ABYC publishes a table of wire gauge based on length and load with 3% Voltage Drop, and 10% voltage drop. Here is a link to the tables in PDF
For example, given a 5A load (the lowest rated load in the table) and AWG 14 wire (what the OP has) the longest allowable run with a 10% voltage drop is 80'. If you want to tighten the spec to 3%, which as fairbank56 pointed out is required for navigation lights, the longest run for AWG 14 is 20'.
If the OP hasn't already purchased the wire, I suggest he consider going down an AWG size.