[Elecraft] Elecraft Digest, Vol 114, Issue 11

[Neal Enault]

If you look at most wire specifications (military in particular), you will note that the temperature rating of the wire is based on the conductor temperature which is the combination of current heating and ambient temperature.  To determine the effects of ambient temperature, one has to do some not so accurate calculations or refer to a document like MIL-W-5088 which also provides guidance for wire ratings in bundles and at various altitudes.
 
Neal WA6OCP
 

________________________________
 From: David Cutter <d.cutter at ntlworld.com>
To: don at w3fpr.com; elecraft at mailman.qth.net 
Sent: Monday, October 7, 2013 3:33 PM
Subject: Re: [Elecraft] Elecraft Digest, Vol 114, Issue 11
  

So far nobody has mentioned insulation material.  The rating of a cable has little to do with melting the wire and mostly to do with melting or softening the insulation which would create a shock and/ or fire hazard. The same size wire insulated with PVC is allowed to rise to a much lower temperature compared to insulation of PTFE or numerous other materials.  And what is good or bad for mains circuits will not be the same in a vehicle or aircraft circuit.

David
G3UNA




----- Original Message ----- From: "Don Wilhelm" <w3fpr at embarqmail.com>
To: <elecraft at mailman.qth.net>
Sent: Monday, October 07, 2013 11:14 PM
Subject: Re: [Elecraft] Elecraft Digest, Vol 114, Issue 11


> OK Joe,  You are correct.  It has been a very long time since I have been involved with heat related science issues.  More current equals more heat (for a given resistance) has been sufficient for me most of the time without getting into the exact math.
> 
> It is good to know that some folks here still have those formulas on the top of their heads.
> 
> 73,
> Don W3FPR
> 
> On 10/7/2013 5:57 PM, Joe Subich, W4TV wrote:
>> 
>> On 10/7/2013 5:31 PM, Don Wilhelm wrote:
>> > Geesh, how is that possible - twice the current equals twice as much
>> > heat and will exceed the capacity of the wire.
>> 
>> Sorry, Don ... twice the current => *four times* the heat. Heat
>> (energy in Joules) = Watts * seconds.  One Joule (one Watt-second)
>> is 2.78e-7 kW*h
>> 
>> 73,
>> 
>>    ... Joe, W4TV
> 
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