From: John De Armond
Subject: Re: Advice on shorepower wiring
Date: Tue, 27 Mar 2001 02:58:39 -0500
Rob Gendreau wrote:
> How long an extension cord can I run safely from a grounded 120v outlet
> (standard issue home outlet) to an adapter and then into my camper's
> 120v/30amp system? I've seen 25' "10/3" cords advertised (does this mean
> 10 gauge, three prong??); that should work safely, shouldn't it? The
> adapter is a short dogbone.
Depends entirely on what type and size load you plan to run. If you
plan on running the AC (single unit), you can probably use up to 100
ft with no problem, depending on the gauge and length of the wire
feeding your convenience outlet. IF you're just running lights, the
'fridge and charging the battery, then there's effectively no limit
Instead of guessing, get an inexpensive plug-in volt meter and watch
your voltage, particularly when the AC is starting. Compare that to
your AC instruction manual specification.
Another thing you must be concerned with is the fact that ordinary
convenience outlets, particularly "contractor spec" outlets offer
relatively high resistance. At full load a cheap outlet will get
hot. If it gets hot enough to relax the tension on the contacts,
the resistance will go up, leading to more heat, etc until something
fails. Usually the first thing to fail is the plug on your
expensive extension cord. One or more blades will get hot enough to
melt the plastic and fail. Voice of experience. If you plan on
running the AC or other near-full load in the circuit, the least you
can do is go to Home Depot or Lowe's and buy an "industrial grade"
20 amp (has the "T" shaped neutral socket) outlet and install it in
place of your spec grade outlet. These outlets use tougher plastic,
heavier conductors and stronger contact springs. Of course, while
you have the outlet apart it's just as easy to install an RV outlet
If you use a conventional extension cord, arrange a strain relief at
the outlet to prevent the cord from pulling on the plug. A large
cup hook screwed in a stud above the outlet will do the trick. It
is vital that the plug remain fully and firmly inserted in the
outlet to avoid getting into the failure mode I described above.
The RV end of the cord should be arranged to be strain-free too. If
you use an adapter on the RV cord and lay it flat on the ground
under the rig so it won't get rained on, that should do it.
I've had a BUNCH of experience with piping power over long distances
under unfavorable conditions with my catering operation. I know
that I can run a couple hundred feet of 10 gauge SO cord
(approximately what those bright yellow construction cords at Home
Depot are wired with) and still get sufficient voltage to run my
heating appliances and/or a microwave oven. However I long ago gave
up on convenience outlet connectors and switched over to twist-lock
plugs. I carry an array of adapters that let me tap just about any
outlet (or breaker panel directly) and convert to twist-lock. I
realize that you probably don't want to do that but do keep an eye
on the integrity of the plug prongs. They will be more limiting
than the voltage drop of a hundred feet of 10 ga cord.
From: John De Armond
Subject: Re: Air conditioner problem
Date: Sun, 20 Jul 2008 16:29:24 -0400
On Sun, 20 Jul 2008 08:55:14 -0500, Elliot Richmond
>After our "plumbing malfunction," the water extraction contractor had
>rigged up a box that plugged in to the dryer outlet in my house. Mine
>is a three wire system and is tied in to two 30 amp breakers strapped
>together.. After looking at his rig and doing a little cogitating, I
>figured out how to make a box that would hold a 30 amp RV plug hooked
>to one leg of the dryer circuit and (probably) two 15 amp plugs tied
>into the other leg. Since the three wire dryer circuit has no ground,
>one would need to get a ground from another cord that plugged into a
>standard outlet, but would just use the ground wire in that circuit.
>This is probably a violation of some code or the other, but it would
> I haven't picked up the parts yet but I think I will build one just
What you're planning will work but it's not necessary. Your AC draws about 12
amps. Look at your manual to be sure but that's ballpark. In a tiny trailer
like that, there is little else to draw power. A couple of amps from the
converter maybe. A small microwave perhaps. Unlikely that you'd use that in
I've spent many a night (and sometimes week at a time) plugged into a 20 amp
outlet. Never tripped one yet. As a precaution I do flip my AC off while
microwaving but that might not be necessary - I've never measured.
If you're tripping a 20 amp breaker then most likely the breaker has gotten
weak. That happens. For some reason high humidity seems to be a cause. The
sub-panel that is mounted in my basement where it used to drip with humidity
would regularly suffer weak breakers. Until I installed a dehumidifier, that
is. Changing the breaker is a lot cheaper and more convenient than making
that lash-up and then plugging and unplugging it every time you need to use
I have to express my concern about making a 240 adapter using a 3 prong plug
outlet. While it will work it will force the safety ground to carry current
continuously. There are several potential problems. One, many older
installations used cable that had two heavy conductors and one small ground
conductor. If your dryer outlet is like that then the ground conductor may
not be large enough. Removing the cover of the dryer outlet will quickly let
you determine that.
