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Jim,
I'm more than a little puzzled at your purpose in cross-
posting your response to the COZY_builders ML. What sort
of grudge do you have that you want a dispute to spill over
into that venue?
Specious? Did you actually comprehend what I wrote - rather
than just skim-read a few key words?
What part of "up to" equated in YOUR mind that such a long
run was required or even advocated?
Moreover,
> 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
In the case of the load on the B+ lead to the alternator,
(the fusing of which was the original topic under discussion)
the precise load is well known. It's the maximum output of
the alternator being used.
And *why* wouldn't you know the "precise" values for the
load(s) of the device(s) you are connecting to the circuit(s)?
Every device has a rating for maximum current draw. That load,
or the sum of the several loads connected to a particular
branch, are what the circuit should be designed to safely
carry.
Dale R.
COZY MkIV #1254
>
> From: Jim Sower <canarder@frontiernet.net>
> Date: 2005/01/23 Sun AM 01:45:54 EST
> To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
> Subject: [FlyRotary] Re: Fuel Pump Load
>
> <... 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.
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