Either I am more daft than I realized or you guys are having fun at my expense. Let me know which one it is because this is not making sense to me.
I'm honestly not. I'm Master Electrician and electrical contractor, also into electronics, especially audio/video, I have been installing car stereo and cruise controls since the days of 8-tracks. I realized back when I was an electrician's helper that I understood electrical theory much better than the guys I worked under, but I did learn the nuts and bolts of electrical work from them.
I started in electronics at age 5 when I built a foxhole radio from a book I got from the school library, I have scratch-built and kit-built many items over the years, modified equipment like adding a headphone jack to my brother's stereo record player, ran phone lines in my parents' house, and wired their shed. I also rebuilt a Corvair engine (my first car) when I was 18.
Sorry for giving you my resume, but I want to assure you my intentions are to educate, not poke fun. You have to keep in mind that theoretical discussions are based on nominal values, not necessarily actual, voltmeter-read voltages. Theory is a tool that must be applied correctly to real-world applications, so all of it is important to understand.
Added: I'm also the fifth-highest poster on the Mike Holt NEC forum.
It is my understanding that the fuse in a circuit, regardless of the actual load in the circuit, should never exceed the maximum capacity of the wiring in that circuit. If the stated 75 watts per light, a total of 150 watts or 12.5 amps per pair, is the actual design limit of the wiring as stated on their website, I don't see how a 15 or 20 amp fuse can safely be used in this circuit. Given that the fuse should always be a value less than what the wiring can handle this limits this circuit to a 10 amp fuse.
Your understanding is correct; it's the application that needs more detail. You always start with the load to be served, and work backwards to design the power supply and its protection. They may be claiming that their wiring is designed to carry a maximum of 150 watts, but that may include a bit of headroom to avoid damaging the wiring, and not that 13 amps will melt the wire.
The whole purpose of circuit protection is to avoid heat damage to the insulation and terminations. In code-compliant wiring, a circuit should be designed to never to carry more than 80% of its capacity on a continuous (more than three hours) basis. That's the built-in headroom that takes variables into account, and helps keep from starting fires (the greatest risk of electricity).
They state on their website that a light bulb of maximum 72 watts can be used in these lights. This equates to 12 amps per pair of lights which would seem to be in support of the 12.5 amp limit they state that their wiring is designed for. They do not list different wire sizes based on light bulb selection that I saw. All of their wiring appears to have the same design limit of 12.5 amps.
The wire may be sized to safely carry 12.5 amps without damage, and not that it's on the verge of destruction, and any more will cause it to burst into flames. I would recommend at least a 14-gauge wire if I had a load of 12.5 amps; 12-gauge would be even better, because voltage drop (caused by current and resistance) is more detrimental (to load operation) at lower voltages.
If they do supply kits equipped with and intended to use 20 amp fuses the same rule of not having a fuse exceed the capacity of the wiring still applies. A 20 amp fuse means that a load in excess of 240 watts would be required before this 20 amp fuse would blow. This amount exceeds their stated wiring design limit by 60%. This seems far to excessive to be reasonable. Even if the stated wiring design limit of 150 watts is only 80% of the total as speculated, that means that the total (100%) would be 188 watts. This is still 52 watts less than the 240 watts needed to blow the 20 amp fuse. Does this not mean that if something were to go awry the wiring will burn up before the fuse blows?
A 20-amp fuse would be a good rating for a 12.5-amp continuous load, but the wiring must be sized to the fuse is the weakest link. None of our bikes have fuses that are loaded near their maximum capacity; that would be poor design. However, there have been plenty of reported issues with some of the factory terminations overheating not related to choice of fuse size.
It's easy to forget that current over time affects materials. All electrical wire and terminations get warm when carrying current. When heated well, metals lose their tempering and spring tension, and loosens connections that depend on springiness to remain tight. Loose connections get hotter, and the effects eventually snowball and open connections and/or melt plastic.
My understanding is that either what is written on the website is incorrect or this circuit should not be exposed to a load exceeding 12.5 amps. I don't know which is correct. I would be tempted to not exceed a 10 amp fuse until the answer is determined. The highest wattage bulbs available from MotoLights is 50 watts. Two of these is an 8.3 amp load. A 10 amp fuse is sufficiently sized to handle this load with a reserve capacity of 1.7 amps safety margin available. I would think that would be sufficient to avoid nuisance failures.
Possibly. Do we know what gauge wire they use? If you were to tell me the specs of a given load, I could design a safe and functional circuit to supply it. For a pair of 50-watt lights, I would use 12-gauge wire and a 15-amp fuse, even though 16-gauge wire is large enough for an 8.3-amp load. Again, it's poor design to use a circuit near its maximum capacity.
I would hate to see the OP use an oversized fuse and risk damage until he knows for sure.
Without more info, one must default to manufacturers instructions and recommendations.