Cable gauge size for 20' run

[tonyiiiafl]

I am thinking of running a separate circuit to the fly bridge of my 1994 CPMY for the electronics. I would make a run from the back of the selector switch which is under the stairs, up the wire chase and under the dash panel. I would place a battery disconnect switch up there so I could effectively shut off all electrical from the electronics while they are not used. I have:
9" RAY e97
12" Ray e127
Quantum 2 Dome
Raystar 130 GPS
Evolution Auto Pilot

What sized cable would you suggest? Would a number 4 be sufficient? Or should I go to a #2?

Thanks!

[Midnightsun]

None of those draw much current and I would suspect a 4 gauge is overkill. Put in the values in this calculator and then add some just to accommodate additions in the future. https://www.wirebarn.com/Wire-Calculator-_ep_41.html

[tonyiiiafl]

After I posted this I was thinking the same. I am going to try that link you sent. I am now thinking a 10 may be sufficient. THANKS!

[tonyiiiafl]

YEP! I am good for a 20AMP circuit with 13.8 volts to a 19’ run with #10, and 28’ with a number 8. 2% current drop. Even the 15 amp is high, but with figuring the 20AMP I have plenty of room. It will also save a few bucks too and make the run easier. Thanks again!

[Midnightsun]

I typically run #6 to any kind of small sub panel. Yes it may be over kill but better more than not enough. Todays accessories draw little in comparison to 20 years ago. Heck even spotlights are LED and draw minimal current.

[bud37]

You didn't say.....but I would run a nice big ground up there with the hot....that way you can have an extra clean ground bus that will come in handy, especially with electronics.......

[tonyiiiafl]

That will be done. Would you ground to the engine block ? I try and stay away from a rats nest of wires to the battery terminals.

[bud37]

Source of the post Would you ground to the engine block

Yes..
Ultimately the best way is to put a ground bus in the engine compartment and run the grounds to it , then just one ground from each of the battery banks and the engines to the bus. Then you can use that bus as a connecting point for the bus at the bridge etc. Just a cleaner way of doing it, makes for good connections.

[tonyiiiafl]

Actually, now that I think of it, I have a bus bar ground for the battery charger connecting the negatives on separate posts for each bank. I used #6 cable for the battery charger so I wouldn’t have any droop or current loss. I can ground there and then run to the fly bridge? I think that would be cleaner and better than going to the engine?

[Midnightsun]

I can ground there and then run to the fly bridge?

That's what I would do.