Most 50W FM mobile radios draw around 8A~9A at full transmit power. Yaesu radios (and I think most others) come with a 14AWG sized harness, usually about 9 feet (1.5 meters) long and are usually fused at either 10A or 15A. Going 12AWG would be fine for most any mobile and is a good idea generally and especially if you power run is longer than 9'.
If you're installing something larger, a 75W mobile or 100W mobile you should bump up to 12AWG if you can swing it. The FT-2900 draws about 15A at full power.
HF 100W mobiles like the FT-857 will draw 20A~22A at full 100W transmit in some modes, so use 25A fuses and at least 12AWG and 10AWG is not a bad idea.
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