Marine wire is indeed UL listed, but my understanding is the UL number is not accepted for RV code purposes. It has to be listed under another UL number for RV use. ...
... marine wire ... is thinned for corrosion purposes and U/L rated ...
OK, there is a LOT of confusion about this subject, so let me clear it up.
First of all Underwriters Laboratories (UL) has nothing to do with the NEC (developed by the National Fire Protection Association, NFPA) and vice-versa. UL testing something does not make it acceptable under the code, nor does anything need to be tested by UL to be acceptable under the code.
UL is a private testing laboratory. It has competitors, including other private testing laboratories in the US and abroad, as well as testing laboratories run by the governments of other nations (the US has a private commercial testing laboratory system, whereas some other countries require products to be tested by government or quasi-governmental agencies). Other common laboratory marks you might see are CSA (Canadian Standards Association), DIN (Deutsches Institut für Normung), TuV (Technischer Überwachungsverein), and many products carry additional acceptance marks such as SAE (Society of Automotive Engineers), CE (Conformite Europeenne), ASTM (American Society for Testing and Materials), etc.
When any agency tests a product, they test it for its suitability and safety for a specific purpose
. The stated purpose is supplied by the manufacturer when the item is submitted for testing. Some items might be submitted for testing to multiple purposes, and if all tests are passed, the item will be certified for all those purposes. An item with a testing hallmark such as "UL" is only certified for the tested purpose. You can have a UL-listed hair dryer and a UL-listed pool heater; the hair dryer is not safe for heating the pool, and the pool heater is not safe for drying hair.
Likewise, if you open the walls of your house you will likely find that your telephone cable is UL listed. It's safe and suitable for carrying phone signals, not 120-VAC power. Similarly, the power wires, while not unsafe, are unsuitable for carrying phone signals (phone wire needs to be twisted-pair, and power wires are untwisted).
UL "listing numbers," by the way, are unique to each product, and do not reference any specific standard. So if a manufacturer of wire wants to mark the wire THHN and THNN, he would submit the wire to UL and ask for testing to both standards. The wire would be issued one single, unique listing number, and only by calling UL and asking them would you be able to learn exactly what standards the wire passed.
So to apply this to the subject at hand: Boat cable is tested and listed by UL and/or other testing agencies for use on boats
. There is a specific standard for this and it is very detailed. It is different from the standard for, say, NM cable, which is approved for use in homes and RVs. Can the same cable meet both standards? Possibly, but here's the thing: it costs tons of money to have UL or anyone else test a product to a specific standard. Moreover, when it comes to wire, you need to also develop (at some expense) the full documentation on how the product must be used. So in this case, for example, you'd need to specify how the wire should be terminated to devices, breakers, etc.
The reality is that the market for boat cable to be used outside of the marine industry is minuscule, and it's just not cost-effective for the manufacturers to pay for this testing. The vast majority of the RV market uses standard NM, and they are simply not going to pay the premium for fine-strand, tinned wire. That leaves a small handful of bus converters who might pay such a premium (though frankly I do not understand why).
What's perhaps more important to note here is that, because it has not been manufactured with the NM standard in mind nor submitted for this testing, we simply don't know whether or not it meets the standard. Perhaps the insulation would not meet the smoke characteristics. Or perhaps the cable can not be made to terminate safely on certain styles of NM-compatible circuit breakers. Without the testing, we don't know, nor do we have proper instructions for connecting it, and therefore can not conclude that such installations are safe.
Particularly problematic here are home-brew bastardizations wherein marine components have been intermixed with residential components. While not code-compliant, an installation using 100% marine materials and conforming to ABYC guidelines (see below) such as Clifford described would likely be safer than such a mixed system.
Electrical on a bus is a cake walk compared to boat RV codes will allow Romex and real cheap @$# fixtures,switches and plugs ...
OK, I just had to respond to this piece, too, because it makes it sound like the RV code is more lax than the practice in the marine industry.
The fact is, there are no codes whatsoever
for pleasure boats. None, zip, zero, nada, period. You can wire your boat with baling wire on knob-and-tube if you want, and there is not a thing anyone can do about it. Lots of people and agencies would like to see this changed, because boat fires due to faulty electrical installations are fairly common and deadly (and, in a marina, much more likely to spread to neighboring vessels).
The closest thing we have in the pleasure boat industry to a "code" is a set of guidelines promulgated by the American Boat and Yacht Council (ABYC). Unlike the NEC for shore-side facilities, which is law in most jurisdictions and thus subject to legal enforcement, these ABYC guidelines are 100% voluntary. I personally know of no pleasure boat manufacturer that follows the ABYC guidelines to the letter in their entirety, and even fewer aftermarket installers, including boat owners. Some of you may know that I am shopping for a boat right now, and I have seen every conceivable kind of unsafe wiring on boats, including circuits with no fuses or breakers at all, and some of it was done at the factory. Think about that the next time your buddy takes you deep-sea fishing.
FWIW, the fixtures, switches, and receptacles used on most pleasure boats are exactly the same "cheap @$#" ones used by the high-end RV industry -- the ones you buy in the bulk pack at Home Depot. (Lower-end RVs use "integrated devices," which are a PITA but not inherently bad). And I would argue that Type NM cable (aka "Romex®") is not any better or worse than Boat Cable when used properly for the intended purpose, just less expensive. Both types will fail if installed improperly, and both types are subject to most of the same limitations and susceptibility to physical damage.
As Clifford says, this subject has been beaten to death here in the past. Lot's of people have the opinion that boat cable is somehow better than NM and therefore they consider it safer to use, code be damned. I can probably argue that braided stainless hoses with aircraft fittings are inherently safer to use than DOT nylon air brake line, too, but if the hose is not stamped DOT it's not legal. Probably no one will notice unless someone dies, and then all hell will break loose.