Wire gauge questions

[courtney]

About to go buy new wire to re-wire the trailer lights and 12V system (lights and fan only) in my 1977 Scamp, and I have some questions about appropriate wire sizes. I’m sure I’m way over-complicating this. Thank you in advance for this community and your willingness to help.

I think what’s throwing me off is the idea of connecting smaller sized wires to the larger sized wires in the junction box. My brain wants these sizes to match. What happens as wire goes from a 10G wire to a 12G wire (or smaller)?

The red, green, yellow, and brown wires from the 7-pin wiring harness run into a junction box and are all 14G. The original wires that are running from the junction box to their appropriate trailer light are all 18G. Should I wire with 18G or 14G?

A 10G white ground wire from the 7-pin wiring harness runs into the junction box. What gauge(s) ground wire do I need to run from the junction box to the various trailer lights, 12V kitchen lights, and MaxxAir fan? Currently, is spliced down into various sizes of smaller gauged wires.

A 10G black positive wire from the 7-pin wiring harness runs into the junction box. Connected to same terminal in JB is a 10G wire running to the battery. What size black wire needs to be run to my 12V lights and fan?

[John in Michigan]

Perfectly reasonable question. The wire gauges for your 7 pin are typical and these are fine.

Always run both a power wire and a ground wire to each branch circuit. Wires running to each circuit need to be sufficient gauge to accommodate the current draw (amperage).

As an example, if your are running a circuit to supply power to ALL "parking/running/side marker light" (typically the green wire) together, it must accommodate the total amperage of all these lights. On your 13 foot trailer, that would be 7 lights (4 side markers, two taillights and the license plate light). If you are planning to use led lights, total amperage would be 1 or 2 amps. If you are planning to use incandescent lights (or want to accomodate this possibility) total amperage would be 10 amps or so.

[John in Michigan]

So in my example of wiring all of the running lights on 1 circuit, I would probably just use 14 gauge stranded copper wire. This would be sufficient for both led and incandescent lights.

EDIT: 18 gauge is not sufficient to power all of the running lights if they are incandescent, but would be sufficient for led.

[courtney]

Originally Posted by John in Michigan

So in my example of wiring all of the running lights on 1 circuit, I would probably just use 14 gauge stranded copper wire. This would be sufficient for both led and incandescent lights.

EDIT: 18 gauge is not sufficient to power all of the running lights if they are incandescent, but would be sufficient for led.

What about the main ground wire vs the spliced that run off of it to ground the various lights/gadgets?

[John in Michigan]

Same rule as noted above. Branch circuit wires need to be sized based on the planned loads for those circuits. On the other hand, "main" ground wires to the tow vehicle and to the battery need to be able to handle higher expected cumulative loads. Branch circuits should be supplied through a fuse panel, in your case inside the PD4135. I think most of the pigtail branch circuit wires in the PD4135 are 14 gauge. I would just run 14 gauge positive and ground to branch circuits from the PD4135.

[John in Michigan]

Per the PD4135 installation manual, the built in fuse panel has "DC rating 1-40A, 1-30A, 4-20A fuse holders". The first one on the list, 40 amps, runs to the battery and protects against attaching the battery in reverse.

[jimknoch]

Originally Posted by courtney

About to go buy new wire to re-wire the trailer lights and 12V system (lights and fan only) in my 1977 Scamp, and I have some questions about appropriate wire sizes. I’m sure I’m way over-complicating this. Thank you in advance for this community and your willingness to help.

I think what’s throwing me off is the idea of connecting smaller sized wires to the larger sized wires in the junction box. My brain wants these sizes to match. What happens as wire goes from a 10G wire to a 12G wire (or smaller)?

The red, green, yellow, and brown wires from the 7-pin wiring harness run into a junction box and are all 14G. The original wires that are running from the junction box to their appropriate trailer light are all 18G. Should I wire with 18G or 14G?





A 10G white ground wire from the 7-pin wiring harness runs into the junction box. What gauge(s) ground wire do I need to run from the junction box to the various trailer lights, 12V kitchen lights, and MaxxAir fan? Currently, is spliced down into various sizes of smaller gauged wires.

