Hello, I have been trying research this on the web but have been having a hard time finding direct answers to my questions.

We have a 12ft trailer that has had all the wiring pulled for a remodel however we left the breaker box. I would like to add 2 6v batteries to the trailer to run the few lights, outlets and an electric burner for cooking.

So my question is, how do you wire the battery up to the breaker box and still leave a plug in for when there is shore power?

What I do know is that we will need and inverter from the battery to make it A/C but the invertors that I see on-line all have a cigarette lighter hook up. Is there one that can be connected to the battery cables? Then how do I wire the inverter to the breaker box? Or is there a special inverter for just this need?

Lastly, how will the batteries re-charge, if I am hooked up to shore power will they re-charge?

Thank you


Tags: a/c, battery, breaker, hook, inverter, power, shore, trailer, up, wire, More…wiring

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Hi Richard: I am wondering if you have the gas cooktop range still from your conversion? I am looking for one as I am going to gas form electric for cooking. thanks, 

There is a great illustration and other info at rvtechlibrary.com that may help.

Tim,I use a double pulley on my high output Ford alternator with matched belts.Also run 2/0 gauge wire for the battery connections.

I love these conversations. To explain my setup for the benefit of the original poster and those interested...

My Cortez is running all 3 systems if you will, LP, 12v DC and 110v AC.

LP runs a catalytic heater (Wave 6) that keeps us nice and toasty with the open arrangement that is the Cortez interior (and only 20ft). I have another Wave 3 I was thinking about adding to the restroom in the rear, but not needed yet. I still have the Gasoline furnace in place, but it's not operational at the moment. Also on LP is the 3 burner stove/oven and the 2 way Fridge (LP/AC). Both the Stovetop and the fridge have electric BBQ type electronic push-button ignition, powered by a single AA battery (per unit).

DC - All lights, 2 outlets, stereo, 2 fans, and water pump run on 12v DC from a 115 a/h deep cycle for the house, and a 85 a/h Marine starting in the coach. I would like to add 2 more 115 a/h to the rear, I have room for them as I have two 3-battery trays one in front and one in the rear. Connected and charged by a 100amp alternator with a high capacity disconnect triggered by the oil pressure switch (only connects front and rear for charging while the engine is running, similar to the disconnect in a 7-round trailer plug) The rear batteries run my dash stereo, and power a 300w inverter that runs a laptop, subwoofer/speakers and a LCD monitor for watching movies on the road. This keeps the stereo from ever discharging my starting battery when dry camping as well. I always have the coach battery at full charge, as it's disconnected from the house (completely) at rest. 

AC - There is a 30 AMP shore power to a 30amp AC service panel. This powers the Fridge (and the stereo/TV when plugged into a separate outlet beside the inverter, unplug and plug into shore outlet. It runs two banks of outlets, the water heater (LP heater exploded from freezing, replaced by 2.5 gallon under sink), the rooftop AC/Heater and the 20 amp DC Converter/Charger. All lights run on DC, so the Converter takes over supplying the 12v and tops off the rear battery bank while plugged into shore power.

My only gripe with this system, without a genset, I am unable to heat water or run the rooftop AC without shorepower. Due to limited space, I have little options to add one built in. I would need to relocate the 20lb LP tank to another locations or install a fixed unit from a vanagon or conversion van underneath the unit, but I like the portability my system allows me in case I need to refill without moving the full unit. Or, I could run the "rear" battery bank in the physical front bank location (still separate circuits) to free up some space in the rear for the Genset, or a combo of both changes to give me a lot of room. 

Then, I could run the genset while driving or camping, to run the Roof AC and keep the water warm. Right now, being as insulated as the water heater is, and typically camping with hookups, I always have hot water, even stopping for lunch along the way. :)


You have two separate electrical systems. You don,t mix the two. Your 115 volt shore power is one and needs a breaker box for safety. The 12 volt or low voltage system needs a small automotive fuse box with perhaps two or three circuits. The wire is automotive for 12 volt. In my camper i use a good 12 volt battery that hooks up through the connection to my truck and charges the batter same as the vehicle battery when i am driving. When at a site with shore power i keep a small electric battery charger which i hook up to top off the charge in the back up if necessary. If you wish to use an inverter you will find that the battery alone will not supply you significant AC Power to run much more than electronic chargers for phones and computers unless you pull vehicle alternator is supplying power while the engine is running. If you have 115 volt power in your camper you will find a limited need for converted power or least thats my experience. A friendly RV repair center can give you more detailed technical advice. Good luck and safe travels

What you need in as inverter/charger which automatically transfers from battery to shore power (when available) and recharges the batteries. The output of the inverter is connected to the breaker box, incoming power goes to the inverter AC input and the inverter is connected (with cables) to the battery.  The inverter is sized to match the biggest load (microwave - AC). Google Magnum, Xantrex, Outback for a selection.

