help needed with 120/220 wiring

[Larry B]

I have not post in a long time, I do follow along as others work through their problems and issues. I do need some guidance with this issue. I am a steamfitter by trade so plumbing, gas and welding are easy compared to this AC WIRING. I have made a schetch of my next attempt at wiring. Will this work as per schetch? Plug A or plug B will be used to power up my elec panel fuse box at plugC. I will physically plug in the coresponding plug to whatever power supply I am using. I know on shore power I will be limited to 30 amp total(only one AC unit). If I want more than one ACunit I need to start the gen. After putting this on paper I can see that when connected to shore power I am grounded to earth by the shore plug in. When using the gen how does the ground get back to earth?(the stuff we walk on) The bus is on rubber, when I step out the door of the bus could I serve as possible grounding contact? Is there a better way to wire this?
     While I have the attention of the electric people am I correct in assuming that if I add a 12volt alternator to my bus engine to keep my house battery charged I would be able to run a roof top AC unit while driving? using my inverter with plugA connected to plug C. I have eliminated the original bus AC. If so has anyone had experience with Unipoint alternators? good or bad reviews. A new 180 amp alt. is about $300.00 I could live with that price to get front roof AC working while driving.It gets very warm in front of them big windshields when driving into the sun even in Canada.  I hope I have not confused others as much as this stuff mixes my up. Thanks in advance. 

[pvcces]

Larry, whenever you step out of any vehicle, you run the risk of providing a ground path. It's important if you get a lightning strike.

When you are on generator, your bus probably won't be grounded. But, you can drive a stake in the ground, if you like. If the ground is wet enough to provide a path, then your bus would be grounded.

If there was a lot of atmospheric static, the ground would probably take it. But, when you step out of the bus just when there is a lot of static in the area, you would be baiting the charge to hit your bus. Why would you want to do that?

I would vote for no ground when not connected to shore power.

For what it's worth.

Tom Caffrey

[bevans6]

At first glance I don't anything wrong with your schematic.  You are using two paths for power to a split-phase 230 volt AC distribution panel but you aren't planning for any 230 volt loads so you are supplying the 120 volt from the pedestal plug and the inverter to both sides of the AC distribution panel, which is fine.  You are supplying split-phase 230 volts from the generator, which is fine.  You are not bonding neutral to ground except at the source, which is fine.  That Honda may or may not have neutral bonded to ground, generators vary on that, so you should make sure the generator connection bonds neutral to ground.  You are using the bus frame for ground, which is fine, and you will bond ground to the pedestal when you plug in to outside power, which is fine.  I agree with the comments above on grounding while not connected to an outside power source.  I would personally use an automatic transfer switch instead of plugs at the final connection to the AC distribution panel.

Your MCI may have the big Delco DN50 alternator running at 24 volts.  If it does I strongly recommend going to a 24 volt inverter and using the DN50 to power it on the road and run your rooftop AC unit.  That's what I do.  Your 2800 watt 12V inverter is a little marginal for a 15Kbtu air conditioner, and it will draw around 225 - 250 amps at 13 volts while running, which is a honking big 12 volt alternator - most alternators aren't rated for continuous output at their peak rated power.  Going to the 24 volt system cuts the amperage in half, and really reduces the size of the wiring required.

Brian

[Cary and Don]

Another consideration would be how you wire your shore power to the panel.  The way you have run it,  all your circuits will invert.  This can be a real disaster if you forget and leave something on when switching to invert.  A big load will draw the batteries down to nothing real fast, big enough and the inverter will fault out.  You will also technically,  be running all the air conditioners through the inverter even though they are on a pass through.  I would take and split the shore power hot, one to the inverter, one to the noninverted loads, seperated loads in the panel. You won't ever forget and leave something on when the inverter starts.  The generator will still work the same.

Don and Cary

[Larry B]

Thanks for all the replies.  Don -- we are seldom connected to shore power, so splitting my panel might be a disadvantage. When I want 120 volt it has to come from inverter or generator. My fridge runs on propane all the time. the microwave is the big use item and only a few minutes at a time. We spend a lot of time at rodeos watching the cowboy play or by a lake waiting for fish to bite. As Brian point out my inverter is small and seldom used, so if I want AC I start gen. Brian-- I do have the large Delco alt on my bus. How large an inverter would you recommend? I do not have original 24 volt Ac or heating fans.  Does it need to be pure sine wave?  The battery supplier I stopped at today while picking up parts for boat said I should have a 4000 watt continuous/ 8000 surge. Is that a little oversize? I would not need one with charge feature. When I put my unit together I purposely installed one Ac unit way up at front for driving on road. I have three Ac units on a 35ft.

[bevans6]

I would say 3000 watts is fine, pure sine is preferred since your loads are motor and microwave, and they are both sensitive to waveform for long term use.  The 2800 watt unit you have will probably run the AC but won't be as reliable, and the big issue is the DC current being so high and not being able to tap into the stock bus cable at the start batteries and run it from the alternator.

There is a good deal going on this unit:  http://www.thehousewaresstore.com/sw-magnmsh4024m.html

Brian

[Oonrahnjay]

...   There is a good deal going on this unit:  http://www.thehousewaresstore.com/sw-magnmsh4024m.html

Brian 

      There are nice specs on that unit.  The description says "Hybrid Series Inverter/Charger, MFG# MSH4024M, 4000 Watt-120VAC-60Hz inverter, 24 Volt-110 Amp charger, with 60A pass-through. Power loads with both AC Power (shore or AC generator) and batteries to run larger loads from smaller generators."

      If that "power loads ..." part is what I think it means, it's very good.  I have an Outback inverter; it works well in many ways but the way it handles big loads isn't very good.  For instance, if I only have a 15A plug and I'm not sure that it's a very good source* and I set my "Max AC Input" at 12Amps, and then someone switches something (microwave, hair dryer, etc.) on and pulls more than 12A, the unit switches off the AC Input and pulls *ALL* the power from the batteries.  I think that the idea is that once the big load gets switched off, then it will recharge the batteries but it will pull your batteries down really fast.

      If this is truly a "power sharing" inverter (i.e. in the above example, it would take the 12A from the shore cord or generator, and only 3A/360Watt from the batteries to make a total of 15A for a 15A load), then this does appear to be a very good unit.  If you're considering buying this unit, I'd get more info on this feature.

      I will mention that the Outback, despite its shortcomings in the power sharing area, does have a very complete and flexible control system and I have to say that I like how it's set up and it is easy to use.  I don't have any experience with a Magnum inverter so maybe people who do have that experience can comment on the Magnum unit specifically. 

HTH,   BH   NC   USA 

*(An additional issue is whether you have access to the breaker for your AC source.  If you only have access to one socket and it appears to be a fairly limited one - or if other people are pulling power from the same circuit - and you have access to the breaker, it's not a big deal.  If you accidentally pull more power than the socket or circuit can handle and the breaker pops, you can "shed some load", reset the breaker and you're good.  However, if you don't have a way to get to the breaker - if it's inside a locked building for example - you may need to set your "Max AC Input" lower to have a better margin for not tripping that breaker, especially if you have to depend on that AC plug in for your power overnight, for example.  A true 'load sharing' - rather than a 'load switching' - inverter may be very helpful in this situation.)