2 Roof Airs

[david l greenberg]

On Sat, 29 Jan 2000 09:58:50 -0500 Richard Waters
writes:
>
>Maybe I'm doing things the hard way, but I leave my 50 amp
>power cord coiled up in the power cord locker. I keep a 50'
>30 amp cord up in the pod. I use that to connect to the outside
>world.

This works fine in cold to moderate weather. Be careful in hot weather,
especially if you are drawing serious amperage. The coiled up cord may
get a bit warm but may not be critical. Someone else knows better than
me.

Sounds like you have a early 76 on a late 75 chassis.

David Lee Greenberg
GMC Motorhome Registry
Dedicated To The Preservation Of The GMC Classic!
www.gmcss.com/registry.htm

 

[david l greenberg]

>Dave
>
>My shore line cable has 40 amp stamped into it, and the book says that
>it
>carries two 20 amp circuits??? But it looks likes a 50 amp cord and
>plug.
>
>Wayne Newland
>
I wonder if a PO replaced the cord for some reason. I am only aware of a
change from 30A to 50A. Anyone have any thoughts?
>

David Lee Greenberg
GMC Motorhome Registry
Dedicated To The Preservation Of The GMC Classic!
www.gmcss.com/registry.htm

 

[wayne newland]

David

No change. Original equipment. Others have talked about a 40 amp cord,
also.

Wayne

> >Dave
> >
> >My shore line cable has 40 amp stamped into it, and the book says that
> >it
> >carries two 20 amp circuits??? But it looks likes a 50 amp cord and
> >plug.
> >
> >Wayne Newland
> >
> I wonder if a PO replaced the cord for some reason. I am only aware of a
> change from 30A to 50A. Anyone have any thoughts?
> >
>
> David Lee Greenberg
> GMC Motorhome Registry
> Dedicated To The Preservation Of The GMC Classic!
> www.gmcss.com/registry.htm

 

[tim dawson]

Actually, this is incorrect. In the GMC, when you are on a 50Amp outlet, the
suppy is 240 Volts (2 legs of 110), and you get the full 40 Amps per leg.
Hence, total power available is 80 Amps, not 40 as suggested below (which would
be true IF the service was only single leg, as is a 30A plug.)

- - Tim

>

>
>
> No change. Original equipment. Others have talked about a 40 amp cord,
> also.
>
> Wayne >>
>
> I think that the reason for the confusion about 50 amp versus 40 amp is that
> the 4 prong receptacle at campgrounds is designed to allow 50 amp of current
> to flow and is called "50 Amp Service". The plug on the GMC fits this
> receptacle but if you look at the breaker box in the GMC you will see that it
> is equipped with a 40 amp main breaker. This means that the GMC will never
> pull over 40 amps because the breaker would trip. This gives 20 amps per 120
> v. side of the breaker. Yes, -- GM did have their cords made up which show
> 40 amps stamped on the cord. Probably due to the fact that they had a 40 amp
> setup in the breaker box. These cords are of heavy enough gauge that they
> can easily carry 50 amps.
>
> My main breaker went bad a couple of years ago and I couldn't find a 40 amp
> one at the time and place so I put in a 50 amp breaker. I never used to trip
> the 40 amp one so I know that my GMC will never use anywhere near 50 amps
> unless I get a direct short on the main line in the box. The air
> conditioner, water heater, receptacles, etc., all have their individual 15
> and 20 amp circuit breakers so I don't think it will make a bit of difference
> to me whether I have a 40 amp or 50 amp main.
>
> Emery Stora
> 77 Kingsley
> Santa Fe, NM
>
> Emery Stora
>

- --
================================================================================
Tim Dawson (tadawson) Owner/Engineer
TPC Services Bellnet: (972)-221-7385
Lewisville, Texas 75067 FAXnet: (972)-221-0393
"The world is complex. Sendmail.cf reflects this...."

 

[travism]

> Tim - you are correct on this. I made a mistake. Because of the way our
> main breaker is connected it will trip both breakers if one line draws
more
> than 40 amps. For 240 volt you could only draw 40 amps total because the
> current flows from one of the two 120 volt leads to the other. However,
we
> don't have 240 circuits in our GMC You are right that the two leads (red
and
> black) will provide 40 amps each with the return through the neutral
(white)
> wire. I had incorrectly said 20 amps each.

