240v to 120v?

Converting Cargo Trailers into TTTs

Shore Power adapter, 240v to 120v?

Postby Karebru » Sat Oct 17, 2020 8:44 am

I've been plugging my 30 amp camper in to an extension cord, plugged in to a 15 amp GFCI. I'd like to be able to plug it in to a 30 amp circuit.
I have two 30 amp/240, 3-prong outlets. (Dryer and table saw.) It make sense to me that with a 4-prong, one hot leg is dropped to get 120 volts.
But I'm not sure how or if I can make up an adapter to do that with a three-prong.
Any help here? My family electrician is no longer with us. :angel:
Thanks!
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Re: Shore Power adapter, 240v to 120v?

Postby GTS225 » Sat Oct 17, 2020 1:19 pm

Just so I/we understand what you're asking; You're looking to plug a four-lug, 240VAC plug into a three-lug, 240V household socket, and need to make a pigtail/adapter in order to do it, correct?

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Re: Shore Power adapter, 240v to 120v?

Postby Karebru » Sat Oct 17, 2020 6:59 pm

Sorry. No.
I want to adapt my 30 amp/120 volt, 3-prong camper to a 30 amp/240 volt, 3-prong outlet.
If the outlet was a 4 wire, I'd know to leave out one hot leg to get 120.
As I understand it, my 3-wire, 240 volt circuits have two 120 volt hots, and one neutral? Or is it ground? ...Or is it both?
I've never understood the relationship between neutral (Or is it common?) and ground. :oops:
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Re: Shore Power adapter, 240v to 120v?

Postby pchast » Sat Oct 17, 2020 9:06 pm

There are countless diagrams and descriptions on line.

However the difference is;
120v plug uses ground, neutral, a hot leg.
240v single phase uses either: 4 wire; ground, hot leg+, neutral, hot leg-, ground
or a 3 wire system which does not carry a separate ground
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Re: Shore Power adapter, 240v to 120v?

Postby rjgimp » Sun Oct 18, 2020 12:40 am

The old 3 prong 240 volt household receptacles had two hots and a neutral. They did not have a ground within the receptacle. If you look on the back of an appliance that came from the factory with one of those plugs (like a kitchen stove or a dryer from the 1970s) there is often a metal tab on the back of the appliance that you could screw a separate ground wire to and connect it to a water pipe or metal conduit. The newer four prong receptacles have two hots, a neutral, and an integrated ground.

Have you tested either receptacle with a volt meter? I'm no electrician, but I believe if you put your meter leads in the two hot terminals you'll get 240 volts but if you have one lead in a hot and one in a neutral you'll get 120 volts. There are only three combinations. Elimination will identify the neutral lead. Just remember when you build your adapter, if you only connect two wires at the end where you plug it into the house you'll only have two at the female end where you'll plug in your camper.
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Re: Shore Power adapter, 240v to 120v?

Postby flboy » Sun Oct 18, 2020 6:30 am

Something like this adapter for a 240VAC Dryer to 50A Rv will work then just pick up the 50A Rv to 30A Rv adapter which are common. Just be aware you will only be picking up one of the hot legs from dryer outlet. This would not require any wiring on your part..

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Re: Shore Power adapter, 240v to 120v?

Postby GTS225 » Sun Oct 18, 2020 6:38 am

Karebru wrote:I want to adapt my 30 amp/120 volt, 3-prong camper to a 30 amp/240 volt, 3-prong outlet.

Ummmm. The above looks like you don't need an adapter. Or did you fat-finger the keyboard, and put a 3 where there should have been a 4?
Just making sure we're all on the same page.

Karebu wrote:I've never understood the relationship between neutral (Or is it common?) and ground. :oops:


Ground= A non-current carrying conductor that is present for the safety of humans. (Represented only by a green colored or bare copper wire.) The only time it should carry any current is when something in the appliance/load goes terribly wrong, and a hot lead comes in contact with any metallic portion of said appliance that could also be in contact with a human.

Neutral (or common)= a current-carrying conductor that is present as a "return path" for the hot lead, in order to complete the circuit and make the load or appliance operate. (Represented only by a white or neutral gray colored wire.) This wire can hurt you just as bad as a hot lead, if your body happens to be a better path to ground than the neutral.

The only place where there is a common connection between the ground and neutral is in the distribution/breaker panel. The neutral is considered a "grounded conductor", and is therefore attached to a ground bus, only in the panel, nowhere else. (There are very specific cases where the grounds and neutral are isolated from each other, but that's another discussion.)

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Re: Shore Power adapter, 240v to 120v?

Postby Karebru » Sun Oct 18, 2020 8:01 am

Thanks for all the responses!
Water heater, dryer and spare garage outlet are all 3-wire. I opened one up and you're right. The ground wire is not connected.
House was built in 1993. My late brother was the electrician. It passed inspection by the code enforcement inspector. (My father.)
I come from a family of electricians. Dad was a lineman, an electrician, and an inspector. I've always hated electricity... except when we don't have it after a hurricane.

Don, you nailed it again. I already have a 50 to 30RV adapter. All I need now is the dryer to 50RV and a 30RV extension cord. :money:
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Re: Shore Power adapter, 240v to 120v?

Postby Ottsville » Sun Oct 18, 2020 10:43 am

To the more electrically inclined here - shouldn't the RV have a ground wire in use for safety reasons?
Wouldn't a better option be to install a 30A/120 outlet piggybacked off the 30A/240 outlet and utilize the ground wire that is not connected in the 240 outlet?
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Re: Shore Power adapter, 240v to 120v?

