Introduction and ???

[hellboyz]

My name is David.  We have bought our first Motor home.  It is 1966 Challenger bus conversion with the Detroit 8V72 and Allison auto tranny which we will use to pull our rock crawler to various off road parks and the beach etc.  Very solid rig for us to upgrade and customize to us.  I'll start with my electrical  ??'s because we could use it as is as long as we have some power, I'm sure to have other ??'s as we start customizing and changing what we don't like on the inside.  The ??'s have probably been asked a hundred time already, but I wanted some answers kinda quick so I can at least get us ready to camp and not wanting to risk their safety due to an electrical fire.  The inside is already wire up and runs into a 50amp service box with the 50 amp male cord running out of it to plug into shore power.  1st ?, I have a 24v inverter to run things while on the road, it is currently not hooked up.  I do not want to wire it straight to the service box, I rather run it to either a switch or female 50 amp receptacle.  This way I do not forget to disconnect inverter and end up feeding shore power into it and frying it.  Inverter has the standard three wire outlet and multiple 110 outlet type connector.  I plan on hard wiring into the inverter.  It being an older rig it doesn't have all the automatic switching or even a manual switch over built in.  2nd question, other than a long 50amp long extension cord what other accessories to do I need to connect to shore power.  Example, some parks have 30amp service and others have 50amp service.  Last time I was there, I noticed the electrical service boxes had either a 30amp or 50amp along with 2 standard electrical outlets.  Stove, oven, heat, water heater all run of propane.  I'm open to all advice and suggestions ya'll have to offer.  Like I said, we are newbies to the motorhome world.  I'm also notice there are a couple different types male plugs, some have 4 blades, some have 3.  Mine has a 4 blade male running to the service box

Thanks ya'll
David

 

[Lou Schneider]

Hi David, Welcome to The RV Forum!

The easiest and safest way to switch between inverter and shore power is to mount a matching receptacle in the compartment where you store the shore power cord and plug the shore cord into the inverter whenever you're not using shore power.  This gives a positive interlock to only having one source connected at a time - with only one plug feeding the rig there's no way to get both shore power and the inverter cross connected.

You'll want to turn off the converter that charges the batteries when you do this, or you'll set up a power wasting loop from the batteries to the inverter, then back into the batteries through the converter.

Most inverters that only have outlets (i.e. not factory designed to be hard wired) put out balanced power, with voltage on both the hot and neutral wires.  The problem is the battery negative post is tied to the RV's chassis, and if the RV's neutral is connected to ground (which is also tied to the chassis) you'll put a direct short on half of the inverter's output.

Get a multimeter and measure the voltage from each of the inverter's output pins to the ground lug on the inverter's frame.  Conventional power has 120 volts from the hot pin to ground and 0 volts from neutral to ground.  If you measure 60 volts from each of the outlet pins to ground the inverter is putting out balanced power and you have to isolate the RV's neutral from ground before you can connect the inverter to the RV.

Standard house breaker panels often have the neutral and ground tied together inside the breaker box.  This is because most house breaker panels are the main panel where neutral is tied to ground.

The RV is considered a sub panel, and according to the electrical code, neutral and ground are kept separate in sub panels.  An RV panel should not have neutral and ground connected, if yours does you'll have to find the interconnection and isolate them.

Measure the resistance between the neutral and ground pins on the power plug.  If there's no continuity you're good, if there is continuity between them you have to find where neutral and ground are tied together and eliminate the cross connect.  Possibilities are the neutral and ground wires sharing a common buss inside the breaker box - if so, the neutrals have to be moved to their own, insulated buss,  or if there are separate neutral and ground buss bars, look for a bonding screw joining them together.

You can go to any RV store or Wal-Mart and get shore power adapters to let you plug your 50 amp shore power cord into a 30 amp socket if that's what is available at your campsite.  Also get a 30 amp to 20 amp adapter to let you further reduce down to a standard 20 amp outlet if needed.  Just be aware that your total draw cannot exceed the rating of the outlet you're plugging into, if you draw too much power you'll blow the source breaker.  But even 20 amps is enough to run your lights and keep your batteries charged.

 

[hellboyz]

Thanks you Lou, the receptacle idea is kinda what I had in mind.  That would make it pretty fool proof.  My 24V inverter have both regular plug in outlets and a 3 wire hard wire panel.  I don't think my bus has a built in battery charger, I gonna have to get one though.  It does have a equalizer that needs to be wired in because the dude that I got it from had mismatched batteries.  I had to replace them on the way home, so now it has nice new matching batteries.  I'm know a little about household ac and the use of 3 wires, but the 4 wire stuff is throwing me off a little bit.  Thanks for the help.  We just got it home Sunday and I have a Army issued back injury that was really agitated by the 600 miles trip home and the replacing the batteries on the way home.  So I haven't really had a chance to start really start exploring every thing, dude jerry rigged some stuff, so I know I'm gonna have to go behind him and redo some things.  After getting out of the Army Intel business, I got a degree in aircraft electrical/mechanical systems and I worked in the field for while until my back injury finally retired me for good.  So I'm can fix or redo any of the things I don't think are done right.  I just wanted to get a idea of what I'm getting into before I get started.  Luckily I have some very good friends that are electricians and diesel mechanics that are willing to help me out with anything I can't figure out.  The are part of the off road crowd I hang out with and we help each other out to keep the cost of maintaining the rigs to a minimum.  Thanks again Lou, I am open to all the info I can get.  I don't like going into things blind.

