As part of correcting a no brake issue on our trailer (small 23 foot), I decided to freshen up and rewire some components of the braking system.I will quickly admit, I knew/know very little when it comes to the intricacies of trailer electrical system(s) and I am learning "on the job". So far, I have replaced the 7 pin cable, junction box, emergency brake cable component and run new brake wiring from the junction box to each of the four, soon to be replaced, brake assemblies. All the wiring has been laid out, but many actual connections are still pending.One of the latest stumbling blocks is a small device at the front of the trailer (previously hidden under an amateurish bo-jack cover which is like a hand formed thin plastic cover, open to the elements at the bottom - original to the trailer). It is attached to the front frame cross member, is about 2 x 1 x 2 inches high, and has two threaded studs, to which four cables are attached by ring terminals. I was unable to fully inspect it, due to impending darkness, but it has BAT etched in it, and one of the wires goes to the black wire of the 7 pin cable. I think another wire goes to the battery, but can't confirm that yet (due to wire loom). It is located right beside, but not connected to, a grounding screw in the frame. Because neither the grounding terminal nor this device are protected from the weather, there is noticeable corrosion.At the very least, I will be cleaning up the corrosion, but don't know yet what else needs doing.However, I am trying to determine what this device is, what it does and, whether it can be just cleaned up or needs replacing. Have done some internet searching and came up with either a busbar or stud type circuit breaker (the closest photo I could find is a Princess Auto 6 to 24v stud type circuit breaker).Any help is greatly appreciated, and sorry for the long post.Marv

There should be markings on the unit as to what amperage it is. It is no doubt a circuit breaker.Many different brands and amperage.circuit breakers

Enblethen,Thanks very much. One of the photos you linked to looks almost exactly like this device, so it looks like it is a circuit breaker. One of the things that caused me to hesitate in thinking it was a circuit breaker is the location (outside, fairly accessible to weather and road splash-up). Always figured circuit beakers were kept in closed, dry locations.Once again, thanks for your help.p.s. travel a lot from Vancouver BC to Denver Co along I-90 and quite like the Moses Lake and surrounding area.Marv

Many are mounted under the trailer tongue area with no protection at all. Some put them in a standard indoor electrical box with splices as well as circuit breaker(s).If you cannot find amperage mark, look at your converter. A common sizing is based off the converter output or the size of the wire between the converter and breaker.Many auto parts stores have them. Another source is marine stores which some have rubber covers for them.

Your description right down to the location under the frame says it's a circuit breaker. This breaker is for your interior wiring, but not the brake wiring. If your lights and electrical items in the trailer work you can put off replacing that breaker. Your brake wires should go straight to the brakes. Go thicker on the wire like at least 10 gauge. Best is to put a waterproof junction box on the tongue or under the front of the trailer. Take the one brake wire from your 7 pin to the junction box and then run 4 separate brake wires from the box to each wheel. Do not run the wire from the 7 pin to the first wheel and then jump from the first wheel to the second and so on. That it a surefire way to get poor braking performance.It's usually cheaper or at least easier to buy complete brake assemblies from someone like etrailer. These are complete units (brakes and backing plates and magnet) You just cut the old wire, remove 4 bolts and pull the whole brake assembly off. Put the new assembly on, tighten the 4 bolts and hook up the brake wire. Figure $200 per axle to replace. maybe it was 100 per axle I it's been a while since I looked.As far as the circuit breaker. You should have one wire going from the breaker to the breakaway box. From the breakaway box you should have a wire going to the waterproof junction box you added for your brake wiring. This wire gets tied in with all the other wires. Yes 6 wires all joined together inside that junction box. The other wire from the breaker is to the wires that feed the electricity into the coach.

