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Ed,
It sounds like your hung up on local issues but the rules were made to cover a much broader range of equipment. They go from hoists and cranes that you don't fuse at all to plating machines that pump 15 volts to an anode that will suck up 900 amps. And God only knows what NASA is doing. So let's start with the equipment. Your going to install a 5000 dollar wizzbang that will use 175 watts or 14 amps. The first thing you want to do is find out if the wizzbang has internal overcurrent protection. If it doesn't then you'll probably want to buy a different product or put an inline fuse at the appliance itself. That will take care of that. That brings us to rule one. I won't go into wire length or wire resistance. You can get that from Bob N. if you want. "Size the wire to 125% or LARGER" than it has to be to carry the 14 amps you are going to need, that being 17.5 amps. So you are going to want the wire to be able to carry 17.5 amps. 14 Ga. wire will do this. Now rule two. Fuse the wire to 80% of it's ampacity ( OR LESS ). So now we take 20% percent off of the 20 amp ampacity of the wire to come up with 16 amps. Obviously a 15 amp breaker or fuse will fit right in there. Now what have we done ? Well, I would like to change the word protect to the word allow. The battery will pump 300 amps thru the system if it is allowed to.
As in short circuit. By putting a wire from the positive terminal to the negative terminal we can see the danger we would put ourselves in if we allowed unlimited current to flow thru our tiny little wires. In summation we want to allow only the amount of current to flow thru those little wires as we need, and no more than that. That is what these two rules accomplish. They were written in the National electrical code and based on physics. And as we know the laws of physics will not be denied, except by quantum mechanics. But we are not building obam aircraft that require quantum mechanics,yet. HTH
Larry Mac Donald
lm4@juno.com
Rochester N.Y.
Do not achcive
Max. current for 14 Ga. wire is 20 amps and
On Fri, 20 Oct 2006 18:12:11 -0400 "Ed Anderson"
<eanderson@carolina.rr.com> writes:
One thing I have never really understood regarding fuses (or CB for that matter) is - why you would use a fuse/CB with the rating set to protect the wire!!! If you have a system operating off a power wire, it is highly likely that the wire will flow much more current than the system requires by quite a bit. So if you select a fuse or CB rating that protects the
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