Yes, but if you are connected to a 120/240 circuit, you are getting 40 amps
at 240 volts phase-to-phase, which is still 9600 volt-amps, and thus
equivalent power to 80 amps at 120 volts. In this instance, since there are
no 240 volt loads in the motorhome, if the loads on the two legs are the
same, there will be no current flow through the neutral. If the two are in
phase, the entire load will return through the neutral, which means trouble
if you are trying to use it anywhere near the 9600 volt-amp capacity.

It is also true that if the two 40 amp breakers are ganged, tripping one
will also trip the other.

Travis

 

[blank]

I didn't want to jump in late because I haven't been closely following this
thread, but I can't hold back any longer. AFAIK, there is no phase
difference between the two 120 volt legs of a split 240 volt line. The two
120 volts legs are the result of a center tap on the 240 volt secondary side
of a single phase transformer. If there were any phase difference, then the
two sides wouldn't add up to 240 volts. It's interesting that the NEC
addresses neutral loading in industrial and residential installations with
load tables. But in the RV section of the code(Art. 551), I couldn't find
any reference other than "load distribution shall ensure a reasonable
current balance between phases" when using a 50 amp panel. I'm looking at a
96 Edition, but I doubt it's changed much since then.

In our installation(with the 50 amp plug), you would need three of the 20
amp branch circuits running near maximum to overload the neutral conductor.
Not very likely IMO. Of course, travelling with my wife and two daughters,
I may get to find out. They always seem able to find interesting ways to
load the system.

Patrick

> -----Original Message-----
> From: EMERYSTORA [mailto:EMERYSTORA]
> Sent: Tuesday, February 01, 2000 10:43 AM
> To: gmcmotorhome
> Subject: Re: GMC: 2 Roof Airs
>
>
>

>
>
> same, there will be no current flow through the neutral. If
> the two are in
>
> phase, the entire load will return through the neutral, which
> means trouble
>
> if you are trying to use it anywhere near the 9600 volt-amp capacity.
>
> >>
>
> Wouldn't it be very unlikely that the load would be the same
> on each of the
> two legs? Also, I believe that running the air conditioner
> which would put
> inductance and capacitance into the equation would change the
> phase somewhat.
>
> Its been 37 years since I've had any EE courses and I've
> never practiced much
> other than rewiring a couple of older homes that I've owned
> over the years.
>
> Thanks for your info.
>
> Emery Stora
> 77 Kingsley
> Santa Fe, NM
>

 

[travism]

> Wouldn't it be very unlikely that the load would be the same on each of
the
> two legs? Also, I believe that running the air conditioner which would
put
> inductance and capacitance into the equation would change the phase
somewhat.
>

Sure, it's extremely unlikely that the loads would be _exactly_ the same,
but to the extent they are, they unload the neutral. If you are running two
roof air units and they are on the opposite phases, their draw, including
the reactive components, cancel each other as far as the neutral is
concerned. That is, the neutral only has to carry the unbalanced portion of
the load, which is never more than one of the phases. That's why the wire
used for the neutral need not be any heavier than either of the phases; this
was my point.

On the other hand, if the service is wired such that the two hot leads are
in phase, then they do _not_ cancel each other but rather add, such that the
neutral has to carry the return for both of them. Thus if you are using a
wire size adequate for 40 amps for each of the hot leads, the neutral needs
to be capable of carrying 80 amps if the two hot leads are in phase.

Whew. I hope I made sense...

Travis

 

[travism]

Au contraire, mon ami; the two 120 volt leads _have_ to be out of phase for
240 volts to be available. 180 degrees out of phase, in fact.

You are correct in pointing out that the 120/240 is supplied by center
tapping a 240 volt transformer, with the center tap being labeled neutral
and grounded. With that mental picture in mind, observe that the two 120
volt leads, which are at opposite ends of the transformer winding, are 180
degrees out of phase with each other. When the one on top is positive, the
one at the bottom is negative and vice versa. This gives 240 volts.

