Another accidental 30 amp into 220

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So as a master electrician... don't you think the neutral and ground wires should be joined on a 3 wire 220 dryer plug
like this ?
I'm not a master electrician but I've studied a lot of code.  My understanding is the answer is No, they are not to be joined.  The ground strap in the 3-wire picture connects the metal frame of the appliance to the neutral, which adds safety for some faults but has some potential problems of its own. That's why the 3-wire dryer hook-ups are no longer approved for new installations.  When you have an older 3-wire appliance but a 4-wire outlet and power cord, the ground wire on the cord is attached directly to the appliance frame, not to the neutral terminal on the appliance.  The neutral-to-frame ground strap is removed.  That method provides a safety ground for the appliance frame while preserving neutral-ground isolation at the appliance.
 
sightseers said:
Since there aren't any 220 circuits in most RV's..  the 50 amp plug could also just have two 50 amp 120 circuits of the same phase.


Sorry this is not true. the 2 50amp circuits cannot be in the same phase because if the were you could overload the neutral wire! If you were drawing 40amps on each leg, and they were in phase, the the neutral would be carrying 80 amps.


ken
 
Ken & Sheila said:
Sorry this is not true. the 2 50amp circuits cannot be in the same phase because if the were you could overload the neutral wire! If you were drawing 40amps on each leg, and they were in phase, the the neutral would be carrying 80 amps.


ken
so... your RV is capable of drawing 100 amps of power ?
 
HueyPilotVN said:
I just wired up a 30 amp dryer outlet the other day for a dryer in the garage for Renae's daughter Kelly and it only uses 3 wires.  The ground, (bare copper), is not connected.  The two 120 hot wires are on both sides and the neutral is in the middle.

You have probably connected the neutral to the dryer chassis ground.  IMO.

Also to note, a standard 30 Amp dryer plug should be a 4 wire recep, not 3 ???
 
Ken & Sheila said:
Sorry this is not true. the 2 50amp circuits cannot be in the same phase because if the were you could overload the neutral wire! If you were drawing 40amps on each leg, and they were in phase, the the neutral would be carrying 80 amps.


ken

Actually, we ran into this situation in a park one time. What brought it to my attention is our power management system would only let 30 amps turn on, because it only saw 120 volts, even though we were plugged into a 50 amp service.
 
Maybe the simplest way to look at it is that a 50 amp 4-wire 120/240 volt RV outlet is identical to a residential 50 amp 4-wire 120/240 volt range outlet. The same NEMA 14-50R outlet is used for both, and both are wired exactly the same.
 
Neal said:
Actually, we ran into this situation in a park one time. What brought it to my attention is our power management system would only let 30 amps turn on, because it only saw 120 volts, even though we were plugged into a 50 amp service.

it's the same thing with the 30 to 50 amp adapter...it supplies both legs with the same 120 phase.

your power mgmt. system knows how to use it.
 
Ken & Sheila said:
Sorry this is not true. the 2 50amp circuits cannot be in the same phase because if the were you could overload the neutral wire! If you were drawing 40amps on each leg, and they were in phase, the the neutral would be carrying 80 amps.


ken
so....what do you think happens when you use a 30 to 50 amp converter plug..?

(both legs get the same 120 vt phase)
 
sightseers said:
so....what do you think happens when you use a 30 to 50 amp converter plug..?

(both legs get the same 120 vt phase)

You're connected to a 30 amp breaker. The most the neutral wire could carry is 30 amps. If you're connected to two 50 amp in phase feeds the neutral would be subject to the possibility of 100 amps.
 
If you're connected to two 50 amp in phase feeds the neutral would be subject to the possibility of 100 amps.
Note the word "possibility".  100A would be really, really rare with an RV, but 65-75 is not far fetched on a hot day with a couple a/c running and maybe a dryer or a microwave. Or a large coach with 3 a/c units running.  Exceeding the current ratings becomes a matter of time (duration). The wire gets warmer and warmer and if the amperage is high enough can eventually melt insulation or even the wire itself.

