Question about appropriate wire gauge

Wow. Faith in the threads!

DC power acts very differently than AC. Distance matters. High quality connections and larger cables rule for branching circuits, but not “too” big so as to add unnecessary resistance.

You need to calculate the load first, then calculate the optimal AWG.

There is good reason the battery is on top of the alternator.

 

https://www.bluesea.com/products/7635/m-LVD_Low_Voltage_Disconnect

 

It isnt actually that hard, but a different way of explaining might help.
(Here is blue sea's 2 part explanation)
https://www.bluesea.com/resources/1437
https://www.bluesea.com/support/art..._Fuse_Holder_For_Your_DC_Product_Installation

There are just 2 fundamental checks,

One, that there isnt excessive voltage drop (and wire overheating) for the wire for when the equipment is operating normally. (Or the design condition assuming X amps drawn from your new added panel).

Two, that an appropriate breaker or fuse is connected to the wire. The current protection device needs to be large enough to not open in normal conditions, but small enough to open the power before the wire melts through its entire length and starts stuff on fire. Think about it like this, if the wire shorts to ground there will be heat and sparks at the short, but in addition to that until the fuse/breaker opens the entire length of the wire is heated up to red-hot and can melt plastics and fabrics on fire.

(This is why 2 small wires is not safe to attach to a single large breaker. Assume that a 14 gauge wire is ok for 20 amps to run a single motor, but we don't have a piece of 14 gauge so we run 2 parallel 18 gauge wires, connecting from a single fuse to a single motor. When the motor is running normal, the motor current is split between the two wires and is less than 10 amps each... all is good. Now assume 1 of the 18 gauge wires shorts to ground. The single small shorted wire starts to get very hot and may not blow the 20 amp fuse before it melts the insulation which endangers the vehicle.)


Welding wire may be fine. Need to make sure it is well stranded (to reduce breakage), and insulation rated at reasonably high temperature (such as at least 105C). Note, welding wire often has very thick rubbery insulation which I DO NOT like.

But I really don't think welding wire is worth the cost savings (and I am a cheap ass) compared to high quality wire. Just an example, I get wire from skycrafters (in orlando) and get Mil-Spec wire for 6 awg for about $1.65/ft https://skycraftsurplus.com/6-awg-aircraft-wire-white-m2275916-6-9-white-6-awg-rp7637.html. The similar gauge welding wire from same location is about a dollar less per foot. https://skycraftsurplus.com/6-awg-welding-cable-red-red-6awg-sk9217-red.html

So for $1 per foot less you get wire that is good for 90C instead of 150C, and wire with insulation OD of 0.37" vs 0.25" for the same amount of copper.

Its just an area where I think paying more is just nicer to work with

 

Try to avoid CCA(copper clad aluminum) it's much cheaper for the same wire size but doesn't have the current capacity of pure stranded copper wire. The stuff is everywhere on the net so be sure of what you buy.

 

Ok, I’ve found myself a little extra time this morning so I’m directing my attention back to this project.

Thanks for the great info so far.

So, my fridge draws 9.3 amps. The round-trip circuit length will be ~30’ so I figure I’ll just round up to the next increment on the chart which is 50’. Adhering to the non-critical load recommendations, I should go with 12awg.

One thing I’m not clear on is the following:

Is the recommended 12awg good for 10-15 amps, or would I move up to 10awg if I were to exceed 10 amps?

The reason I ask is that, my fridge being well over 20 years old, I haven’t been able to source a 12v element with exactly the same specs. I’ve found them with slightly higher or lower wattage, so I’m thinking I should factor a possible higher draw should I go with a higher wattage element should I need to replace the one currently in the fridge. So, should I run a 10 or 15 amp circuit? Seems that over-doing it and going with the next thicker gauge (10) wouldn’t cause any concerning resistance.

 

The 12awg should be fine for up to 20 amp, even at 30’ it would be fine for your ~9 amp fridge. If you want to go with 10 awg that’s not going to hurt either. The element I imagine says what wattage it is at 12v which can give you the rough amp draw. Fuse accordingly.

 

There is no harm in going to a larger gauge wire.

 

Good. I will go with 10awg.

Next, and I think last question which is more a matter of preference is:

Should I run this dedicated fridge circuit, or should I run, say, a 30 or even 50 amp circuit to a fuse box with room to expand, to the bed area, and run the fridge circuit from that box?