Travis

- ----- Original Message -----
From:
To:
Sent: Tuesday, February 01, 2000 10:33 AM
Subject: RE: GMC: 2 Roof Airs

> I didn't want to jump in late because I haven't been closely following
this
> thread, but I can't hold back any longer. AFAIK, there is no phase
> difference between the two 120 volt legs of a split 240 volt line. The
two
> 120 volts legs are the result of a center tap on the 240 volt secondary
side
> of a single phase transformer. If there were any phase difference, then
the
> two sides wouldn't add up to 240 volts. It's interesting that the NEC
> addresses neutral loading in industrial and residential installations with
> load tables. But in the RV section of the code(Art. 551), I couldn't find
> any reference other than "load distribution shall ensure a reasonable
> current balance between phases" when using a 50 amp panel. I'm looking at
a
> 96 Edition, but I doubt it's changed much since then.
>
> In our installation(with the 50 amp plug), you would need three of the 20
> amp branch circuits running near maximum to overload the neutral
conductor.
> Not very likely IMO. Of course, travelling with my wife and two
daughters,
> I may get to find out. They always seem able to find interesting ways to
> load the system.
>
> Patrick
>
> > -----Original Message-----
> > From: EMERYSTORA [mailto:EMERYSTORA]
> > Sent: Tuesday, February 01, 2000 10:43 AM
> > To: gmcmotorhome
> > Subject: Re: GMC: 2 Roof Airs
> >
> >
> >

> >
> > >
> > same, there will be no current flow through the neutral. If
> > the two are in
> >
> > phase, the entire load will return through the neutral, which
> > means trouble
> >
> > if you are trying to use it anywhere near the 9600 volt-amp capacity.
> >
> > >>
> >
> > Wouldn't it be very unlikely that the load would be the same
> > on each of the
> > two legs? Also, I believe that running the air conditioner
> > which would put
> > inductance and capacitance into the equation would change the
> > phase somewhat.
> >
> > Its been 37 years since I've had any EE courses and I've
> > never practiced much
> > other than rewiring a couple of older homes that I've owned
> > over the years.
> >
> > Thanks for your info.
> >
> > Emery Stora
> > 77 Kingsley
> > Santa Fe, NM
> >

 

[darren paget]

I am a little lost on this. AFAIK nothing on the GM motor home runs off 240. All
power required is 120. Just curious. Darren

> Au contraire, mon ami; the two 120 volt leads _have_ to be out of phase for
> 240 volts to be available. 180 degrees out of phase, in fact.
>
> You are correct in pointing out that the 120/240 is supplied by center
> tapping a 240 volt transformer, with the center tap being labeled neutral
> and grounded. With that mental picture in mind, observe that the two 120
> volt leads, which are at opposite ends of the transformer winding, are 180
> degrees out of phase with each other. When the one on top is positive, the
> one at the bottom is negative and vice versa. This gives 240 volts.
>
> Travis
>
> ----- Original Message -----
> From:
> To:
> Sent: Tuesday, February 01, 2000 10:33 AM
> Subject: RE: GMC: 2 Roof Airs
>
> > I didn't want to jump in late because I haven't been closely following
> this
> > thread, but I can't hold back any longer. AFAIK, there is no phase
> > difference between the two 120 volt legs of a split 240 volt line. The
> two
> > 120 volts legs are the result of a center tap on the 240 volt secondary
> side
> > of a single phase transformer. If there were any phase difference, then
> the
> > two sides wouldn't add up to 240 volts. It's interesting that the NEC
> > addresses neutral loading in industrial and residential installations with
> > load tables. But in the RV section of the code(Art. 551), I couldn't find
> > any reference other than "load distribution shall ensure a reasonable
> > current balance between phases" when using a 50 amp panel. I'm looking at
> a
> > 96 Edition, but I doubt it's changed much since then.
> >
> > In our installation(with the 50 amp plug), you would need three of the 20
> > amp branch circuits running near maximum to overload the neutral
> conductor.
> > Not very likely IMO. Of course, travelling with my wife and two
> daughters,
> > I may get to find out. They always seem able to find interesting ways to
> > load the system.
> >
> > Patrick
> >
> > > -----Original Message-----
> > > From: EMERYSTORA [mailto:EMERYSTORA]
> > > Sent: Tuesday, February 01, 2000 10:43 AM
> > > To: gmcmotorhome
> > > Subject: Re: GMC: 2 Roof Airs
> > >
> > >
> > >

