Some electrical questions, if I may...

I'll admit up front that though I'm an avid camper (2004 Dodge Ram 2500 4x4 Quad Cab with 5.7 Hemi and 4 man tent and companion canine) that I don't own an RV. I did however just purchase a new 8.5 x 16 Haulmark cargo trailer. It's intended purpose is to be a home away from home while on a jobsite. I just received a WFCO 35 amp power converter to hook into my trailer for both 110 and 12v needs. The only problem is, my skill level with things electrical is not quite up to my skills with things...wooden.
I'm wondering if you fine folks mind if I park here a little while and ask some pretty basic (to you) questions about wiring this thing.
Re: Some electrical questions, if I may...

Welcome to the forum, Rob, but that's a tough home even away from home. What will it feel like tucked in around all them 4x2's? ;)
Re: Some electrical questions, if I may...

Hey, I figure if I can sleep on an air mattress, in a tent for 17 days in northern Maine, I can sleep on a cot if I have to for an afternoon siesta.
Thanks for the welcome.
Re: Some electrical questions, if I may...

Any time you are starting an electrical project (or any other project) start at the END and work your way to the START; in the planning stage. You being an experienced carpenter means you already know how to do that. (How many 4x2's does a wall need? Well, ... how big is your wall?) Same thing with electrical stuff.

What size wire? How big is your load? What will you be powering with electricity? Which ones will be AC? Which ones will be DC? How big is your battery system?

You say "converter" in your post. A CONVERTER converts AC into DC. You might have a CONVERTER powering your CB radio if you put one in your house so many years ago. (A power supply.)

An INVERTER changes DC from the battery into AC for appliances. It isn't clear (without making some assumptions) from your post whether you have an INVERTER or a CONVERTER. Again, size matters.

AND the distance that you run electrical wire makes so much difference, too. (Just like in 4x2's)

Now, that's enough for you to ponder for awhile. Pass some of that info, and we'll ask more questions.
Re: Some electrical questions, if I may...

I will be installing one 8' cold start fluoresent light fixture (double tube) for my main 110V lighting needs.
I will have 2-3 110V outlets on each side of the interior. I will keep these at 15 amp as the largest draw any of my portable power equipment will need is 15 amps. Plus, chances are the customer is not going to have anything higher than that as an outside receptacle. All the 110V Romex will be put inside the flexible conduit.
My 12 volt needs are...
Charge the Interstate Marine/RV HD24-DP 405CCA, 505MCA from the 45 amp power center (Power CONVERTER) I just purchased from Best
Power miscellanious interior lights
Power 2 exterior flood lights
Power a coffee make
Power a radio
Power misc other gasgets I may install.
The CONVERTER came with a 35 amp breaker and 2 15 amp breakers. I have 11 slots for DC fuses.
I'm making a case to house the CONVERTER, which will be on top of a seperate case to house the batteries. The distance from the CONVERTER to the batteries will probably be no more than 2 feet.
Is that enough to start with?
The instructions that came with the converter are a little sketchy at best.
The tow vehicle is a 2004 Dodge Ram 2500 with 5.7 Hemi, Quad Cab, 4x4 with towing package.
Re: Some electrical questions, if I may...

You definately need to ground the trailer frame, because you definately need to have GFI protection. I would install GFI breakers in a disconnect box for the main AC coming in. Be sure and provide an easy way to hook up an external generator.

You said 115AC only, but there is a way to ALLOW FOR a 220 connection in your disconnect box. That way all of your appliances are on one "leg", but you have another "leg" available in case you want to add a 220 air conditioner or something. Just a thought.

Here's a good link for your DC connections. Do the wiring with conduit just like your AC (not with the same receptacles!) This link provides a good way to standardize on your DC connections while you are constructing.

I would probably use #10 red/black zip wire for the accessories and lights. Batteries are batteries so use battery cable to your distribution point.


Senior Member
Re: Some electrical questions, if I may...

A couple of things to keep in mind about batteries.

