Stepping down wire gauge sizes for aux power

[DaveM]

I am planning on installing this RV roof connector into the base of my Maggiolina RTT so that I can have it plugged to Aux power full time and still be removable. The plug I ordered will work great but its 12 gauge wire and I probably only need 18 gauge to the actual light in the tent.

Anybody here know how I can step down the wire size? Can I just solder some 18 gauge to the 12 gauge wire and seal it with heat shrink tube?

Also, if I were to want more than one circuit running off this line in the tent (ie 12v light and an aux socket), can I just split the wire in the tent and have it all run off one fuse back at the aux fuse box?

thanks,
dave

 

[spencyg]

You won't hurt anything by running a larger wire gauge. I'd keep the size at 12ga as far as you can and leave the smaller stuff for wiring specific components off your distribution network (gang blocks, panel, etc). If you're set on keeping the wiring small right from the plug, than using 18ga in place of 12ga won't present any problems other than limiting the available current. I would make sure the supply plug is fused correctly for the size wire you end up using.

Spence

 

[DaveM]

The reason I want to use 18 gauge wire is because that’s what the tent light is already using and the thicker 12 gauge wire has less flexibility which it needs since it will open and close with the tent top (plug is on bottom of tent, light is in top of tent; wire moves up & down 2 feet).

What I was hoping to do was run 18 gauge from the aux fuse box to the plug (under the chassis and up though the side vent). Cut the inside pug wires short and splice in the 18 gauge light wires. So the only part that would be 12 gauge would be about 2 or 3 inches right at the plug in the tent bottom, everything else is 18 gauge and the fuse would be back at the aux fuse box under the hood.

I could run 12 gauge all the way to the plug incase I want to expand that circuit and just step down the light wires inside the tent about 2 inches above the plug.

The real question here is, is it ok too directly solder 18 gauge wire to 12 gauge wire for running a small 12v light?

 

[overlander]

12 gauge is plenty flexible, and you'll regret going to 18 gauge if you start using high demand items in that aux outlet your thinking of. bigger is better when it comes to wiring. Do it once, do it right.

On another note, where did you get that bulkhead plug? I've been searching all over for something like that? :jumping:

 

[DaveM]

12 gauge is plenty flexible, and you'll regret going to 18 gauge if you start using high demand items in that aux outlet your thinking of. bigger is better when it comes to wiring. Do it once, do it right.

On another note, where did you get that bulkhead plug? I've been searching all over for something like that? :jumping:

ask and you shall receive

On the wire bit, the other reason I wanted to splice in the light wire is because its already there, run out from under the headliner. If I go to a thick wire I'll have to open up the headliner to run it into the box the light is mounted into (hard to describe unless you've see the inside of these tents). Your right though, I should do it once and set it up for an extra circuit.

 

[madizell]

To answer the question asked, whether you can solder 18ga to 12ga and heat shrink the joint, the answer is yes. The industry does it all the time, and there are many places in a standard auto harness where such joints are made. They are usually covered with tape or a loom, so you don't see them, but they are there. The wire used needs to be sufficient for the intended load. If 18ga is large enough to handle maximum anticipated load, you can do what you suggested by splicing to the 12ga pigtails on the plug.

Your other question is whether you can split the wire and use only one fuse. Again the answer is yes, as long as the fuse is light enough to work as a fuse for whatever is on one or the other of the wires, and not rated to handle the load on both wires (unless the fuse and wires are well within the working range of the total load). In other words, if you have two systems running off the same source, and both are, say, 10 amp max loads, you could use a 10 amp fuse before the split. However, in doing this, you would not be able to use both systems at full rating because you would overload the fuse. If you use a 20 amp fuse to protect both, you run the risk of the fuse being too large for the wire, which could be a heat problem or a fire hazard. Better to use two fuses, one in each line, rated to the anticipated load, but you will still need to verify that the max load on both systems is within the handling range of the 18ga wire you want to use, if the wire used up to the point of the split is also 18ga.