> > >
> > > > >
> > > same, there will be no current flow through the neutral. If
> > > the two are in
> > >
> > > phase, the entire load will return through the neutral, which
> > > means trouble
> > >
> > > if you are trying to use it anywhere near the 9600 volt-amp capacity.
> > >
> > > >>
> > >
> > > Wouldn't it be very unlikely that the load would be the same
> > > on each of the
> > > two legs? Also, I believe that running the air conditioner
> > > which would put
> > > inductance and capacitance into the equation would change the
> > > phase somewhat.
> > >
> > > Its been 37 years since I've had any EE courses and I've
> > > never practiced much
> > > other than rewiring a couple of older homes that I've owned
> > > over the years.
> > >
> > > Thanks for your info.
> > >
> > > Emery Stora
> > > 77 Kingsley
> > > Santa Fe, NM
> > >

- --
Darren Paget
76 Experimental
Another Fab Day
http://www.TZEplus.com

 

[blank]

Travis,

Maybe we're just bashing semantics here, but you're confusing "phase" with
"potential". There is only one waveform present, therefore it's impossible
for the taps to be "out of phase". As for "unloading the neutral" that's a
totally new concept to me - I'll look into it.

Patrick

> -----Original Message-----
> From: Travis Martin [mailto:travism]
> Sent: Tuesday, February 01, 2000 1:01 PM
> To: gmcmotorhome
> Subject: Re: GMC: 2 Roof Airs
>
>
> Au contraire, mon ami; the two 120 volt leads _have_ to be
> out of phase for
> 240 volts to be available. 180 degrees out of phase, in fact.
>
> You are correct in pointing out that the 120/240 is supplied by center
> tapping a 240 volt transformer, with the center tap being
> labeled neutral
> and grounded. With that mental picture in mind, observe that
> the two 120
> volt leads, which are at opposite ends of the transformer
> winding, are 180
> degrees out of phase with each other. When the one on top is
> positive, the
> one at the bottom is negative and vice versa. This gives 240 volts.
>
> Travis

 

[blank]

Darren,

The early GM finished coaches had a 50 amp plug designed to be plugged into
a split 240 volt circuit supplying 120 volts on each leg. This enters the
coach service panel and goes through a pair of 40 amp breakers. It then
splits into four 20 amp branch circuits(two on each side).

HTH,
Patrick

> -----Original Message-----
> From: Darren Paget [mailtoaget]
> Sent: Tuesday, February 01, 2000 1:13 PM
> To: gmcmotorhome
> Subject: Re: GMC: 2 Roof Airs
>
>
> I am a little lost on this. AFAIK nothing on the GM motor
> home runs off 240. All power required is 120. Just curious.
> Darren

 

[darren paget]

I realize that, but it still is only pertinent when plugged into into 50 amp
power. Nothing in the coach runs off 240. That is both sides of the panel. They
are separate 120 sides. The generator only supplies 120 to each side not 240.
That is why I am wondering. Darren

> Darren,
>
> The early GM finished coaches had a 50 amp plug designed to be plugged into
> a split 240 volt circuit supplying 120 volts on each leg. This enters the
> coach service panel and goes through a pair of 40 amp breakers. It then
> splits into four 20 amp branch circuits(two on each side).
>
> HTH,
> Patrick
>
> > -----Original Message-----
> > From: Darren Paget [mailtoaget]
> > Sent: Tuesday, February 01, 2000 1:13 PM
> > To: gmcmotorhome
> > Subject: Re: GMC: 2 Roof Airs
> >
> >
> > I am a little lost on this. AFAIK nothing on the GM motor
> > home runs off 240. All power required is 120. Just curious.
> > Darren

- --
Darren Paget
76 Experimental
Another Fab Day
http://www.TZEplus.com

 

[blank]

They are separate except they share a common neutral connection which is the
center point of the discussion between Travis and myself. I didn't realize
this thread only related to use of the generator.

