Feedback Requested on Electrical Concept

[Sam 4106]

Hi Mike,

I too applaud your research. For me, part of the enjoyment I get form working on the bus is the learning experience.

Yesterday I offered some bad advice in my post. I just learned that the Magnum MS 4024 has a 30 amp transfer switch in it so option 1 is not viable. If you used that option you would not be able to utilize the benefit of 50 amp shore power. My suggestion now would be option 2.

Another thought for powering your 2nd A/C would be to feed it from an outlet on the panel after the transfer switch and when on 30 amp shore power unplug the A/C from the outlet and plug it into a 20 amp cord from the campground pedestal. That would only be feasible if you are putting your panels in a bay. Do you know that there are adaptors for plugging your 50 amp shore cord into a 30 amp outlet? No need for a 30 amp shore cord too. There are also adaptors to go from 30 amp to 20 amp.

Do you know that with 50 amp 120/240 shore power that you have 50 amps on each leg for a total of 100 amps at 120 volts? So more than 3 times what you have from 30 amps. Again, sorry if this is being redundant, just trying to help.

Sorry for my poor advise, Sam 

[Midwilshire]

Another thought for powering your 2nd A/C would be to feed it from an outlet on the panel after the transfer switch and when on 30 amp shore power unplug the A/C from the outlet and plug it into a 20 amp cord from the campground pedestal.

Do you know that with 50 amp 120/240 shore power that you have 50 amps on each leg for a total of 100 amps at 120 volts? So more than 3 times what you have from 30 amps. Again, sorry if this is being redundant, just trying to help.

Thanks for being redundant; I need it and it does help!

Right now, the following is giving me insomnia:

1.  The neutral and ground has to be bonded, unless connected to shore power, in which case they must be unbonded.  Okay, how do I do that?  Sounds like a big NC relay would be in order.  Is it that simple?

2.  I can use [30amp/120v] shore power just like I'd use my generator.  Great, that's what I was expecting.  Or, I can use the 240 service, which gives me [50amp/120v]+[50amp/120v], which sounds fabulous, except they must be "load balanced."  Now I'm confused, since, they'll never be completely in balance, and worse, if I run the inverter on one leg and an A/C on the other leg, I'll blow the beaker as soon as I turn off that A/C.  What's the solution -- shut everything down, switch to 30amp, and start up again?

Is there a magic box into which I can plug the 240 service and have it spit out [100amp/120v] so I need not worry about load balancing?   Would this even be desirable?

I've read Sean's Odyssey website numerous times.  I would like to fabricate an automatic transfer thingamajig like his, but I just don't get it (yet).
 

[Oonrahnjay]

(snip)

1.  The neutral and ground has to be bonded, unless connected to shore power, in which case they must be unbonded.  Okay, how do I do that?  Sounds like a big NC relay would be in order.  Is it that simple?

2.  I can use [30amp/120v] shore power just like I'd use my generator.  Great, that's what I was expecting.  Or, I can use the 240 service, which gives me [50amp/120v]+[50amp/120v], which sounds fabulous, except they must be "load balanced."  Now I'm confused, since, they'll never be completely in balance, and worse, if I run the inverter on one leg and an A/C on the other leg, I'll blow the beaker as soon as I turn off that A/C.  What's the solution -- shut everything down, switch to 30amp, and start up again?  

    Just a couple of quick remarks.  I'm not really good on electrical stuff so FWIW ...

1)  If I understand what you're saying,  the neutral and ground must be unbonded when on shore power and connected to the neutral and ground circuits, respectively, of the shore power.  Due to the circuits and timing necessary a simple relay won't do it, but there are "automatic" transfer switches available commercially which do this.  How you select the right model and design for your needs, I have no clue.  Sean can probably suggest an appropriate type for your needs; it may be that he built one "because he can".

2)  "Load balancing" is an approximate thing.  If you're pulling 35 amps off "one leg" and 37 amps off the other leg, it's not a big deal.  If you're pulling 50 amps off one leg and 0 off the other, you're way out of balance and could be running into trouble.  And since you'll be constantly switching appliances and other electrical loads on and off (or they may switch themselves on and off automatically if they're on controls like thermostats, etc.), your load balancing won't be exact but if it's close, you're OK.  As I understand it, a main problem with load imbalance is that the "return power" that's not balanced goes back down the neutral line into and through the power pole circuit in the campground (or other supply circuit).  In the example above, with 35 and 37 amp loads, you're only putting 2 amps down the neutral (this is negligible); the 50/0 amp loads would put the whole 50 amps down the neutral.

   I hope that these little items help clarify details (and that I'm right on them).

[thomasinnv]

Most transfer switches unless you build your own will give priority to the generator by default.  Why would you run the generator if you have shore power? 

many people will run the generator periodically as part of a maintenance schedule whether plugged in or not. In order for this to be effective the generator must have a load applied. With the transfer switch wired as suggested all you have to do is push the start switch and it will take care of itself. Otherwise, you have to manually switch something or plug/unplug somewhere in order to place the load on the generator.  If it ain't broke......

[belfert]

Most transfer switches unless you build your own will give priority to the generator by default.  Why would you run the generator if you have shore power? 

many people will run the generator periodically as part of a maintenance schedule whether plugged in or not. In order for this to be effective the generator must have a load applied. With the transfer switch wired as suggested all you have to do is push the start switch and it will take care of itself. Otherwise, you have to manually switch something or plug/unplug somewhere in order to place the load on the generator.  If it ain't broke......

