Re: Which fuses should I use?
The advice to size the fuse based on protecting the wire is not wise. Almost all electronic equipment is designed to have a fuse protect that piece of equipment from catching fire. Putting in a larger fuse will surely cause major grief one day. The fuse must be sized to the equipment need, not the wire size.<br /><br />Let me give you an example. All VHF radios have a diode across the power leads to protect against the user wiring the leads in reverse. Without the diode, the reverse voltage would destroy the entire radio. With the diode though, the reverse polarity causes the diode to short out and as such, the voltage stays near zero (really .6 volts), avoiding damage to the radio. However, the diode is only rated at a few watts and if the reverse voltage is maintained across its leads for too long, it would heat up to the point of potentially starting a fire and almost assured lead to distruction of the diod in the shorted out state (i.e. the radio will no longer work even if you properly wire it up). The inline fuse for the radio plays a critical role here by blowing up once it faces the current accross the diode and hence, there is no chane of the diode heating up.<br /><br />In the above example, the fuse would be rated at say, 6 amps. If you try to protect the 16 guage wire leading to the radio with a 15 amp wire for example, you would surely lead to shorting out the above diode and major risk of fire due to heat generated. I can't tell you have many VHF radios I have repaired that had completely charred the circuit board after it shorted out.<br /><br />The proper way to wire such a circuit is to use a circuit break to protect the wire going to a fusebox. Then use the appropriate fuses to protect each electronic device. Alternatively, you can use inline fuses but that gets messy as there is no convenient way to find them all should something go wrong.<br /><br />And as been mentioned, you must look up the correct fuse size for each device. For lamps though, you don't have the above issues above as there is little chance of them shorting out so protecting the wire should be OK.<br /><br /><br />Amir
. But the example would still stand if someone protected the wire with a thermal breaker which has a trip time of 1 second or more. In this case, you will be driving some 170 watts into the diode for a full second, rather than 70 watts for a fraction of a second wit the fuse. A 15 amp breaker would be more than compliant with the code to protect the wire, but clearly not a good idea in this case.<br /><br />I could also go on about many other scenarios where something could internally short out and use up above 6 amps but not more than 15 amps. And the increased heat generated could easily mean the difference between a fire and not.<br /><br />The section quoted from bluesea btw, is not applicable to the point I am making. What they are saying is factually correct in keeping the boat wiring from catching on fire and preventing false trips. They say nothing about what could happen to the equipment itself, should it fault. <br /><br />If the idea of putting the same fuse on every electronic gear were correct, then why on earth would every equipment manufacturer spec a different value? Surely they know more about the equipment they have designed than bluesea 