1) If it has a 'cranking rating' (CCA, MCA), it is not a true 'deep cycle' battery. Cold Cranking Amps implies the battery is supposed to provide a large amount of current for a short amount of time and then immediately be brought back to full charge. A true 'deep cycle' battery, which is designed to provide a 'small' amount of current for a long time, is rated with Amp Hours (AH).

2) For the longest battery life, try not to discharge the battery more than 50% before recharging it. You can guess at at how long this is by dividing the AH rating of the battery by the current you expect to use (in Amps). So if you have 100 AH, and use an average of 10 Amps, then the batteries can theoretically provide 10 hours before being completely discharged. Of course, this will probably not happen in 'real life'. So in the sample case, I would run for 3 to 4 hours before recharge. Not long enough? Use less current or get more AH or add solar panels to assist the batteries... (to figure out the current draw, look at the ratings of the devices you will be using. They will be rated in Amps, Milliamps (1/1000 of an Amp) or Watts (voltage times current, so divide the watts by 12 to get the amps).

3) When recharging a 'wet' battery (one which has ports through which water can be added), the water can be converted to Hydrogen and Oxigen gas, which is a good source of explosions. Make sure the battery compartment is vented to the outside, and that you keep any source of sparks as far away as you can (ie, don't have the converter in the same compartment or right above the batteries). Also, you need to check the water level on a regular basis. Ways to eliminate or reduce these problems are available, but they cost a bunch.

4) The reason to have a batteries in the system is to provide power when no external power is available. How often and how long will the trailer be unplugged with a need for powered devices?


Senior Member
Re: Some electrical questions, if I may...

A few things about the converter and hook up.

1) The converter is designed for 30 Amp input. You might want to go ahead and use a 30 amp RV cable. Extentions are fairly cheap and available everywhere (including WalMart). Then at the end, use a 'dogbone' to convert the 30 amp plug to a 15 amp plug (I also made one which converts the 30 amp plug to a 20 amp plug in case 20 amp is available). The RV cable would probably be easier to handle than romex in conduit, although it might not be as well protected. It would also handle long runs better due to less loss in the cable. And if you do have 30 amps available, you could use them. How long will you be staying at each location and what ranges of distances from the power outlet? If you do decide to stay with 15 amp cable or 20 amp cable, make sure the main breaker in the converter is rated at 15 or 20 amps, respectively, not 30 amps (I'm surprised they provide a 35 amp breaker, the instruction manual says 30 amps is the max you can have, and it talks about a 30 amp power cord as the source).

2) The 'branch' breakers can add up to more than the main breaker, but cannot be bigger than the main, and should not add up to more than perhaps twice the main. Thus if you have a 15 amp main, I'd use only 1 15 amp branch (because the converter uses power too. You might get by with 2 15 amp circuits if you are careful during use. With a 30 amp main, you could easily have 2 15 amp branches and perhaps as many as 4 with care (the number in the power center). Note that if you have a 30 amp main and 30 amp wire, you won't have a problem if you try to draw more than 15 amps, but the breaker on the external source outlet will pop.

3) According to the manual you pointed to, there is no provision for 220. I agree, it is a pretty poor manual, but it does seem to be clear that it is single phase power only.

4) You can get breakers with GFI built in. Or you can have the first outlet attached to the breaker be GFI and all breakers 'downstream' from that outlet will be protected. If it was me, I would probably have one outlet which is not GFI, if possible on a separate breaker which is normally 'off', since some devices seem to 'fight' with GFI. Then I would only use that outlet if I had to.

5) I'd put a 110v outlet or 2 outside (weather protected, of course) on a separate breaker so I could leave them off unless I needed them.

6) Remember, if you have 5 15 amp sockets connected with 15 amp wire to a 15 amp breaker, you can't draw 15 amps out of all of them at the same time. You can draw 15 amps from 1 or 3 amps from each one. Also, just because you plug this into a 15 amp outlet, doesn't mean you can draw 15 amps if the breaker which powers that outlet has other loads on it.