Postby MtnDon » Sun Oct 18, 2020 12:51 pm

The old style 3 wire dryer and range plugs do not have a ground wire. As has been stated they have two hots and one neutral. The reason for the neutral wire is to allow the operation of the 120 VAC lights or motors that are present and to allow the breaker to trip when an overload is present.

The 4 wire configuration was adopted in 1995 in most locations in the US. It was determined long ago that attaching the exposed metal of an appliance to a neutral conductor that is carrying current is a bad idea. That moving current is always looking for the path of least resistance back to its source, and under the right circumstances, such as a homeowner standing barefoot on a wet basement floor, that current on the exposed metal of that appliance may just briefly decide that a human being in contact with that exposed metal is that path of least resistance.

With that in mind it is probably a bad idea to make an adapter to provide 120 VAC for any general purposes such as plugging in an RV or whatever, because that adapted circuit has no real ground. The shock danger increases when more current is being drawn on one of the two hot wires in that circuit. It used to be that we never gave the mismatch too much thought. But as more features that operate on 120 VAC were added to ranges and dryers, the mismatch between the hot legs grew. The current difference bewteen the two hot wires would flow through the neutral wire. That increases the danger if something was to go wrong.

By tapping off a single hot to use the 120 VAC we would be setting up a guaranteed mismatch. And that mismatch could be conducting a lot of amps.

So.... bad idea, if you want to be electrically safe.

If you want, or need, a 30 amp 120 VAC outlet you will be much better to install a proper circuit to provide that 120 VAC, 30 amps.

In addition, if the home brewed adapter system was to cause a fire or injury the insurance adjuster would be having a careful look. Since the home brewed adapter would not be a code approved item they could and probably would, deny the claim.
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Re: Shore Power adapter, 240v to 120v?

Postby rjgimp » Sun Oct 18, 2020 4:13 pm

LOL! Back in the 70s in the kitchen of our cabin we had a combination of an old 'rounded' style fridge and an electric oven that had tall legs under it. Under certain circumstances if you touched both of them (they were about 4 feet apart) you would get a jolt similar to peeing on an electric fence. Yeah, go ahead... ask me how I know what THAT feels like!

:shock: :shock: :shock:

We also had a couple different wringer washers over the years, one of which would also give a bit of a jolt on occasion.
:frightened:
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Re: Shore Power adapter, 240v to 120v?

Postby Karebru » Sun Oct 18, 2020 5:02 pm

rjgimp wrote: ...a jolt similar to peeing on an electric fence. Yeah, go ahead... ask me how I know what THAT feels like!

I don't have to ask. My playground was cow pastures growing up... "I'm not gonna pee on it. YOU pee on it." :whistle:

MtnDon; If you want, or need, a 30 amp 120 VAC outlet you will be much better to install a proper circuit to provide that 120 VAC, 30 amps.

Of course, you're right but... I'm thinking it would be easy to just replace the dryer outlet and cord with 50 amp, 4-prongs. The ground wire is already there in the box. I'd just have to make sure it was hooked up at the panel end.
It wouldn't be a dedicated circuit, but with my 50 to 30 adapter, I'd be grounded.

While were talking electricity... My 120 V system ground isn't tied to the trailer frame. Should it be? I can imagine reasons not to.
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Re: Shore Power adapter, 240v to 120v?

Postby MtnDon » Sun Oct 18, 2020 7:55 pm

While were talking electricity... My 120 V system ground isn't tied to the trailer frame. Should it be? I can imagine reasons not to.


In an RV application the ground wire of the AC service should be grounded to the metal frame.

https://www.rvtravel.com/rv-electricity ... und-power/

https://www.rvtravel.com/diy-safe-way-t ... he%20frame.

https://perens.com/2017/05/31/how-an-am ... -on-an-rv/
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Re: Shore Power adapter, 240v to 120v?

Postby Karebru » Mon Oct 19, 2020 3:57 pm

OK. Thanks. I did read all that... Did I mention that I hate electricity? :?

I will ground my panel to the frame.
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Re: Shore Power adapter, 240v to 120v?

Postby troubleScottie » Mon Oct 19, 2020 7:39 pm

Just to repeat. A 30 amp 240 outlet really is not convertible to a 3 prone 120VAC outlet. I would worry about all sorts of issues. Just run a new wire and outlet. Many homes have 20Amp lines. A 30amp line is/should just be a heavier gauge wire with appropriate breakers. You can do the dry work yourself and have an electrician do an inspection and final hookup to save money. Technically, this might need a permit/inspection by the city/county.

The 3 prone dryer/electric range has 240 volt across the two hot wires. It is the made up of two circuits about 180 degrees or there about out of phase with each other which is how the power comes to the house. In a typical house, half the appliances are connected to one circuit, half to the other. This is visible in the 2 rows of breakers. One side is one, one is the other.

Also grounding your AC to the frame is not grounding your AC system. You have to connect the ground to the actual ground. To really do this, you would need a grounding stake in the soil and electrically attached to the ground. For a light weight trailer frame that is not welded assembly, the electrical connection is not always consistent over the entire trailer. Grounding the AC to the frame is not needed. Most people do not add a ground stake either. If you are using VHF/UHF/SW antenna, you might want a real ground.
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