This is only true if you've wired the circuit with wire of a heavier gauge than necessary for the load and run length. Generally, I prefer to oversize the wiring on my boat by a bit...going up a size in wire gauge, since oversized wiring is generally not the problem undersized wiring is, and it reduces the voltage drop at the terminal end as a bonus.
SD, this is where real "know how" is important on electrical systems and unfortunately where people make mistakes. Please don't
take that as me saying that your statement is incorrect. BTW, I totally agree with you on oversizing & the 80% factor.
People need to properly fuse and protect equipment. Saying that fusing is done accroding to wire size, is incorrect. Fusing depends on the location as well as the situation. A circuit with a single load might be fused differently than a circuit with multiple loads. Further more, circuits with electric motors on them are fused completely different. I always recommend, when installing a single load on a circuit, read the manufacturer's recommendation
regarding fusing. It is much better to take the time to get really informed or pay some one to do the job, since electricity is serious business and an improper installation can lead to problems, including fire.