Of more concern is the fact that if some fault develops in the ground wire -
corrosion or a loose connection, the safety ground on your RV can have line
voltage on it. Remember that the ground wire normally carries no current and
so a fault might not be evident.
I'd not have much concern and wouldn't say anything if the adapter was for
inside use. Wood flooring, shoes, etc., provide enough insulation that you'd
probably not get more than a tingle. Outside, though, you'll be touching the
RV while standing on dew-wet grass or whatever.
If you really want to use the dryer outlet then you might look at one of these
For little more than the bits and pieces necessary to make your adapter would
cost, you can buy a 1.5 or 2 kVA transformer. The transformer will let your
safety ground do what it's supposed to do - be a safety. I've recommended
this vendor to EV drivers who needed to run a 240 volt battery charger from
120 volts. They report that the product is of high quality and works as
If it were me, I'd do nothing. The 20 amp outlet is fine.
From: John De Armond
Subject: Re: Air conditioner problem
Date: Sun, 20 Jul 2008 22:18:37 -0400
On Sun, 20 Jul 2008 17:29:29 -0500, Elliot Richmond
>On Sun, 20 Jul 2008 16:09:53 -0500, firstname.lastname@example.org wrote:
>>The dryer outlet is a 220 volt service - that means that you have two
>>hot wires with 220 volts across them, and a ground. There is NO
>Partly incorrect. In a three wire dryer plug, there is NO ground.
>There are two hot wires and a neutral. In a four wire dryer cord,
>there are two hot wires, a neutral and a ground.
>In either case, you will have 220 volts between the two hot wires and
>110 volts between either hot wire and the neutral.
If you remove the cover on the dryer where the cord attaches (assumes the
dryer was made in the last 30 years or so) you'll find two hot leads and a
chassis ground. The cord has two hot leads and a green ground lead.
In theory, the difference between neutral and ground is largely a semantic one
as long as the neutral isn't also connected to the frame. In practice there
frequently IS a difference because in past times, the ground conductor could
be much smaller than the power conductor. It wasn't uncommon for the main
conductors to be #6 or #4 gauge while the ground conductor is an uninsulated
#14 or #12. That very configuration, 6-2 with #14 ground, is how my cabin's
dryer outlet is wired. My cabin was built in 1970 when that was acceptable.
Since everything inside my dryer (and probably yours too) operates on 240,
there is no need for a neutral and none is provided. The 4 wire dryer plug is
just another example of code overkill.
My concern with your proposal isn't per se that your ground/neutral lead will
be carrying current. It is that your RV's safety ground and neutral will be
connected together at your adapter. Any fault anywhere down the line in the
ground/neutral lead can elevate your RV's chassis to line potential.
If you make this adapter, please do it like your disaster-recovery man did and
provide a separate safety ground lead. Or a separate neutral lead. I'd
prefer the latter since in that case, neutral current will be returning to the
panel through the neutral leg of the 120 volt outlet. True, the 30 amp
breaker is too large for the 120 volt neutral wire but that's code overkill
again. It wasn't THAT long ago that 30 amp fuses were used on #12 and
sometimes even #14 branch circuit wire. The first house my parents bought
after I was born was wired that way and it was built in about 1950.
Like I said earlier, you don't need to do anything - the 20 amp outlet is
fine. If you just want to do the adapter, please do it safely. Remember that
this is Neon John, the antithesis of the safety nazi speaking! When I think
something is unsafe, it really is unsafe.
I casually knew a telephone company wireman who was killed about 20 years ago
by touching an energized mobile home frame that was wired similar to what
you're describing. Neutral and ground buses connected together in the mobile
home panel and a 3 wire cable run to the pole panel. Neutral opened and that
let 120 volts appear on the "grounded" frame.
In typical mega-corp over-reaction, they issued every lineman this huge
voltage detector that he had to carry around on his tool belt and test every
metallic object that he had to work near. It couldn't be one of those nice
pocket detectors. Noooo, this thing's like a sword. I have one, if you'd
like to see a photo.
From: John De Armond
Subject: Re: Air conditioner problem
Date: Mon, 21 Jul 2008 19:46:46 -0400
On Mon, 21 Jul 2008 08:31:30 -0400, Matt Colie <email@example.com> wrote:
>According to my NEMA books, a standard 6-30 plug/receptacle has three
>connections, L1, L2 and G (not N). The code required machine ground is
>king of a given as I understand the current code. This does not change
>the fact that may stoves and dryers with three wire connections do use
>the ground as a neutral to supply 120 components like lights and times.
