As Ryan posted, an
'ampacity calculation' is certainly the
by the book method to do so, where you make sure your ampacity can handle the maximum worst case amp load. That 'worst case' is where every circuit is energized or ON. Most items list their amp draw, but if you have something like a light rated in watts, simply divide the wattage by 12.6 to get the amps.
Also remember that the total ‘ampacity’ for your circuits will be the total length
FROM the battery
TO the load and
BACK again, so if 10’ away, that’s at least a 20’ run for factoring the correct wire size.
Too many miss this key detail ... of doubling the circuit length!
If critical circuits, like Nav lights, VHF radio or bilge pump, use the 3% loss value coulmn. NOTE on my bigger boats, I run two main circuits, for critical items and then house items (deck or cockpit lights, am/fm radio, or livewell pump etc.), otherwise considered as non-essential to the boat's running and primary safe operation.
But FWIW I've equipped many a small boat to 14' of typical wired accessories with main runs of 10 AWG without issue.
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