I thought about running the generator periodically to keep it lubed and such as a reason to run it while one has shore power, but it doesn't explain why the OP wants shore power as the priority in a transfer switch.

[Midwilshire]

I thought about running the generator periodically to keep it lubed and such as a reason to run it while one has shore power, but it doesn't explain why the OP wants shore power as the priority in a transfer switch.

I must have used the word 'priority' incorrectly in this context.  What I mean to say is:  I only want to use the generator when shore power is not available or is otherwise inadequate.

-Mike

[belfert]

I must have used the word 'priority' incorrectly in this context.  What I mean to say is:  I only want to use the generator when shore power is not available or is otherwise inadequate.

Most RV transfer switches have shore power as the default and will switch to the generator when they detect power on that input.  Most don't actually switch over for one to two minutes to allow the voltage to stabilize.  They will switch to generator even if there is shore power present.

[Sam 4106]

Hi Mike,

You are right that neutral and ground must be bonded at each source of power and you must keep the neutrals and grounds separate in the bus. Typically the neutrals go on the insulated bus bar in the panel and the grounds go on a terminal strip that is screwed to the metal panel enclosure. The N/G (neutral ground) bond is provided through the shore cord when plugged in, the generator when that is used, or the inverter. The shore/generator transfer switch switches the neutrals along with the hots to keep the neutral and ground separate with those sources of power. The Magnum MS 4024 inverter/charger also switches off the neutral/ground bond when there is power to the inverter/charger and makes the neutral/ground bond when the inverter is suppling the power. So if everything is wired correctly the neutral/ground bond is handled automatically. However, you can't use the generator and shore power at the same time through the transfer switch since the contacters are interlocked electrically or mechanically or sometimes both. That is why I suggested putting an outlet on the panel for the 2nd A/C and using a 20 amp shore cord to power it when on 30 amp shore power.

As Bruce said load balancing doesn't have to be exact. It is more important with a 120/240 VAC generator but I would guess that your generator will be 120 VAC only. There is no magic combiner to create 100 amps of 120 VAC from 50 amps of 240 VAC. If you try that you will be making sparks and smoke.

[thomasinnv]

As Bruce said load balancing doesn't have to be exact. It is more important with a 120/240 VAC generator but I would guess that your generator will be 120 VAC only. There is no magic combiner to create 100 amps of 120 VAC from 50 amps of 240 VAC. If you try that you will be making sparks and smoke.

if you are referring to shore power, you are correct. But, if you are referring to generator power, many of the 240v generators can be reconfigured to do exactly that. No need for magic.

[Uglydog56]

Here are two other sites that have some good info when you are designing an rv electrical system:

http://handybobsolar.wordpress.com/the-rv-battery-charging-puzzle-2/

http://www.jackdanmayer.com/

[Midwilshire]

Here are two other sites that have some good info when you are designing an rv electrical system:

http://handybobsolar.wordpress.com/the-rv-battery-charging-puzzle-2/

http://www.jackdanmayer.com/

Thanks for that!  If you happen to come across any schematics for RV installations, I'd appreciate that as well. 

[belfert]

As Bruce said load balancing doesn't have to be exact. It is more important with a 120/240 VAC generator but I would guess that your generator will be 120 VAC only. There is no magic combiner to create 100 amps of 120 VAC from 50 amps of 240 VAC. If you try that you will be making sparks and smoke.

if you are referring to shore power, you are correct. But, if you are referring to generator power, many of the 240v generators can be reconfigured to do exactly that. No need for magic.

If one were to connect both connections from a 50 amp circuit together inside the RV's subpanel would that cause issues?  Would this be considered a short and trip the breaker in the pedestal?

[thomasinnv]

As Bruce said load balancing doesn't have to be exact. It is more important with a 120/240 VAC generator but I would guess that your generator will be 120 VAC only. There is no magic combiner to create 100 amps of 120 VAC from 50 amps of 240 VAC. If you try that you will be making sparks and smoke.

if you are referring to shore power, you are correct. But, if you are referring to generator power, many of the 240v generators can be reconfigured to do exactly that. No need for magic.

If one were to connect both connections from a 50 amp circuit together inside the RV's subpanel would that cause issues?  Would this be considered a short and trip the breaker in the pedestal?

you definately don't want to do that. If you are plugged into a 50/240 shore power you would have problems. Generators that are configured in this fashion (combining both legs of 120 for a single higher amp output of 120) are usually configured at the generator head, not the panel.

My generator is a little different. The head is not re-configurable and to get the maximum output on a single 120 output, both legs are simply "jumpered" together at the transfer switch on the generator side. Most generators you don't want to do this, it will cause problems.

[Midwilshire]

Could anyone answer a very basic question for me? 

Can I use both the 30 amp and the 50 amp shore power at the same time?  

What I mean is, if I were running a 50 amp coach, but brought along some other stand-alone device, like an air compressor, for example, could I plug this other device in at the pedestal on the 30 or 20 amp circuits if I felt like it, while the rest of the coach is humming along on the 50 amp? 

Thanks....

[thomasinnv]

Basic answer...yes.