A 10G black positive wire from the 7-pin wiring harness runs into the junction box. Connected to same terminal in JB is a 10G wire running to the battery. What size black wire needs to be run to my 12V lights and fan?

courtney, In regards to joining smaller wires to larger ones, If you will think of the electrical system as a plumbing system, it may be easier for you to understand.The taps off the main line (the big wire like a water main) and the taps (smaller wire like the small water line running to the house.) Ultimately, you need to size the wire to carry the electrical load without getting too hot or dropping too much voltage, so that the device it is feeding will operate. Similarly, your house probably has a 3/4" water line feeding it and the individual faucets are likely fed with 1/2" or 3/8" supply line. If the lines are too small or too long, the water will not have enough pressure to flow properly and all you get is a trickle.


The big #10 white wire is ultimately bolted to the chasis and the chasis may be used as the neutral return for the 12V system. So the taps may go to other locations on the chasis or be wired directly to the Neutral wire. In either case, the wires going to the loads only need to be the size of the wires feeding the loads on the +12V tap wires. (Personally, I( wouldn't use anything less than a #14 wire for any of the+12V taps or Neut returns. #18 is very small and easily breaks and #16 is not much better and can only carry light electrical loads(think cheap extension cord) With regards to what size wire for your lights & other electrical loads, look up the specifications for the light bulbs you are using (typical incandescent 12V bulb is 500mA= 1/2 Amp) add up the loads on the circuit and that will tell you what the total expected amperage is going to be on that circuit. Then just pick a fuse big enough for about 150% of that value and there you have it. I would recommend that you measure the amps pulled by the exhaust fans, water heater, and refrigerator if so equipped.


I hope this helps with your project. Message me if you have questions.


Jim

[Terry in Fowler]

What you lisred in your first post sounds like the original wiring and should handle the loads listed. There is no need to change the 18 guage to 14 guage. If I replace a 18 guage wire I usually use 16 guage because with my hands it is easier to work with, not because it is needed. From what you have posted you probably know this. It is best to make as few connections as possible and keep the connections accessible. Thay are the weak point.

[Kenny Strong]

Courtney
In 2018 I put a PD4045 in my 93 B13.5 Bigfoot People on this site suggested I run a 10 gauge line from the 12volt + DC connection in the 4045 straight to the battery; which I did.
There is also a original equipment positive bussbar that the 7 pin goes to under the front bench, with a 14 gauge line going to the battery; redundant ??
By running the separate 10 gauge wire to the battery I don't worry about any over heating.
I also have a 40w solar panel wired directly to the battery.
battery reads 13.3volts at this moment.
I have 2 factory wired 120v outlets. One outlet, has 2 usb ports; factory wired into it.
I wired a cigarette lighter outlet under rear bench into the 4045.
I have a green oem 4 gauge negative out of the 4045 to the frame
Later Kenny

[courtney]

Thanks, Kenny!

[courtney]

Originally Posted by John in Michigan

Perfectly reasonable question. The wire gauges for your 7 pin are typical and these are fine.



Always run both a power wire and a ground wire to each branch circuit. Wires running to each circuit need to be sufficient gauge to accommodate the current draw (amperage).



As an example, if your are running a circuit to supply power to ALL "parking/running/side marker light" (typically the green wire) together, it must accommodate the total amperage of all these lights. On your 13 foot trailer, that would be 7 lights (4 side markers, two taillights and the license plate light). If you are planning to use led lights, total amperage would be 1 or 2 amps. If you are planning to use incandescent lights (or want to accomodate this possibility) total amperage would be 10 amps or so.

Thanks for your response. I will surely have follow up questions soon!

[jwcolby123]

Keep in mind that "back in the day", particularly as old as your scamp is, while lights were incandescent, other loads were non-existent or minimal. Today with all the modern gadgets the loads are getting bigger and bigger.

Just as an example, the "Cig lighter" 12v outlets today will often be hosting a dual USB adapter plug. "back in the day", waaaayyyy after your scamp was built, a usb adapter was 700 milliamps (7/10th of one amp) at 5 volts. Then they moved to 3500 ma (3.6 amps) at 12v. Then quick charge with 12v 1.5 amps.

Etc.Etc.

The point is that things only get bigger, faster and more power hungry.

So... If It Were Me (and I am currently rewiring my '95 19' scamp), I am going bigger. Replacing ALL 12V CIRCUITS with 12 gauge wire. The question is, why not? Doing so means I can with ease, and never worrying about wires heating up, throw in a fuse block at the far end and run half a dozen new circuits to ... whatever I need, whenever I need, OTHER THAN INVERTERS.

Remember too that Power = Volts*Amps or Amps = Power / Volts, which is important because anything DC 12v high power can quickly become high current. Further complicating the issue is wire temperature.

The following is a good chart to help with all this.

12 Volt Wiring: Wire Gauge to Amps |