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Old 02-14-2008
rhaley
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Quote:
Originally Posted by xtatico1404 View Post
catmount, you got the general idea.

Yes, the fuse is intended to protect the wire AND to a certain point, the load (whatever is connected to it). I say to a certain point, because it protects the load to faults (short citcuits) outside of the load itself. It will not protect the load from internal faults.

Also, the fuse size is NOT based or related to the the wire size/gauge. The fuse size depends on the amperage of the load that is connected to it. Say you have a lighting load that draws 5 Amps. According to wire ampacity tables http://www.ancorproducts.com/support..._Amperage.html
you can wire it with #18AWG (if less than 3 conductors and not taking into account wire length), which will give you 20 Amps outside engine areas, but your fuse will have to be a 5 Amps fuse at a minimum. However, recommended practices call for a fuse or breaker to operate at 75-80% of it's rated capacity. So 5Amps + 20% = 6 Amps, so you would have to go with the next size up which is 7.5 Amp fuse.

Also, please first look at the recommendation of the equipment manufacturer as to wire size & fuse size. When you are talking about electric motors (pumps, cooling/heating equipment, windlass, etc) the protection requirement is much higher. A respectable manufacturer will tell you what wire & fuse size to use.

As to why starting on the battery, because that's where you have all your enegy stored onboard. Say you accidentally short the wires on a battery, if the fuse is not in place, the short circuit will continue until the battery depletes it's energy, and once again we go back to the big bonfire thing
Think of it as the lock on your front door, you have to start protecting from the very begining.

Just to give you a good idea of the energy dissipated on a short citcuit:
http://teslamania.delete.org/frames/...#500_kV_Switch
http://teslamania.delete.org/frames/longarc.htm#Blowup - the fire you see is caused by the oil inside the transformer that escapes under pressure and gets ignited

And just in case you thought that it only happens at high voltages, here is a 480V short circuit...the lower the voltage, the higher the current.
http://teslamania.delete.org/frames/...volt_arc_flash

Serious stuff, so take electrical systems seriously. Be safe please.
There are a few points that I disagree with here, one of which is that the fuse size is not related to the wire size. The wire size most certainly dictates the maximum size fuse. You may fuse the wire at a lower current rating than the wire itself, but the fuse protects the wire, not the load. The 80% rule of thumb for the protective device applies to the rating of the wire, not the device on the other end. As before, you can fuse lower if you prefer. A short circuit between the source and the load will expend it's energy before the current reaches the intended load (hence the name) and at the relative voltage levels found on most boats, will have no effect on the load other than not providing it with enough energy to function.

One point I absolutely agree on is the fusing of the battery as close to the battery bank as possible. As xtatico1404 points out, this is the beginning of all your stored energy and a short circuit there has a huge fire risk. This should be non-negotiable and standard practice every time you start to layout an electrical system.

Good Luck!

Last edited by rhaley; 02-14-2008 at 12:00 PM.
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