 

What would that be in this install? 10 or 15amp fuse?

 

As close to max power draw as possible but not less than, 10A. Running larger allows for larger current flow to reach the device. Go too high and you could get smoke. You want the fuse to go first. If you add more later you'll need to adjust fuse rating. You should also add the fuse closer to the battery than the bed, most do this anyway.

 

Perfect, thanks.

 

...I should mention that I plan to install oba for sure and want room to grow.

So I guess the real question is:

Is there any significant benefit to running the dedicated fridge circuit. I’m thinking pretty much no, and that I should just run a 50amp circuit to the bed and wire my fridge to that sub panel...

 

No not really. Itll be less wire running to run a main line and then distribution in the bed. Just make sure its fused properly. Fuse the main line near the battery and then a distribution block in the bed with each lead out being fused to the devices proper max draw current.

 

Good deal.

 

You would take watts divided by volts to get amps.

 

I just want to chime in and highlight what bagelboy said here. Fuses are more of and old school PIA way of dealing with installing a new circuit. Use a breaker. Commonly found at car stereo shops, they're much easier to deal with than replacing a fuse and if you need to isolate the circuit you can just press the test button to "trip" the breaker and service whatever you need.
In addition, when building a new system like this, I would not recommend fusing as close as possible. Fuse at 25% more than your expected max load. That way if you add something in the future you've got a little room to work and that also gives you a little wiggle room if the components are exactly at their ratings- something rated to pull 5 amps is pulling 5.5 let's say.

 

Fuses and breakers are identical in that their most important quality is protecting a down current wires or panels. I'd amend twolane's assessment to say that the breaker spec should not exceed the wire gauge or a fuse panel rating. Panels have a max total and max individual circuit current rating so a breaker for a 100A panel should not be more than 100A. An individual circuit breaker can exceed the rated current draw of a device so long as it does not exceed either the wire gauge rating or the individual circuit rating of the panel. Don't get a 100A breaker for a 100A Blue Sea fuse panel unless you also run wire to the panel rated for 100A. It's perfectly fine to run awg10 to the panel but the breaker that serves it needs to be matched to the lower rated wire.

 

I like this but only for the main power line and if he added more devices later he'd have to replace the breaker rather than a fuse, cost is not in favor of this. Cost would be very prohibitive if he wanted to put breakers on a distribution panel in the bed for multiple outlets. Pulling fuses on my set up takes about 20-30 seconds, so yes a breaker is nice, but wasn't worth it for me since the possibility of changing things later would mean new breaker installs and greater cost. You really want the weak link, be it fuse of breaker to be just at the max load rating specific to the device, main supply should be cumulative and device leads specific to the device. The devices don't operate there often and the rating is for safety concerns of the device. Running a fuse that allows more than that means the device can get more power than it was intended to have. For years I have ran my amps with fuses that match the max load on my amps and never blown a fuse.

Also you'd have to run considerably low draw devices to add 1 fuse safely leaving room for more devices on the same lead and fuse later. Even then its safer to run separately fused leads to each device for their safety. If you fused a lead at 9 amps say for the fridge and then used a 25% higher fuse at 11-12A, added a second device that runs at 3A you would not likely run into a constant draw being too high to blow fuses constantly. What you would have is a lead that allows the 3A device to see 11-12A before the fuse pops. At 4x the intended supply current, that device isnt going to last, especially since it will see more power as the fridge will be pulling it.

 

Fuses are to protect wiring not devices. They are there to fail before the wire fails and you have a carbeque. A short anywhere in the circuit and the fuse needs to fail. You certainly don't want to run excessively heavy wire just to be able to use a heavier fuse, that increases the likelihood that the device becomes an ignition source. Be prudent, use enough wire to handle the device draw, keep the fuse rating at or slightly above expected device draw while at or below wire rating.

 

Totally agree that devices should be fused separately but it's the device that determines current draw, not the fuse. The only way that other device would see fridge current is if it's wired in series with the fridge, in parallel each draws it's rated current in isolation except for the terminal both are connected to. The problem with sharing terminals is you can end up with mismatched wires/loads. That 3A device calls for awg18 but shares a terminal with a 9A fridge wired with awg14. If there's a short in the 3A circuit that awg18 wire will fail before the fuse blows > carbeque.