Patrick

> -----Original Message-----
> From: Darren Paget [mailtoaget]
> Sent: Tuesday, February 01, 2000 2:20 PM
> To: gmcmotorhome
> Subject: Re: GMC: 2 Roof Airs
>
>
> I realize that, but it still is only pertinent when plugged
> into into 50 amp
> power. Nothing in the coach runs off 240. That is both sides
> of the panel. They
> are separate 120 sides. The generator only supplies 120 to
> each side not 240.
> That is why I am wondering. Darren
>

>
> > Darren,
> >
> > The early GM finished coaches had a 50 amp plug designed to
> be plugged into
> > a split 240 volt circuit supplying 120 volts on each leg.
> This enters the
> > coach service panel and goes through a pair of 40 amp
> breakers. It then
> > splits into four 20 amp branch circuits(two on each side).
> >
> > HTH,
> > Patrick
> >
> > > -----Original Message-----
> > > From: Darren Paget [mailtoaget]
> > > Sent: Tuesday, February 01, 2000 1:13 PM
> > > To: gmcmotorhome
> > > Subject: Re: GMC: 2 Roof Airs
> > >
> > >
> > > I am a little lost on this. AFAIK nothing on the GM motor
> > > home runs off 240. All power required is 120. Just curious.
> > > Darren
>
> --
> Darren Paget
> 76 Experimental
> Another Fab Day
> http://www.TZEplus.com
>
>

 

[dave1]

>Kara
>If the AC that is noisy and shakes on start up is the original, check to=
see
>if the com pressor tie down bolts were ever loosened. Some got out that
>weren't.
>Justin

How do you get the shroud off of the unit? I have looked at mine and cant=20
figure out how its attached. There do not seem to be any bolts or screws=20
visible.

Dave
Ann Arbor, MI.
73 Sequoia (26'/455/EFI/HEI/160=B0)

 

[travism]

> There is a question about the power produced by the generator, whether it
> is as above with two single phase 120 volt circuits 180 out of phase or
> one just a single line. A generator could certainly produce either one
> but according to the book the onan is single phase two wire.
> However it is wired with three wires, two hot one neutral like a 240 volt
> system !. So to be sure you would have to test it. I haven't so I guess
> I don't know. (betting on onlyt 120 though)

Yep, the generator is factory wired so that the two 120 volt windings are in
phase, thus no 240 is available. Several people have modified them by
reversing one winding though so that 240 is available, although I don't
recall hearing anyone say why. Perhaps they were using the GMC's generator
to power a saw or something.

Travis

 

[patrick flowers]

>
> Potential is what has phase, potential is measured between two points
> (one might be "ground") the phase of the taps we are talking about here
> is the phase of the potential of each tap to the neutral. This phase
> is 180 out. This is elementary EE.

I guess it was too elementary for Georgia Tech in the 70's.

BTW, reset your computer's date. It's not 1969 anymore.

Patrick
- --
Patrick Flowers
Mailtoatrick

The GMC Motorhome Page
http://www.gmcmotorhome.com

 

[patrick flowers]

>
> BTW, reset your computer's date. It's not 1969 anymore.

Now this is weird. The email shows a date of 1 Feb O and Netscape
displays it as 12/31/69. My first Y2K experience.

Patrick
- --
Patrick Flowers
Mailtoatrick

The GMC Motorhome Page
http://www.gmcmotorhome.com

 

[gene]

hip hip Horrrrray

Good stroke.

gene

>In a message dated 02/01/2000 9:07:28 PM Central Standard Time,

>
>> Now this is weird. The email shows a date of 1 Feb O and Netscape
>> displays it as 12/31/69. My first Y2K experience.
>>
>Hi all
>
>Well, I am back on my desktop rig. Got it back tonight. No Y2K problem
>it turns out. Just a high resistance conductor in the ribbon wire that hooks
>everything inside the computer. I have all my drives back and all data is
>intact. Best of all it only cost me $22.50.
>
>Take Care
>Arch
>
Genef -- 77PB/ore/ca
GMC MOTORHOME INFORMATION
mr.erf
http://www.california.com/~eagle/