 

[DaveM]

Thanks for the help guys. I'm not too confident with wiring so I need the input. Since it seems like I got some smart people on this thread I'm going to post up two JPEGs of what I think my circuit should be doing based on the input Im getting (the aux fuse block and the tent hook up). Let me know if I got this set up right and make recommendations on wire gauge or fuse size. In particular I don't know what gauge the wires should be from the batt to the fuse block.

dave

 

[overlander]

I would go 10 gauge myself from the battery through the relay to the 6 point fuze block. 14-18 ga is fine to relay from ignition source. You may want to put a 2 way switch to activate the relay, so you can either activate the relay with ignition source (daily use) or battery direct, so you don't have to keep your key in the ignition for the relay to work such as in camp sights. of course if you do that, you need to watch your battery, and have a voltmeter active as well.

 

[madizell]

Based on your diagrams, I would second the use of 10ga wiring all the way out to the final circuitry, because you have specified 50amp and 30 amp loads. 18ga wire will not handle a 50 amp load. It would glow first. If you actually expect to see a continuous 50 amp load, 10ga might even be a bit light.

Also a really good suggestion was the one of bypassing your ignition switch as a necessary to using the tent system. You don't want to leave your key turned on for hours at a time because it provides power to various secondary circuits you won't be using (inside the vehicle) and will drain your battery faster. If it were mine, I would not even try to make the tent circuit dependent on key position. I would wire it directly from battery source with a kill switch or relay kill circuit. Presumably, if you are using the tent, you won't be driving the car and probably won't even be in the car. I would put the power controls where they can be accessed, but not where they will be in the way or subject to weather, abuse, or accidental contact.

 

[madizell]

In another thread, Robthebrit posted this information. I don't know how to cross link to other threads or I would have simply posted the link:

"I can't tell you if you need to upgrade because I have no idea what size cable Toyota use. If you are running 15-20 amps over 15 feet of cable then you need 10 guage cable.

In an ideal world 3% is considered to be the maximum voltage drop for a 12v system (0.36 volts). For a 15 foot run, here is what I worked out for wire sizes that will give less than 3% volt drop. The currents involved are significantly below the limits of the cable sizes so there is lots of safety built in but make sure to use a fuse of some kind.

amps gauge (AWG)
5 16
10 12
15 10
20 10
25 8
30 8
40 6
50 6


Rob"

As you can see, by Rob's recommendations, which look right to me, a 30 amp circuit would need 8ga wire and a 50 would need 6ga, whereas your diagrams in a few places show a 30amp circuit feeding direct to appliances through 12ga or lighter wiring. You can still do this, but each of those light circuits with a 30amp feed HAVE to be adequately fused so that no appliance or combination of appliances can draw more amperage than the lightest wire component of the system can handle. For feed lines (diagram one feeding battery to relay and relay to block, the wiring simply must be able to handle the maximum amperage transferable through the feed, and if you are using 50amp as a maximum, you will need the heavier wire listed above, and even then each feed should have a fuse or fusible link built in.

 

[DaveM]

I like the idea of having the tent circuit switchable without the need for the key to be in the ignition. But I want the aux fuse box to be ignition dependent.

So I guess I can either have the tent circuit separate from the aux fuse box, or find a relay that can be both ignition activated or switch activated? Does that even exist?

As for the wire gauge, I got 50amp from another post on the forum somewhere, the fuse box itself may spec something lighter, I don't have it yet so I don't know. The 30amp rating just means each circuit on the aux box can handle that much current but I would put in fuses that matched what was hooked up to each circuit.

To be honest, the aux fuse box is for later anyway. I may just start with a dedicated line to the tent, with a relay switch in the cab somewhere. Can one of you guys describe how you would set that circuit up? Keeping in mind that the bulkhead plug I have is already 12 ga and I want to hook up a single 12v cabin light and one 12v aux socket to it inside the tent.

