Wire gauge - amp ratings chart help!


Hello all,

Trying to understand 12V electrical wiring and having a tough time of it. I'm planning out various wiring upgrades and figuring out what size/gauge wire I need and have found that the charts seem to vary wildly.

For example:

Show different gauge wires!!!

Just take for example a 20' run and 20 amp capacity. The charts show I need 8, 12 and 6 gauge! Three different charts, three different answers.

Or if I was trying to figure out what size wire to run for my fridge... 10' run at 10amps.. The charts show I should use 14, 18, 14 gauge respectively.

Can someone help me out here please! What's the deal?

Thanks in advance!


Expedition Leader
Inherent in these calculations for a wire size is the amount of voltage drop that can be tolerated - none of these charts indicate what percentage was used.

The lower the voltage drop the larger the wire - I would go with the chart which shows the largest wire size (smallest number gauge) required as its not usually worth it to undersize wires.


Expedition Leader
Also - the top and middle charts do not specify if the length used on the chart is one way (i.e. two conductors) or if it needs to be used with a round trip number (total of both the negative and positive wire lengths). In the automotive and RV industry, it used to be common to use the chassis for the negative return path - so some of the charts were for positive length only and considered the negative to not have losses. This is not really a good way to wire in loads.

I generally size loads for a maximum of 2% loss for DC systems. Again, better to go too large with wiring then too small.


How do I know how much voltage drop is acceptable?

FWIW, I've got an aux fuse panel in the back of my LC, fed from my aux battery. 4# power and ground run to a Bluesea Safetyhub 100. Trying to determine wire size for the following:

1. 10amp ARB fridge. 10' total run roundtrip.
2. 55W/4.6amp Hella backup light. ~20' total run
3. ?? amp. Circuit run up to center console to power 12v outlet, dual USB charger socket and CB radio. ~15' total run
4. suresine 300W/25 amp inverter. 10' total run roundtrip.


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I've always used this little calculator with good results (it lets you input your exact amps and circuit length, then gives you a voltage drop percentage for the size wire you select).

I agree with Haf-E, 2% drop or less is a good number to shoot for (especially for things like inverters), though your fridge, CB, and the like should be fine with up to 3% loss. For lights I wouldn't go over 5% loss.