Thanks everyone for the continued comments (massive learning curve for me, so lots of good advice).Initially, it was my intention to just replace the 7 pin cable and brake wiring. But as I got into it, I could see a bunch of corrosion locations, which I figured would bite me later so figured I would re-do/replace some obvious items. That is how I came upon the circuit breaker and as it was accessible, figured I may as well replace that too. Funny thing was, there were no markings at all on the old one, so I took it into an RV place and was advised to just get one which was the same amperage as the pedestal amperage for the trailer (in my case it is 30 amp), which is what I did.As the junction box was a residential box and it was absolutely crammed with wiring, all connected with typical residential marrettes and a lot of corrosion, I purchased an RV waterproof box and re-did the wiring connections.As to the brake wiring, I replaced with the same gauge wire and ran a single blue and white from the box to the centre of the axle(s) then pigtailed off a wire to each wheel. So, each brake has exactly the same length of wire running to it (don't know if that is relevant but the OCD in me said to do it that way).I did purchase a brake assembly for each wheel, and my next step will be to replace the old with the new.Marv

Electrical Help Needed | Good Sam Community

Gdetrailer
Explorer III
Sep 11, 2022
Grit dog wrote:
Are you actually picking an argument over the finer points of copper wire? Lol

When one well meaning and well intentioned person says a smaller in diameter wire "handles more current" there is a problem that needs to be addressed.

Also should point out, DC doesn't care how much "surface area" the wire has, DC does not play by the "skin effect" like AC can and uses the entire depth of the wire and not the surface.

SKIN EFFECT ON WIRE

"The Skin Depth of Copper in Electrical Engineering

As previously mentioned, the skin effect is where alternating current tends to avoid travel through the center of a solid conductor, limiting itself to conduction near the surface.

This effectively limits the cross-sectional conductor area available to carry alternating electron flow, increasing the resistance of that conductor above what it would normally be for direct current
The electrical resistance of the conductor with all its cross-sectional area in use is known as the “DC resistance.” The “AC resistance” of the same conductor refers to a higher figure resulting from the skin effect.

As you can see, at high frequencies the AC current avoids traveling through most of the conductor’s cross-sectional area. For the purpose of conducting current, the wire might as well be hollow!
"

Learned that in "Basic DC/AC Circuits" way back in Tech School..

As I mentioned, the OP bought a 30A breaker, as long as the breaker doesn't constantly trip, it is a very safe bet.. If they have issues with the breaker cycling on/off then 40A would be to max I would recommend.. Sure, the wire might handle 50A, but doing so will result in more energy being lost as heat than it is worth it.. That heat may result in melted insulation which is why there is ampacity charts..
Grit_dog
Navigator
Sep 10, 2022
Are you actually picking an argument over the finer points of copper wire? Lol
Gdetrailer
Explorer III
Sep 10, 2022
enblethen wrote:
SAE have more circular mill copper wire!

SAE is LESS diameter than AWG.

SAE wire was developed for low voltage applications (60V or lower), typically for the automotive industry (Hint, SAE is an abbreviation for Society of Automotive Engineers).

I have some 12 SAE on hand.

Click For Full-Size Image.

It measures 5/64" (1.98 mm) diameter

Compared to 12 AWG which measures 3/32" (2.38mm).

The last time I checked, 5/64" (1.98mm) is STILL smaller than 3/32" (2.38mm).

12 SAE will handle LESS current than 12 AWG unless you are fine with higher wire resistance, more voltage drop for the current and more heat generated through the wire.

Wire resistance is a real problem with 12V items.

Additionally, the 12 SAE I have is also marked with 60C temp and has a PVC jacket..

In the OPs case, they bought a 30A breaker for "8" wire, that is a safe bet. My TT used unmarked 8 ga wire, might have been SAE or could have been building wire or something in between.. But the RV manufacturer used a 40A breaker on the tongue and the converter side was fused at 40A..

So, I would say the OP can safely stick with the 30A breaker or possibly exchange it for a 40A breaker only if they run in to having the breaker cycle on/off all the time..
enblethen
Nomad
Sep 10, 2022
SAE have more circular mill copper wire!
Gdetrailer
Explorer III
Sep 10, 2022
enblethen wrote:
Wire size and type of wire is determination of circuit breaker or fuse size.
Must remember that the wire in question is fed from two directions. Battery feeds wire toward the 12-volt DC power distribution panel. The converter feeds wire to the 12-volt distribution panel then to the battery.
SAE type wire can carry more amperage than AWG building wire.

Ampacity depends on type of insulation and if it is enclosed or open air, lots of factors goes into what is deemed as safe or not.

Ampacity is highest without any insulation, but then you would now be back to knob and tube..

If you look at a ampacity chart it does vary with the type of insulation and type of wire (copper or Aluminum)..

Found HERE

Click For Full-Size Image.