7) You can get 12 volt 'outlets' (like cigarette lighter sockets) and install them around for those things which have the 'cigarette lighter' plugs. I would probably hardwire in a few of the internal lights, all of the external lights and a vent fan, since those types of plugs are not very reliable. I would try to have 1 fuse of an appropriate rating for each 12 volt load, but I would try to leave 2 or 3 fuses unused for future expansion. So make a list of all the 12 volt things you want to power and separate them into 8 categories based on relationship and/or location, then figure out the value for the 8 fuses. This will leave 3 for future use. For instance:
A) interior lights
B) or B&H) exterior lights
C) vent fan
D) 'bedroom' outlets (radio & personal fan)
E) 'kitchen' outlet (coffee pot)
F) front outlet
G) rear outlet


Senior Member
Re: Some electrical questions, if I may...

oops, all of that and I didn't answer your actual questions :)

Do you want to ground the unit to your trailer? Actually there are 2 grounds to be concerned with, the AC ground and the DC ground. The ground bus is definitely for the AC side, since it show all the AC lines connected to it. However, I can find no place in the manual where it refers to or implies the the DC ground, so the only conclusion I can draw is that it expects the chassis to provide the return path (ground) for 12V, which means you have to connect the battery minus to the chassis. See if you can trace the wiring inside to see if the battery minus terminal is connected directly to the ground bus. If so, it seems you should connect the ground bus to the chassis. If not, you may need to make other provisions for the 12V return path. But still, consider connecting the ground bus to the chassis as 'TC' suggests.

For going from the 12V fuse to the device, the guage depends on the current draw and the distance. As stated, 10 ga is a good general purpose diameter for the distances you will be dealing with. You might get by with 12 ga for low power devices. The terminals accept 2 to 14 ga, so never use anything smaller than 14 ga.

What guage between the battery and the converter? Hard to say, bigger is better. I'd want something which would handle at least 60 amps for whatever length the wires are. Probably more than '2 feet', since you need to isolate the potential sparks in the converter from the potential hydrogen from the batteries... Also, compare the terminals to connect to the batteries with the terminals to connect to the 12v loads. Since the latter are rated from 2 to 14 ga, this might provide a clue at what the former are capable of handling. Also I would not under any circumstances use wire smaller than the wire they use to go between the connection board and the main board.
Re: Some electrical questions, if I may...

I agree with all of what John has said and have the same questions.

The reason I say to wire the frame of the trailer to a ground rod stuck into the ground is because I've been shocked once by the lack of such an arrangement. There was improper wiring elsewhere, but with a ground link I wouldn't have received a shock.

I didn't look at the converter manual, but there is no real ELECTRICAL need to use the frame as a DC negative return. I personally would always keep the DC side as isolated from the AC side as I possibly could.
Re: Some electrical questions, if I may...

