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<... fuse/breaker is there to protect the wire ...>
Agreed
<... wire MUST be large enough to handle all the loads connected to it - plus a "fudge" factor of about 10 - 30% ...>
MUST is key here. 10% - 30% "fudge" factor becomes 30% - 50% if we are unsure of *precise* loads (which is almost always the case). Also, using the higher safety factor will reduce line losses to a manageable level
<...If fuse is too big for the wire, the wire will protect the fuse ...>
But only the wire is too small for the circuit - which can happen only if you select wire that is so small that although it will [juuust] carry the load, you will incur unacceptable line losses.
<... A #6 wire will carry about 54A - up to *33 feet* - without an excessive rise in temperature (35*C over ambient). A #4 wire will carry up to 72A under the same conditions ...>
My B+ is under FOUR feet. IIRC we are supposed to size wire to reduce line losses. Your 54A for 33 ft example would involve an unacceptable voltage drop in the line.
I think your example is a little specious ... Jim S.
Dale Rogers wrote:
Wendell Voto <jwvoto@itlnet.net> wrote:
Okay, I'm conviced, I'll go for 100% over rated current draw.
Wendell,
You are on the verge of being a victim of over-simplification. I'll try to be as succinct as possible and still cover the important considerations.
Rule number 1: The fuse/breaker is there to protect the wire. If too much current is pushed through the wire, it will melt. You don't want that to happen.
Rule number 2: The wire MUST be large enough to handle all the loads connected to it - plus a "fudge" factor of about 10 - 30%. Remember: larger wire == more weight, so you want to keep your wire sizes pretty close to the actual loads they'll be carrying. Over on one of the canard lists, someone put up a chart of how much current each wire size will carry. Probably Aeroelectric has a similar chart. Rule number 3: If your fuse is too big for the wire, the wire will protect the fuse. You don't want that to happen.
Here's an example of how to pick your wire/fuse combination:
The maximum output of the alternator is 65 Amps. A #6 wire will carry about 54A - up to 33 feet - without an excessive rise in temperature (35*C over ambient). A #4 wire will carry up to 72A under the same conditions.
Since your worst-case normal operation only passes 65A - and is usually a lot less - a 70A breaker should never trip unless something is seriously awry, yet still trip before your wire gets too hot from a short. Hope this is some help.
Dale R.
COZY MkIV #1254
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