Yes, you must have a converter to make this work. You can buy them at many sources, but Vintage Trailer Supply is your best bet. Check their online catalog for the one you need. The converter will be hardwired to your breaker box when done. There is a dedicated line that gies out to a battery(ies). The converter recgarges your battery while plugged in to shore power. Most old cmpees came with 15 amp service since people didn't haul around the junk we "need" today, and many things we must have with us didn't exist "in the day". Since you are doing major restoration, you should upgrade to 30 amp at a minimum...50 amp service would be best. That way you can plug into the 50 amp utility post at campgrounds...you likely would never pull 50 amps, but too much source is never an issue. The issue happens when you try to pull too much power resulting in thriwn circuit breakers and blown fuzes. You can blow your own fuzes or the utility post fuze by making coffee, toasting bread, and microwaving while your wife dries her hair. VTS sells Performance Dynamics converters that combine circuit breaker box, circuit breaker lines, and multiple 12 volt circuits. The dedicated battery line has a "smart" device that monitors and adjusts charging rates as needed. You can buy them for 30, 45, and 50 amp services. Pick the one you want.
I tecommend yo change out an old circuit breaker box, anyhow as some were designed and built dangerously. The one in my derelict Fireball was like that. I googled the company and fiund it was not only out of business, it used proprietary circuit breakers and had a history of causing fires. The government essentialky shut them down after their boxes exploded or erupted into flames. I installed the Performance Dynamics 9230 converter but kept a NEW circuit breaker box rather an integrated converter. Also a 12 volt fuze block fir up to about nine, 12 volt circuits. Too easy! Longest part was nstalling a new, 30 amp power cord running outside the camper! As you rewire, dump the 14/2 gauge wires you will find. Old campers typically have 14/2 gauge wire which is fine fir lighting, but not approvee by the NEC and ASWG for appliances. Wheras my derelict had a single, 15 amp sustem, it now has four separate circuits with on dedicated to the new Dometic low profile AC. Th battery wizard s a pendant that monitors and charges your battery. VTS sells a book on 12 volt systems but which gies into excruciating detail about 110 and 220 volt systems and even hiw to connect solar panels in case you go boondocking. Just please, please determine and use the citrect wire gauge so as to not hurt yourself or burn down your camper and a campground! There are still some nutjobs on these websites who tell you 14/2 gauge is fine fir all your circuitry; and that 12 gauge is fine to run power into your camper and then to your battery. The size of your coach; number and current draw appliances; size, shape, and number of wires going into your jnction box; and other factors drive wire gauge size! Se the 12 volt book from VTS as your "bible" (little b) rather than trusting questionable advice. And get any decent DIY book on wiring (Lowes, HD, Books a Million, Walmart, etc) to verify info on wires, how to crimp lines, how to connect outlets, etc. Never trust your memory unless you are a true professional and not even then! You will find most electricians don't understand wiring in campers since they don't follow the same pricesses as in a house. Too many cases of a licensed electrician wiring campers wrong...one of the best electricians in my area did that to me because a camper 30 amp plug looks similar to an AC plug, but isn't the same. Goid luck and best wishes,

Maximum ampacity for #12 copper is 20. If you have a 30 amp AC (as in alternating current) shore power connection, the conductors from main plug to breaker box must be at least #10. If you have a 50 amp plug, you must use #6! (The maximum ampacity of THHN conductors is actually dependent on maximum ambient temperatures so check your NEC before installing). # 6 is very large cross section and it's uncommon to find flexible SO cord of that size at the local hardware store. A 30 amp service should be more than adequate for most vintage RV applications, considering the number and type of circuits they were built with. Just use common sense when powering-up that air conditioner and electric hot plate at the same time and you'll be OK.

Agree mostly. A vintage camper restored and ugraded for conveniences we consider "required" today, however, go against your statement. Modern campers come with either 30 amp for lower to moderate expense and size. Larger campers with modernized kitchens, entertainment centers, electric faux fireplaces, and such will come with 50 amp service. Yes, you can get along with a 30 amp service most of the time these days. But it is easy to forget the 30 amp power budget when going about daily functions. My wife causes either camper or campground utility post circuit breakers: infrequently, since she is very intelligent and an exoerienced camoer, but it happens. No one counts amps before flipping a switch. You don't have to...the disappearance of power is proof "you done messed up". And as for 6 and larger gauge wire not being immediately available....that is true only if you aren't near "big box" building supply stires (Lowes, HD, Ace, etc, etc). Where do you think home builders get their building supplies? At those and other building supply centers. Don't forget: there is nothing mystical and magical about camper building supplies. Six gauge wire fir a house is the same six gauge wire used in a camper. But yes, it is harder to find larger gauge stranded wire than solid copper conductors. The camper manufacturers of modern campers with 50 amp services have to follow NEC, ASWG, and RVIA standards in order to sell their products. As for ACs being 30 amp so that 12 gauge can't power them: you must be talking about some AC type not installed nofmally in a camper. My new AC for our Fireball is a Dometic made for campers....it calls for twelve gauge wire. Yes, it is fine to jump up to a larger than minimum gauge wire IAW NEC and ASWG!

