30 Amp Wiring Question

[RustyJC]

Did anyone happen to notice the topic in the tech section of this month's Trailer Life dealing with the individual who has rigged a pigtail to plug his RV into one phase of a dryer outlet at his house? The magazine guru gave this arrangement his blessing.

I don't recall the dryer plug having a separate terminal for ground - it uses a grounded neutral if I recall correctly. I wonder what the individual is doing with his RV ground lead? Without knowing, this sounds like a pretty good way to light up one's eyeballs!

Rusty

[RustyJC]

Did anyone happen to notice the topic in the tech section of this month's Trailer Life dealing with the individual who has rigged a pigtail to plug his RV into one phase of a dryer outlet at his house? The magazine guru gave this arrangement his blessing.

I don't recall the dryer plug having a separate terminal for ground - it uses a grounded neutral if I recall correctly. I wonder what the individual is doing with his RV ground lead? Without knowing, this sounds like a pretty good way to light up one's eyeballs!

Rusty

[Ken Lenger]

I think there are two kinds of dryer outlets. One has 3 prongs (H, H, G) and the other has 4 prongs (H, H, C, G). Assuming that you have a 4 prong outlet, and connect between one of the hot legs and common, there should be no problem. I did this exact thing at the townhouse I lived in since I had a gas dryer.

[RustyJC]

Now that I think about it, you're probably right. We recently bought a new washer & dryer at Lowe's and they asked us if the dryer had a 3 or 4 prong outlet. Since our house is more than 5 years old, it had a 3 prong - that's what I was thinking of. The difference, of course, is that an RV uses an isolated ground where our 3-prong dryer outlet doesn't.

I guess all you folks with new houses don't have a problem. I'm not sure I would want to try this with the 3-prong outlet with the grounded neutral, though!!

Rusty

[RV Wizard]

I will again recommend that all owners invest in a Surge Guard for the service of their rigs. I cannot remember the numbers of rigs that have lost almos all electronics from plugging into a dryer outlet. If it is a four prong receptacle and the adapter and receptacle are wired properly, there should not be a problem but if not; with out an overvoltage protection device you will fry the rigs electrical systems.

[aa6vh]

The difference between the three prong and four prong 240V plug is the additional "common" wire (as described above). The reason why the common wire is added is to allow for 120 volt operation for the device as well.

Some dryers (or other 240 volt appliances) have controllers or other such stuff that require 120 volts, whereas the main heater/whatever requires 240. Actually the 240 is done by having two 120 hot lines exactly out of phase with each other, so the cummulative effect is 240 volts.

The bottom line is yes, if you do have a four prong 240 plug (with the common wire), then one hot lead and the common wire will supply 120 volts. Be sure to use the common wire, not the ground!

If you have any questions on this at all, then I recomend getting a qualified electrician to assist you.

Dave

[lionrampant]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by RV Wizard:
I will again recommend that all owners invest in a Surge Guard for the service of their rigs. I cannot remember the numbers of rigs that have lost almos all electronics from plugging into a dryer outlet. If it is a four prong receptacle and the adapter and receptacle are wired properly, there should not be a problem but if not; with out an overvoltage protection device you will fry the rigs electrical systems. </div></BLOCKQUOTE>

Mike--

I agree wholeheartedly! Probably one of the best investments you can make in the long term health of your RV. I just installed a 50A hard-wired Surge Guard in my 5th wheel this past weekend. The hardest part of the whole operation was figuring out where to hang it! The installation itself was a snap.

Ken

[RustyJC]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by aa6vh:
The difference between the three prong and four prong 240V plug is the additional "common" wire (as described above). The reason why the common wire is added is to allow for 120 volt operation for the device as well. </div></BLOCKQUOTE>Dave,

With all respect, what was added in the 4-prong plug was an isolated ground. The 3-prong plug has L1, L2 and N with neutral (N) normally being grounded at the breaker box. In the 3-prong plug, L1-L2 will provide 240VAC, and either L1-N or L2-N will provide 120VAC. As you state, L1 and L2 are 180* out of phase.

Rusty

[aa6vh]

Rusty,

We both could be right, so to speak. I have seen 240 volt circuits (my house A/C for example) where there is no neutral wire, just two hots and a ground. I suspect there are other, espcially older circuits that do not have the ground, and the third wire is neutral.

According to current electrical codes, you must have a separate ground wire (at least in my area).

With respect,

Dave

[RustyJC]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by aa6vh:
According to current electrical codes, you must have a separate ground wire (at least in my area).

With respect,

Dave </div></BLOCKQUOTE>Yep, that's why newer homes (built within, say, the last 5 years) here in the Houston area went to the 4 prong plug - it added the isolated ground wire instead of relying on a grounded neutral that could be grounded at the breaker box 100' or more from the appliance.

Rusty

[Cummins Hot Rod]

So what do we have when when plugging in a 30amp adapter on the end of our 50amp RV cord. It doesnt have the 4th wire so is it H,H,G?
Or is it H,C,G. I suspect it would be the first configuration or some of the RVs electrical would be non functioning due to load splitting in the main RV panel. If this is correct how does it function without the 4th common wire.
Good topic as I am making a extra 50amp cord from a unused welding extension cord that is only 3 wire.
John

[RustyJC]

John,

The 30 amp "dogbone" is L1, N, G. It internally jumpers the 2 hot legs of the RV's 50 amp service together so that they are both powered, but are no longer 180* out of phase. Using the 30 amp adapter, a 50 amp RV only has 30% of the theoretical ampacity that is has on 50 amp service where it has 50 amps x 2 legs = 100 amps.

One thing to consider when using these dogbones is that the 2 each 50A main breakers in the RV distribution box are no longer protecting the RV (actually, the dogbone adapter) in the sense that they're overrated for the available power supply - you're relying on the 30A breaker on the shore power pole for overcurrent protection.

Rusty

[Cummins Hot Rod]

Good to know how the 30amp adapter works, hated to cut one apart to find out.
For many years I have done many different types of wiring jobs and almost all main panels (residential and commercial) combine the neutral wire and the ground wire on the same buss bar. Many old houses didnt have a ground circuit at all. I was told by an electrician working for the power company that the neutral is the ground supplied by the power company and the panel ground is supplied by the individual installation.
I hope I havnt hijacked this thread with these questions but there is a need for this knowledge here I believe.
Thanks
John

[RustyJC]

For those that may be wondering, one reason that it's so essential that the RV have a separate, isolated ground (typically, a ground rod at the shore power box) is that it's sitting on rubber tires. Now, if I have my jack pads on plastic Lynx levelers, this local, isolated ground is grounding the RV chassis through the power cord, thus preventing any stray current from knocking one flat if they were to, say, step onto the metal steps barefoot with one foot still on the ground!

Rusty