Folks, just a gentle reminder that breakers protect
wires, not devices. I worry especially that some of the linked-to articles may be written for more skilled readers, and novices might interpret them the wrong way, especially in the context of RVs.
Breakers prevent electrical fires by preventing wires from getting so hot that they melt and/or creep out from things like screw terminals and cause arcing. You size wires to devices, and breakers to wires. It seems like a nuance, but there's some language here and in the linked articles that makes it sound like you size breakers to devices, and that could lead to some dangerous decisions. One of the articles makes it sound like breaker selection is a flexible thing. Once a wire is installed, it's absolutely not flexible. A breaker should never be up-sized to support a larger load without replacing the wire. It could start a fire.
The "80% rule" is not (directly) about safety. The folks who author the NEC think about this stuff on multiple levels and they incorporate records of past problems in their guidelines. "Nuisance trips" is one of those things. There's no mechanical/electrical reason a breaker MUST be undersized e.g. to 80%. It's perfectly fine ("safe") if it trips after a few hours. Your house won't burn down. But breakers/fuses tripping too often have led to problems in the past when homeowners would shove pennies or screwdrivers in there to stop it them from tripping, just so they could get their clothes dry for work tomorrow. They make bad decisions under pressure, and reducing that is still a safety improvement even if the breaker, wire, and device were fine either way. So they made allowances for that with very explicit language like the "Continuous load" (3 hour) rule.
The bulk of the NEC and its tables are "prescriptive". All of this stuff can be determined much more accurately with actual math, but it takes a long time to do it right and it's really easy to make mistakes, so they boiled all this down and gave us rules and tables to follow. If we stick to them, we have plenty of safety margin built in, and an electrician can extend a circuit and a homeowner can pay a reasonable $200 for that. The electrician can say "well it meets code, and the inspector approved it" and sleep well that night, and the homeowner can grumble about the $200 without realizing it could have been "$1000 plus you might still die in a fire," and everybody wins, even if they don't always see it that way.
Unfortunately, RVs throw all of this right out the window. RV manufacturers do not need to comply with the NEC, and some like Heartland seem to almost gleefully defy it. The wiring in the basement of our Milestone would make an electrician weep. You really can't trust or rely on anything in an RV because they just run things however they want, and nothing gets inspected. Every single circuit requires investigation and careful thought, and I would just suggest that we all be thoughtful about quoting the NEC or "my professional electrician friend" because while they might be right, they might not even realize how many corners the manufacturer cut.
I worry that some readers might try to apply "NEC" rules to a system that never complied with the NEC in the first place, and assume they're safe. When we link to articles that support these positions (but were written focusing on household systems our RVs rarely comply with anyway) it might add weight to something that was wrong to begin with - not our fault, of course, but still. Food for thought. Anyway, sorry this was so long, but it might be worth considering.
