Join Date: Aug 2006
Location: Arlington, VA
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Re: Fusing to the size of the wire....
Fusing to the size of the wire is a good overall concept, but it should not be applied slavishly. As Eric intimated, there are times when you would want to use a smaller -- even much smaller -- fuse than the wire size itself would otherwise dictate.
For example, I routinely install SSB radios having a maximum peak draw of 30A using AWG6 cable for runs up to 20' and AWG4 cable for runs over 20'. These cables have an ampacity of 120 and 160 amps, respectively (outside engine spaces). But, as these are cables dedicated to the SSB and it's maximum draw is 30A it would be foolish to use a big fuse or breaker for this purpose. I use 40A ANL and, sometimes, 30A or 40A MRBF (terminal) fuses.
Nothing wrong with using fuses or breakers rated LESS THAN the wire's ampacity, so long as you have definitive knowledge of the anticipated load(s) to be placed on that cable.
Never use fuses or breakers rated HIGHER than the ampacity of the cable they are to protect.