The ubiquitous Onan 7500/8000 diesel genset produces two in-phase hot legs but its max output is about 33 amps/leg, so the neutral maxes out about 66A.
A professionally wired campground outlet that utilized in-phase hot legs is often limited to 40A/leg, bringing the possible amperage on the neutral down to 80A. Maybe even 30A/leg, for a max of 60A.
 
ChasA..

I curious....how many A/C units does your rig have that it draws 100 amps. ?
 
sightseers said:
So as a master electrician... don't you think the neutral and ground wires should be joined on a 3 wire 220 dryer plug
like this ?

That is no longer allowed by the national electric code. It was perfectly fine for many years except in mobile homes which required 4 wire connections for many years. Ranges are the same. The determining factor is whether the appliance uses both 120 AND 240. 120 requires a neutral (white) wire. 240 does not. The motor and controls in a dryer use 120 and the clock, timer, and controls in a range also use 120. A welder for instance does not use 120. So it needs no neutral, just the two hots. And of course all of them need a ground. The code prevents connecting a neutral to the frame of the appliance for safety reasons. I hope this helps.
 
Being that the neutral bus bar and ground wire connections are the same electrical point inside the breaker box....what would be the "Safety issue" connecting them together at the dryer chassis ??? ?
 
My RV has 3 AC's on the roof, an Induction cooktop that requires 50 amp service (or generator) to run, and a residential refer, just to name a few.  So pulling 80 to 90 amps for me is easy to do. 

I was running the rig on a 30 amp plug for the last month and using 2 AC's. Although the power management system worked well, the temps in my location have topped 110 degrees on some days.  There were days when we would return to the rig and the Auto Gen Start had kicked in to take up the demand. 

We were finally able to get an electrician over to install a new 50 amp circuit and now all appliances run happily regardless of the load. The DW is happy. 
 
SargeW said:
My RV has 3 AC's on the roof, an Induction cooktop that requires 50 amp service (or generator) to run, and a residential refer, just to name a few.  So pulling 80 to 90 amps for me is easy to do. 

I was running the rig on a 30 amp plug for the last month and using 2 AC's. Although the power management system worked well, the temps in my location have topped 110 degrees on some days.  There were days when we would return to the rig and the Auto Gen Start had kicked in to take up the demand. 

We were finally able to get an electrician over to install a new 50 amp circuit and now all appliances run happily regardless of the load. The DW is happy.

Sarge..quick question, with 3 A/Cs maxed out what is the temp drop inside your rig ? 

most RV's never get much more than 15 degrees difference from outside temp
 
A lot has to do with the out side temps and how well we keep the inside closed up. Today we had 113 degree temps in Havasu, but the inside stayed a comfortable low 80's.  We keep the window awning down on the outside, and the blinds all down inside.  My patio side faces due west, so when I can put the awning out it helps keep the sun off of the pass side wall.  However, the winds often come up in Havasu, and my wind sensor will retract the patio awning back to the edge of the slide out.

That is with 3 roof AC's running. I did goof up yesterday and left a galley vent open while Diane was cooking, and forgot to close it. I didn't catch it until this morning, and it had run all evening and night. That vent pulled a lot of the cool air from the middle AC unit right out the ceiling vent. After I closed it, I noted an immediate improvement in efficiency. 

In the hot mid afternoons we will also put the door black out shade down to cut the heat. Every little bit helps.
 
sightseers said:
Being that the neutral bus bar and ground wire connections are the same electrical point inside the breaker box....what would be the "Safety issue" connecting them together at the dryer chassis ??? ?

It's due to the voltage drop along the wires.  The same thing that causes voltage sag at the end of a long extension cord when you draw current through it.

The ground wire is kept seperate from the neutral so no current flows through it.  Without current flow both ends stay at 0 volts (ground potential).

Tie both ends of the ground wire to neutral and it's now a current carrying conductor carrying part of the neutral current.  If there's a fault that causes an abnormally large amount of current flow, the neutral line (and the appliance chassis if neutral is tied to ground there) can rise up to half of the supply voltage in the event of a dead short.  Circuit breakers take a finite amount of time to trip, long enough to give someone a deadly shock if they happen to be touching the appliance when this happens.

With a separate ground wire, only the internal parts will rise above ground.  The grounded external parts stay at ground potential.
 
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