 

[DaveM]

here's what I came up with:

 

[DaveM]

In another thread, Robthebrit posted this information. I don't know how to cross link to other threads or I would have simply posted the link:

"I can't tell you if you need to upgrade because I have no idea what size cable Toyota use. If you are running 15-20 amps over 15 feet of cable then you need 10 guage cable.

In an ideal world 3% is considered to be the maximum voltage drop for a 12v system (0.36 volts). For a 15 foot run, here is what I worked out for wire sizes that will give less than 3% volt drop. The currents involved are significantly below the limits of the cable sizes so there is lots of safety built in but make sure to use a fuse of some kind.

amps gauge (AWG)
5 16
10 12
15 10
20 10
25 8
30 8
40 6
50 6


Rob"

As you can see, by Rob's recommendations, which look right to me, a 30 amp circuit would need 8ga wire and a 50 would need 6ga, whereas your diagrams in a few places show a 30amp circuit feeding direct to appliances through 12ga or lighter wiring. You can still do this, but each of those light circuits with a 30amp feed HAVE to be adequately fused so that no appliance or combination of appliances can draw more amperage than the lightest wire component of the system can handle. For feed lines (diagram one feeding battery to relay and relay to block, the wiring simply must be able to handle the maximum amperage transferable through the feed, and if you are using 50amp as a maximum, you will need the heavier wire listed above, and even then each feed should have a fuse or fusible link built in.

just saw this. ok so my new diagram for a dedicated tent circuit is probably overkill. How many amps does a single 12v dome light use anyway? The aux socket would probably be used to power a small RV fan or an iPod occasionally.

Edit to add: I found some references for higher amps than listed above however:
http://www.stu-offroad.com/electrical/elec1/wire-1.htm
http://www.powerstream.com/Wire_Size.htm

 

[madizell]

Both excellent references. Note that there are caveats with each chart, including Rob's. All say they are being conservative, which means there is no real basis for comparison since we don't know how conservative each is being. But each chart shows that as load increases, wire size must increase.

Also, each of the two references states a difference in ratings where wires are bundled versus open air, and the second chart, while stating a drop in carry capacity for chassis wiring also states in the notes that these are still open air ratings, not bundled, even though chassis wiring is almost always bundled.

I think it is probably a pursuit of detail that is not necessary for what you are doing. 50amp is probably overkill. You are thinking of powering overhead lights, perhaps an iPod, but probably not a toaster or coffee pot. A 15 or 20 amp circuit is probably all you will ever need. Overhead lights can be rated using the electrical formula: W=VA, where W is watts, V is volts, and A is amps. You can use this same formula to solve for any one of the three variables. An unsupported 12V circuit (motor off, battery only) might make a bit more than 12V, but lets say 12 just for reference. Assuming each overhead lamp is 15W, the amperage draw for each lamp is 1.25 amps (W/V=A or 15/12=1.25).

To use the limit of a 20A circuit, you would need to run 16 overhead lamps at 15 watts each. So, perhaps 50A is overkill unless you are thinking of a tanning salon on your tent. (Sorry. It was just the thought of 16 lights... )

Note: the only 50A circuit I have ever found in an automobile is the one in my Audi for running the multiple cooling fans which draw a lot of power.

 

[Robthebrit]

If you doubled all the currents in my table it would be perfectly safe. My table was being conservative enough to not drop 3% over 15 feet, assuming 12v DC and multistrand cable. My wire sizes are very oversized by comparison to modern vehicles. You can safely get 100A at 12V DC down an 8 guage cable but anything over a couple of feet and you'll drop a lot of voltage. The wasted power is converted to heat in the cable, in fact the limiting factor of a cable is how fast it can dissipate heat, if a cable feels warm then its too small.

Modern vehicles use the minimum size necessary to save $$$. Most cabling in a modern vehicle is safe from heat point of view but undersized for the load they are carrying. They also assume a starting voltage of 14.4v which makes fair amount of difference. If your new circuit is intended to be primarily powered while the vehicle is running then this is a fair assumption.

Rob