  WOW, I see this topic is being re visited. I posted long ago and maybe its time for an update and to answer the posters actual questions. He talks bout a 12 FOOT TRAILER NOTTTTTTTTTTT SOME HUGE ENERGY HOG DUAL AC MOTORHOME.

"We have a 12ft trailer............ I would like to add 2 6v batteries to the trailer to run the few lights, outlets and an electric burner for cooking."


 ANSWER: For only a 12 foot trailer and you mentioned 2 6 volt batteries, that should suffice well. I recommend they be true deep cycle golf cart batteries connected, of course, in series which would provide around 220 Amp Hours of 12 VDC energy storage.

 "So my question is, how do you wire the battery up to the breaker box and still leave a plug in for when there is shore power?"

 Typically there would be a 12 VDC distribution panel with a few automotive blade type fuses in the 10 to 15 amp range for a few circuits to vent fans or 12 VDC lights or water pump etc etc. Often there is a COMBINATION 120 VAC (with circuit breakers) and 12 VDC (with blade auto fuses) often with a Converter/Charger built in which provides DC to charge the battery when plugged into shore power. The DC feed is wired from the DC distribution panel to the battery bank and the incoming AC feed serves the AC distribution panel to prove power cvia circuit breakers to AC and microwave and fridge and convenience receptacles and the 120 VAC to 13 + - VDC Converter/Charger

 "What I do know is that we will need and inverter from the battery to make it A/C but the invertors that I see on-line all have a cigarette lighter hook up."

 Those cigarette lighter fed inverters are for low power (say 200 to 300 watt range) electronics and chargers and computers etc. Id use at least a 400 watt PURE SINE WAVE  (PSW) Inverter hard wired and fuse protected to the battery bank output. That will run a TV and computer and chargers etc etc small electronics but NOT any huge loads.  

  "Is there one that can be connected to the battery cables?

 YES see above hard wire a say 400 Watt PSW Inverter

  "Then how do I wire the inverter to the breaker box? Or is there a special inverter for just this need?"

  You could wire it to a 12 VDC fuse in your 12 VDC panel IFFFFFFFFFFFFFFFF it can handle a big enough fuse orrrrrrrrr wire it direct to the battery bank output with its own fuse and NOT feed it via the 12 VDC panel, that's how I did mine.

 IT ALL DEPENDS ON THE SIZE OF THE INVERTER YOU CHOOSE. a Cigarette lighter type is smaller maybe 200 to 300 watts max ( and often MSW not PSW)  while a 400 watt (I prefer a PSW) is often wired to the batteries direct.  

  "Lastly, how will the batteries re-charge, if I am hooked up to shore power will they re-charge?"

 They recharge when on shore power via a  cheaper often built into a 12 VDV 120 VAC Combination panel Converter/Charger which is fed by a 120 VAC panel circuit breaker. HOWEVER to increase battery life I prefer a SMART 3 or 4 stage charger then the cheaper more like a constant 13.4 to 13.6  voltage source DUMB single stage charger.


 I dont see a 12 foot trailer needing a 50 amp service whatsoever. Ive owned and bought and sold RV's and used them over 40 years and if only one AC a 30 amp 120 VAC service suits me fine. My 29 Ft Class C has one AC, Microwave, LP/Electric Fridge. I have four deep cycle golf cart batteries 460 Amp Hours, 400 watts of rooftop solar panels, a 2000 watt and a 400 watt PSW Inverter and an Onan 4 KW Genset.

CAUTION if you install a 120 VAC Panel DO NOTTTTTTTTTTTTT BOND THE NEUTRAL AND GROUND BUSSES  The Neutral must be insulated and isolated off Ground Buss and case/frame.

A combination Inverter/Charger while convenient is expensive and may be overkill for a 12 foot trailer. I chose a Progressive Dynamics 4 stage SMART charger and the PSW Inverters mentioned above. I removed the old hot noisy buzzing  DUMB Converter/Charger

 John T Retired AC Power Distribution Electrical Engineer

Right-on, John. I had a 12' trailer 45 years ago that had 2 propane lamps and 3 12 volt lamps. No AC at all and the fridge was an icebox. Water was supplied by a marine hand pump and the head was a porta-potty. People these days are used to so many creature comforts! ;-) The emphasis in the 50's trailers was on lightness and simplicity, which is an excellent thing in very small camp trailers.

I also have a Progressive Dynamics smart charger and monitor, which is a huge improvement over the original Dumb Charger. 60's - 70's electronics leave much to be desired.

The flexible shore power cord required for boats and RV's alike is waterproof SO cord, not residential stranded conductors. 50 amps is a LOT of current, and equals 6,000 Watts @ 120 VAC. 30 amps is bad enough @ 3,600 Watts. In my occupation as a off-grid power systems designer/installer I have to (or had to) deal with large SO cord a lot and I can assure you that # 6 isn't something commonly available outside of wholesale electrical warehouses. # 8, not so scarce.

By AC I mean Alternating Current, not Air Conditioner. My ancient 10,000 BTU FrigiKing off-road air conditioner requires a 15 amp circuit, so I assume that it runs at around 12 amps on high - but I've never actuall clamped it to find out.




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