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To: flyrotary@lancaironline.net
Date: Sat, 21 Oct 2006 12:21:06 -0400
Subject: Re: [FlyRotary] Fuse Ratings for Wiring??
Message-ID: <20061021.122106.1032.0.lm4@juno.com>
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From: Larry Mac Donald <lm4@juno.com>

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