240.4(B) is one of the LIES in National Electrical Code. Almost every if not all manufaturers of thermal magnetic circuits breakers state that using a conductor that is less than the rating of the circuit breaker will result in the breaker tripping at a lower current level than it should. As someone who has extensive industrial electrical experience I have to know what Code rules are a bit of a lie. A lot of Code rules are based on acceptance of a life of 10 or 15 years in heavy service. This is not good for production if you have to require once every 10 years.

When reparing a 100 amp electrical service I insist on using #2 copper or #0 aluminum. A 100 amp service is either very lightly loaded or very heavily taxed and very rarely in between.

Remind your customers than when they heat up a wire that is "Code" size what they are doing is paying for electricity and not getting it. More electricity will reach the load if the wire is bigger than Code. Circuits will always live longer if the wire size is a bit bigger. In a corrosive location you also need extra wire size so that the conductor can be 1/3 eaten before replacement is required. Some corrosive such as ammonia gas are very hard on copper wire and even if you RTV ( silicone sealant ) the heck out of box lugs there still might be a way for corrosive gases to get inside of the cable.

Another LIE in NEC is that the cord for an underwater swimming pool light has to be long enough to permit replamping without draining a pool. If you have ever fixed a boat dock in 3 to 5 feet of water you find out very quickly that buoyancy keeps you from turning a wrench or a screwdriver. A pool is what NASA uses for zero gravity training. The problem is that when changing an underwater light you need both scuba gear and enough weights to be a poor excuse for the Titanic. There are only so many times that a homeowner can spend $500 on changing a light bulb before going out and buying a fiberoptic refit kit.

Pentair still makes 120 volt and 12 volt underwater incandescent light fixtures so that that can sell to cheapskates who cannot resist an alleged bargain. I am still trying to figure out who makes fiberoptic refit kits but I am sure that someone makes them.

Mike Cole mc5w at earthlink dot net