 

[John From Detroit]

On the shore power cord a 3=blade (2 flat one round or U/D shaped) is a 30 amp cord the power is 120 volts at 30 amps.

on a 50 amp it has 4 prongs, 3 flats and one U, D or Round shaped. This is a 120/240 volt connection.

There are adapters to go either way, The adadaper that you would need to plug your 50 amp rig into a 30 amp outlet only delivers 30 amps of 120 volt however.. Since that's all there is.

As for your inverter.. I'm assuming you plan on hooking this to a 24 volt chassis system.. here is hoping you have the batteries for it.... 

Easiest way is to install a second breaker box, this box is of a type called a "Sub Panel"  Depending on the size of the inverter, You put in a breaker that is perhaps slightly larger (I'm guessing it's a 2000 or 3000 watt inverter, for the breaker divide that by 100 for a start)  Run a lead off this breaker to an outlet like the twist lock on the inverter,  Park it next to the inverter.

Now run a 2nd line from a plug (matching) in the inverter's home, to the sub panel.. You move devices you want to run off the inverter,, (Say TV, Microwave, GFCI chain) to the sub panel.. you can move the breakers too if you get the right sub panel. Just block off where they used to be.

To use inverter, MOVE PLUG

Alternative. You can get a manual switch, 30 amp, double pole for about 50=100 bucks.. You can get cheaper switches that claim to be able to handle that load.... But do not believe the claim.. This one is designed specifically for this job.

Hookup is much the same.

 

[Lou Schneider]

I'm know a little about household ac and the use of 3 wires, but the 4 wire stuff is throwing me off a little bit. 

The 4 wire is simply two hot wires that are out of phase with each other, so that one is going Positive at the instant the other is going Negative.  It's the same as a regular household feed where you get 120 volts from each hot leg to neutral and 240 volts between the two hot legs.

Inside the breaker box, you'll see the two hot lines feed alternate breaker slots.  The first slot is on Line 1, the second is on Line 2, etc.  All breakers will measure 120 volts from the breaker to neutral, but two breakers adjacent to each other will have 240 volts between them.

If you have a 120 volt inverter, wire the 50 amp outlet so that both hot leads on the socket connect to the single hot lead on the inverter.

This will give you 120 volts from each hot leg to neutral, so all of your 120 volt stuff works normally.  But you'll get 0 volts, not 240, from one hot leg to the other because they're both connected to the same point and are in phase, not out of phase, with each other.  If you have any 240 volt stuff (unlikely) it won't operate from the inverter.

The same thing happens when you use an adapter to reduce down from the 50 amp 4 wire plug to a 30 amp or 20 amp 3 wire receptacle.  Both hot leads get 120 volts fed from the same point (the single hot lead on the adapter) so you get voltage to all your 120 volt loads but nothing on 240 volt loads connected from one hot lead to the other.

Or as an alternative, only wire the inverter to one of the hot pins on the 50 amp outlet and leave the other one empty.  This will send 120 volts to one of the two hot buses in the breaker box and leave the other unpowered.  Arrange your breakers so the loads you want to power from the inverter are on the powered buss and the loads you don't want on the inverter are on the other buss.

When you plug into the inverter outlet, only the breakers on the active buss will receive power.  When you're plugged into shore power, both sides will be active.

 

[hellboyz]

Great info.  Thanks you all so much.  It's becoming a little clearer to me how things work. Thanks again ya'll.  Keep the info coming.

David 

 

[hellboyz]

I have decide that I'm going to wire in a 50 amp female receptacle to my inverter.  That way I have to physically unplug it from the inverter receptacle to plug into shore power thus making sure I don't screw something up.  To make sure I have this straight, a 50 amp receptacle has 4 poles, 2 hots out of phase, a neutral and a ground. I run the 1 hot, neutral and ground to the inverter .  Since only one leg of the hot will be hooked up, I need to have the stuff I want to run while going down the road wired into the energized buss.  I need to make sure not to put to much of a load on hot side.  There is only one air conditioner in the rig (which we will need, NC is hot in the summer) and it will only run off ac current.  So I would only want it and some of the receptacles (for tv, dvd player etc for the kids) in the rig to be wired into the hot side being powered by the inverter so I don't put to much of a load on the system.  Is this about right?  Some of the campgrounds have 30amp service so I'm going to need a 30 amp to 50 amp adapter and a 50 ft  50 amp extension cord to make sure I have I'm got everything covered for connect into shore power.  Does the 30 amp plug only have one hot side too?  Does this sound about right?    This just came to mind, could 2 inverters be run to the receptacle in order to have to hots so as to run the whole service panel or does them running out of phase make this a no go?  I really appreciate ya'lls help.

Take care ya'll
David 

 

[Gary RV_Wizard]

You can wire the one hot from the inverter to both the hot terminals on the 50A outlet. That makes all the RV circuits active but of course doesn't change the max power limit, which is determined by what the inverter can deliver. Whether you can run an a/c through it depends on inverter size and the DC power source.

 

[hellboyz]

Thank you.  To run both legs do I just split the hot wire off to both busses.  The previous owner used this inverter and he said he was able to power everything in the bus with ease.    It's a 24V inverter that should have plenty of power while going doing the road.  The inverter will only be use when the bus is running.  The 350 amp alternator should provide plenty of power to everything. .  I only plan on running the ac, tv and maybe a couple of things on the receptacles while on the move.  Next year I will get AC/heater unit that runs off both ac/dc.  This bus was converted in the 90's so it needs some upgrades as money and time allow.  I just want it to get us thru this year.

Take care ya'll
David