Yeah, the stove here in my kitchen did that. Of course, it was made in 1954.
After the built-in outlet (with its own 30 amp screw-in fuse) knocked the pee
out of Mom a couple of times, Dad removed the wires from the outlet. End of
that problem. (Technically the stove itself knocked the pee out of Mom but it
was the electric skillet plugged into the outlet that hotted up the chassis
because of a faulty ground/neutral.)
There is now a green ground wire attached to the stove's chassis which runs to
the water pipe at the sink. A pipe that I know is electrically conductive all
the way back to the well pump casing. I did that oh, 35 years ago, just in
case something else in there like the timer clock passes neutral current.
The code is now a bloated mish-mash of special interest crap designed to sell
special interest hardware but from back then, some things did need to be
changed. Such as not letting the safety ground carry load current.
From: John De Armond
Subject: Re: Dryer connection (was Re: Air conditioner problem)
Date: Mon, 21 Jul 2008 20:10:37 -0400
On Mon, 21 Jul 2008 10:55:36 -0500, Elliot Richmond
>On Mon, 21 Jul 2008 08:31:30 -0400, Matt Colie <firstname.lastname@example.org>
>>According to my NEMA books, a standard 6-30 plug/receptacle has three
>>connections, L1, L2 and G (not N). The code required machine ground is
>>king of a given as I understand the current code. This does not change
>>the fact that may stoves and dryers with three wire connections do use
>>the ground as a neutral to supply 120 components like lights and times.
>Well, there seems to be a lot of confusion about this issue. I think
>the confusion arises partly because in US wiring, the neutral leg of
>the supply is physically grounded at the house. Also, we may talking
>about different NEMA standards (see below).
There shouldn't be. UNTIL A WIRE GETS CONNECTED TO SOMETHING THAT SOMEONE CAN
TOUCH, the distinction between neutral and ground is strictly semantic.
They're one and the same, electrically. ONLY when something that someone can
touch is grounded do the two separate, for the reason I explained earlier. The
safety ground (protects people) cannot be safely merged with the neutral
>Here is what I know: In the unused dryer plug in my house, there are
>three wires coming straight from the breaker box which is only a yard
>away. They appear to be in conduit. There is a red wire, a black wire,
>and a white wire. There is no green or bare wire. The red and black
>wires are connected to the slanted terminals (as they should be) while
>the white (according to code, this would be neutral) is connected to
>the "L" shaped terminal. This terminal will show continuity to ground,
>because the neutral leg is grounded.
Until recently, the conduit was allowed to be the safety ground. I think that
it still is in some circumstances but not in the home. The white wire
indicates that the third terminal is to be neutral and the conduit safety
ground. It was common a few years ago for there to be a green colored screw
on the back of the dryer labeled "ground". A green wire was run from this
terminal to a screw on the dryer outlet to provide the safety ground through
Or if no conduit, the green wire was run over to a water pipe feeding the
adjacent washer. That was before PVC was even dreamed of being installed in a
house. Washers and dryers came with a ground wire as part of the accessory
bundle. Usually about 8 ft long with a spade lug on each end. Some even came
with pipe ground clamps.
I actually liked that system better than the current one. The ground was in
place even before the plug was inserted. The ground was also visible so it
could be verified as intact.
>I don't have an electric dryer to examine, but it is my understanding
>that the only thing in the dryer that requires 220 V is the heating
>element. The rest of the bits, such as lights, timers, etc. use 110 V.
>I do not think an appliance would be set up to use the ground (always
>a green or bare wire) as the return leg for a 110 V bit. They would
>use the neutral leg for that.
That was the case years ago. Some dryers still have 120 volt loads but they
also have 4 wire hookups. There was a period (when my dryer was made) after
neutral/ground merging was recognized as a bad idea but before the Great NEMA
Harmonization (TM) (when 4 wire plugs came about) when all 120 volt loads were
removed and the third wire was safety ground.
>Now, for those of you that are concerned that I may electrocute
>myself, thanks for the concern. I could perhaps sell tickets to the
>event and my life insurance is all paid up. But, with the rig I
>envision, I will have a separate ground circuit from the camper cord,
>through the box, into the ground terminal on a standard plug. GFCI
>considerations aside, I do want a robust, separate ground circuit to
>my trailer frame.
That was the tidbit that you left out.
The natural question arises, since your breaker panel is only a couple of feet
away, why not just wire in a proper 30 amp RV outlet? You could put a surface
mount box on the wall beside the panel for the same or less money and about
the same effort as your gizmo.