I just want advice for the conversion I just did before I plug it all in.
1. I have a two-phase 220V Sears Craftsman compressor which uses a 220 volt
plug with one blade sideways (otherwise it looks like a normal 120V
2. I have no sockets in the house which fit that 220 volt plug.
3. I have a three-pronged dryer socket in the garage with 220 volts (two
hots and a neutral).
4. I just attached a new 3-pronged male dryer cord to a female 220 volt
plug with the one blade sideways.
Before I plug in the compressor to the recepticle to the dryer cord to the
recepticle, would you have any concerns about safety or other?
I assumed the neutral in the compressor plug was the center wire (looks
like a ground pin) while I assumed the neutral on the 3-pronged dryer cord
is the L-shaped center pin. Is that the correct assumption for the shared
On Jul 25, 2:06 am, Elmo <dcdraftwo...@Use-Author-Supplied-
Ummm yeah... Why would you want to plug a 20 amp 240 volt appliance
which uses a NEMA 6-20 cord configuration into a 30 amp outlet made
a NEMA 10-30 cord configuration by means of using a cord adapter which
you have created yourself...
There is a reason why the outlets for vastly different amperage
not fit into each other... SAFETY...
Why not have a correct 20 amp 240 volt NEMA 6-20 outlet installed in
home so you can use your compressor safely on a circuit which is not
capable of supplying more power than your appliance is rated for which
could damage it or cause an accident of some kind...
I wondered about that. Does the "code" cover temperary adapters?
Doesn't the code only apply to the wiring in the walls?
This is an adapter, which is not permanently connected.
And, the wiring is the same, either way, as far as I can tell. The only
ground/neutral difference I see is the NEMA 6-20P ground for the compressor
is connected to the NEMA 10-30R ground/neutral for the dryer. But isn't
that dryer neutral connected directly to ground anyway at the panel on the
other side of the wall?
And, the 20-amp rating for the compressor is connected to a 30-amp breaker
instead of a 20-amp breaker. But, that will still protect the wiring in the
wall from 30 amp surges (it just won't protect the compressor).
snipped-for-privacy@Use-Author-Supplied-Address.invalid (Elmo) writes:
| I wondered about that. Does the "code" cover temperary adapters?
That's an interesting question. I've noticed that marine stores sell
a lot of twist-lock adapters and the "odd" configurations (like 30A male
with 15A female) are not UL listed (or say something like "made with
UL-listed parts) while the matched amperage ones have the UL hologram.
So maybe the answer is that UL covers this, but nothing stops people
from selling, buying, and using adapters that UL won't list.
On 7/25/2010 15:00, email@example.com wrote:
UL isn't a government agency, but some jurisdictions may require
electrical products to be listed by UL or similar organization in order
to be sold there.
The UL listing only means that the product is deemed safe *for its
intended purpose.* A listed 18-gauge extension code is safe when run
along a wall for a table lamp, while it can be a fire hazard if run
under a carpet to power an air conditioner and microwave oven.
On Sun, 25 Jul 2010 00:36:55 -0700 (PDT), Evan wrote:
On Sun, 25 Jul 2010 00:36:55 -0700 (PDT), Evan wrote:
I agree with Evan, the right way to power the compressor with the NEMA
6-20P plug (20amp rating), is to hire an electrician to add a dedicated
220V NEMA 6-20R receptacle with a 30 amp dedicated circuit breaker.
Or to have that electrician swap out the existing NEMA 6-20R with a NEMA
6-20R and to swap out the 30 amp breaker with one with only a 20 amp
However, the cost of hiring the electrician to change the circuit is more
than that of buying a new compressor with, say, standard 110 volt power.
I use the compressor maybe once every six months. Unknowns in the adapter
setup are what I'm asking about here.
The unknowns to overcome by asking this post are:
a) How do I test if the NEMA10-30R is on a sub panel or not?
b) Did I select the right pin ground in the NEMA 6-20R receptacle as the
L-shaped neutral in the NEMA10-30P plug?
Evan - sorry but your ignorance is showing. Virtually every item you
plug into a 15 or 20 amp 115 volt receptacle in your home is rated
MUCH below the 15 or 20 amps the receptacle is rated at, and the
breaker is designed for. It is NOT a safety issue. Your computer draws
something like 2 amps, the monitor 1/2 amp, a 100 watt light bult less
than an amp, your fancy little transistor table radio about 250
The "right" way to do it would be a "fused adapter" with a 30 amp plug
that fits the dryier receptacle, and a 20 amp receptacle to fit the
compressor plug, with a 20 amp fuse or breaker between the two. This
would protect the wiring to the compressor motor - which, in all
likelihood, already has a thermal shutdown protection device built
into it - making the fused adapter redundant anyways.