Romex has 60C temp rating and hence is limited to 40A.

"Building wire however has insulation ratings of 75C and 90C and ampacity of 50A and 55A respectively..

However, RV manufacturers do not use either Romex or Building wire, they use the most dirt cheapest plastic covered insulated wire as possible.. It is a much lower grade of insulation (and unmarked) which has a far lower melting point than Romex.

I would not trust RV 8 ga wire above Romex wire ampacity.

There is "Primary" wire typically used for automotive uses which you basically mentioned as "SAE", yes it has a higher temp insulation rated as 105C (the real SAE wire), but, from my experiences, RV manufacturers are using really cheap wire that doesn't come close to 105C melting point and isn't SAE wire.. I pulled lots of the RV wiring out of my current TT when rebuilding it and scrapped that wire. If you wanted to solder a connection the insulation easily melted back, much easier than building wire or even Romex.
enblethen
Nomad
Sep 10, 2022
Wire size and type of wire is determination of circuit breaker or fuse size.
Must remember that the wire in question is fed from two directions. Battery feeds wire toward the 12-volt DC power distribution panel. The converter feeds wire to the 12-volt distribution panel then to the battery.
SAE type wire can carry more amperage than AWG building wire.
opnspaces
Navigator II
Sep 10, 2022
The determining factor is the gauge of wire on the trailer side at the breaker. The breaker is to protect the wire between the breaker and the converter.

I don't know the gauge wire on my trailer. But I do know it has a 25 amp fuse on the tongue and I have never blown it in over 15 years. Breakers are cheap. Fuse holders and fuses are cheap. I would put a 25 amp breaker or fuse on it and see if it ever blows.
Gdetrailer
Explorer III
Sep 10, 2022
marpel wrote:
First, again, many thanks for the continued information. I have/am learning a heck of a lot as I work through this stuff.

Regarding what I was informed by the RV employee, I realize the pedestal power is AC and the battery system is DC, but as the lady who I spoke with has been in the parts section for at least 15 years, I thought she was knowledgeable and that there was some sort of relationship between the DC system amperage and the pedestal amperage. And I showed her the old breaker and told her it was connected to the battery...Go figure.

I checked the Converter and it appears it has eight gauge wire and from manufacturer details on this Converter (WF-8955) the output power (rated DC current) is 55A and max input current is 11A. Not sure which number is relevant, but if it is the output power, does that mean I need a 60 amp breaker?? Afidel, not sure this is the same as "charging circuit" and "charger rating" that you mentioned.

Thanks,

Marv

No, do not use a 60A breaker or fuse.

You want the fuse or breaker to protect the wire from overload and potential fire.

8 Ga wire, 40A fuse or breaker..

That is typically the amperage RV manufacturers use.

The fuse or breaker on the tongue is there to protect the wire connected to the battery, on the converter end there should be a dedicated fuse for the battery. The idea is you have two power sources and because of that you should have a fuse or breaker on each end of the wire nearest the power source.

You can of coarse under size the breaker, 30A is perfectly fine and well within the current rating of your wiring.. The only issue may be occasional breaker trips if you are using amperages close to the breaker rating.
marpel
Explorer
Sep 10, 2022
First, again, many thanks for the continued information. I have/am learning a heck of a lot as I work through this stuff.

Regarding what I was informed by the RV employee, I realize the pedestal power is AC and the battery system is DC, but as the lady who I spoke with has been in the parts section for at least 15 years, I thought she was knowledgeable and that there was some sort of relationship between the DC system amperage and the pedestal amperage. And I showed her the old breaker and told her it was connected to the battery...Go figure.

I checked the Converter and it appears it has eight gauge wire and from manufacturer details on this Converter (WF-8955) the output power (rated DC current) is 55A and max input current is 11A. Not sure which number is relevant, but if it is the output power, does that mean I need a 60 amp breaker?? Afidel, not sure this is the same as "charging circuit" and "charger rating" that you mentioned.

Thanks,

Marv
afidel
Explorer II
Sep 09, 2022
The rating is based on the gauge of wire being protected, not the size of your AC power input. 30A is probably a bit small as the charging circuit in the converter is probably larger than that. Check your DC wire gauge to the converter and the charger rating to make sure you don't have nuisance trips.