Wow! Thanks for taking the time to explain all this to me. I really appreciate it.
1) My post should have read that it came with a a 30 amp breaker, not a 35. Yup, looked at the 30 amp wire and will go with a 50' section of that and the dogbone to bring it down to 15 amp plug.
I guess what I'm trying to do with this trailer is cover as many bases as I can. I had a 6x10 cargo trailer before this and I almost killed myself tripping and stumbling over everything. This one, I had an extra 12" in height added and it's huge compared to the last one.
I do a little bit of everything and need to carry all that comes with that with me, so plumbing, electrical (believe it or not), drywall, painting, etc. The trailer will have plenty of light. There is nothing worse than being someplace that doesn't have power and not have light to see to get something or pack up at the end of the day. I want to have plenty of 110V outlets also. Putting one or two on the outside is a good idea. I was thinking that I would have 1 15 amp circuit just for a couple of fluorescent lights. I would go down each interior side of the trailer with a few outlets and have each side on its own 15 amp of 3 15 amp fuses. What I have to keep in mind is that I will only be able to do as much as if I'm plugged into an outlet outside, like I do now. I have a power strip that will have 3-4 tools on it but I work alone, so only 1 tool is on at a time. I think the heaviest draw is my circular saw and miter saw, each at 15 amps each. I would only be inside the trailer using one or the other if it's raining or is 110* in the shade PLUS a fluorescent light. There will be times when I have to spend some time in the trailer and may want a radio going or a coffee pot. I realize that if I'm gonna use the miter saw, I'll have to turn the coffee pot off. I will only have as much power in the trailer as the outlet to which I'm plugged in has.
7) Yup, wondering about that. I have a 30" 12v fluorescent fixture I bought and never used. That will get hardwired in. I also have a fan which I thought I'd do the same with. The back, top portion of the trailer will have 2 12v floodlights...hardwired. I'm assuming the only fuse for each particular item would be the one back at the convertyer, or should I install in-line fuses closer to the appliance?
As far as ground, presently the battery that I ordered with the trailer has the negative terminal fastened to the chassis of the trailer. So, I'm assuming that I can use the chassis for grounding certain things like floodlights and not have to run a negative wire back to the converter...yes? On the 12v side of the converter, there are connecting screws for each place a fuse/appliance would go. Am I to assume I can either twist the negatives all together and use the 12v negative connection or should I wire in a seperate negative bus and use that? Make sense?
Understand about the battery. I plan on being plugged in as much as possible with this, so the question....should I get another identical battery as I have and wire them together for a little more time or buy two genuine deep cells. I guess time and usage will give me the answer on that.
Again, thank you all for the help. My Dad was an ace when it came to this stuff. I got a lot of his woodworking talent and NONE of his electrical expertise.


Senior Member
Re: Some electrical questions, if I may...

Texas, I agree that keeping the AC and DC separate is better. However I could find no place in the manual to hook the returns from the 12V devices. Could set one up I guess to go directly to the battery negative, but would be additional work.

Rob, your proposed layout sounds ok to me, although I would put all the internal outlets (except possibly one which was NOT GFI) on the same breaker. But you can split the sides if you like, I just don't see any benefit to doing so. You can put the external outlets on a separate breaker so you can turn them off when not in use.

As long as the 12v fuses in the converter are appropriate sizes, there is no real need for 'inline' fuses between the panel and the devices. If you have several devices running from 1 fuse, then it can be useful to then have smaller fuses for each device, close to the device or at a second 'fuse' location for ease of service. Just keep this fact in mind when you go to troubleshoot a device which has stopped working.

Yes, you can use the frame for your 12v return/ground. Every car on the road does :) Some electronics may have problems with noise introduced by having 120v AC grounded there as well. Another option is to use actual wires to get back to the battery negative (directly or through the converter panel). More costly and more work, but 'better'.

As I say, I could find no place in the manual where it indicates where to hook the 12v returns. I would be leary of sticking wires in the same hole where the battery negative is attached, but there does appear to be a spare hole in that connector which could be used. I wouldn't 'twist wires together' and use that hole; a couple of big wires together would work, but a bunch of little ones might have one pop loose, or worse, have intermittant contact. I'd use a big wire to a separate ground bus or termanal strip, then hook the individual wires to that.

The battery you have now may work adequately for a while, so trashing it and replacing it with 2 deep cycle batteries may be overkill. On the other hand, getting another just like it might be wasteful unless they are real cheap. If you do go with 2 batteries, they need to be as similar as practical (both electrical characteristics and age). In any case, I would find the deep cycle batteries I might eventually use, and build the battery compartment to handle them, so I could go to them at some time in the future when my needs increased or the old one(s) wear out.

Keep in mind that deep cycle batteries are available in 12v, so you would put 2 in parallel, but the 'best' ones are 6v, so you would need to put the 2 in series. Use BIG wires between the batteries, like the cables going to the battery in your truck.
Re: Some electrical questions, if I may...

Thanks again. I hooked it up last night in my shop to test things out before I start any custom cab for it and do the installation. Nothing blew up and I got 13.6 volts DC on my meter, which is what I was supposed to get.
Thanks to you folks, I feel pretty confident that I can get this installed without killing myself or setting my trailer or truck on fire.
Thanks again!
You'll probably hear from me again when I actually start to run wires. Just too damn cold out there right now.
See ya!