On Jul 25, 1:12 pm, firstname.lastname@example.org wrote:
Umm... No... It is not ignorance... The CONVENIENCE outlets
in your home are designed for that purpose...
When you are talking about dedicated circuits for non-convenience
loads your logic about that is faulty...
If it isn't designed to fit into that outlet it shouldn't be used with
Instead of assuming such things, why not get a meter that measures
resistance and figure out whicih wire goes to which prong in each
cord? Hint: There is no neutral in a 220 volt appliance or its cord.
Two wires will be hot relative to ground and the ground wire should
not be connected to the other two.
Also get a meter that goes up to 250 VAC and measure the voltage at
all three slots of the receptacle, relative to ground. Then you'll
know which 2 slos are hot, and which two prongs of the cord will be
hot, when connected to the appliance, and which need to be connected
to each other electrically (in the appliance).
This is not to say I think the project is a good idea. No comment on
I did use the Fluke DMM to measure the connections; I was just double
checking which was the ground/neutral because I didn't wire the house in
the first place.
The ground in the two-phase 220v compressor NEMA 6-20P plug should be the
center pin and the ground/neutral in the NEMA10-30R dryer receptacle should
be the L-shaped center prong.
The two questions I would like to ask here are:
a) How do I know if the NEMA10-30R is on a sub panel (it's inches from the
main panel on the other side of the wall outside the garage)?
b) Are my tests correct that the ground on the 20-amp NEMA 6-20P plug is
"equivalent" to the ground/neutral on the 30-amp NEMA10-30R receptacle?
Okay. Good. I jumped to the conclusion that you hadn't.
You keep using "ground/neutral" wrt the receptacle. I think you mean
ground. The neutral is one of the two main slots in a *110* volt
receptacle. It carries full current when something is using the
recept. But it's voltage wrt ground is zero. But that's not the
same as being a ground. It's a neutral.
OTOH, in *220*, two of the wires are hot though at different
potentials wrt ground. Only the ground is without voltage, but if you
call it neutral, I'm pretty sure you will confuse some people, or
their reply to you will confuse you, and that could lead to a mistake.
You are correct sir!
I only belatedly, half way through this thread, realized that the green
ground wire on the 240 volt compressor was connecting to the white ground
wire of the 240 volt dryer receptacle.
If a DRYER were hooked up, then that white third wire in the dryer
receptacle would be doing double duty as a ground (for the 240 volt
circuitry of the dryer) and neutral (for the 120 volt circuitry of the
But, I learned in this thread, that the dryer receptacle in the wall has
its white ground/neutral wire hooked to ground in the panel. So, even
though that white wire does double duty (when a dryer is hooked up), for
the compressor, I'm hooking ground to ground so there is no additional
danger because current is not being carried in this ground wire.
Thanks for all your help. The compressor worked flawlessly when I hooked it
up. I only got one comment on the pictures of the adapter so I'd appreciate
if anyone could tell me how to improve it.
A picture of the adapter cord and compressor is here:
Let's look at this from a "safety" standpoint, which is your point.
The "proper" way to do this, all will agree, is to spend more than the
compressor is worth to install a 20-amp dedicated NEMA 6-20R receptacle.
However, from a safety standpoint, if anyone can show me how plugging the
20-amp NEMA 6-20P through an adapter to the 30-amp NEMA10-30P receptacle is
inherently more dangerous, then I'll listen.
Of course, I do realize that the "equipment" won't be protected to 20 amps
(it will be protected to 30 amps); but I'm not at all worried about the
30-year old compressor burning up.
The "house" wiring is still protected to 30 amps, which is what matters.
And "safety" is the same (as far as I can tell) since we're using exactly
the same two hots and the same ground/neutral.
Or did I miss something critical in that analysis?
Advice always welcome.
The compressor motor will have it's own overload protection.
That's all the circuit breakers protect against for anything plugged
into them; there's no difference in the case of the dryer being plugged
into the outlet as the compressor; the internal dryer motor isn't a 30A
device by any stretch, either.
Correct, and in fact w/ the compressor being a 240V load only, there is
no need for and no neutral current. The shared conductor is serving
only the ground function at that time, just as would the third conductor
in a 20A plug connection.
No...the others are simply incorrect in understanding of Code and
perhaps even function of three-wire dryer circuit...
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