Figuring out wire sizes

[AzTonopah]

I guess this question could go into the" Beginners "group or in 'Wiring". Here's my system as it stands now. 400 watts , 3 - 12 volt AGM 100 amp batteries, 40 amp MPPT charge controller powering 12 volt lights and fan in camper trailer no inverter installed. My question is how do I know my gauge of wire at each system? Batteries size wired in parallel , wire size from batteries to 100 amp breaker to fuse block. Do I make wire calculations at each system. I see a lot of systems where they use 2/0 wire to connect the battery bank. I understand the AC wiring sizing but is 12 volt different?

 

[offgriddle]

Great question, the same one I have grappled with for my entire life. A search on the Internet brings up endless charts, graphs, theory, fact and discussion, agreement and disagreement on the subject! A couple rules of thumb hold true, the higher the voltage the smaller the wire is required to carry the same amperage. Also, copper wire conducts better than aluminum so aluminum wire has to be bigger to carry the same amperage as copper wire. From what I know to be true, this chart seems to be accurate in regards to DC wire gauge and amperage. https://www.tech.txdi.org/dcwiringchart

 

[L-P-G]

Wire gauge is mostly related to amperage. Simplest rule is take the max amps your system is capable of delivering and use that to figure out gauge, in your case you can use watts law to get that.

I = 400W/12V = 33.3A

(Note that I used the rated watts your inverter can deliver, you could also use the max amps your batteries can deliver if you plan to add more inverter capacity later on)

At 33.3A (lets round up to 40) you're looking at 6-8 AWG depending on distance ran. The distance comes into play due to voltage drop, the longer the cable the more your voltage will drop so bigger cable helps prevent the voltage drop (NEC calls for 3% max v/drop). Now that you have your numbers you can use the chart below to figure out what gauge you'll need.

http://assets.bluesea.com/files/resources/newsletter/images/DC_wire_selection_chartlg.jpg

 

[svetz]

My favorite of course is from the Frequently Asked Questions: in the link What wire gauge should I use?