Date:         Mon, 31 Aug 2009 18:49:49 -0400
Reply-To:     David Beierl <dbeierl@ATTGLOBAL.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         David Beierl <dbeierl@ATTGLOBAL.NET>
Subject:      Re: Wire size question.
Comments: To: Rocket J Squirrel <camping.elliott@GMAIL.COM>
In-Reply-To:  <4A9C410F.4020209@gmail.com>
Content-Type: text/plain; charset="us-ascii"; format=flowed

At 05:30 PM 8/31/2009, Rocket J Squirrel wrote:
>The differences between your ampacity chart and the one I referred to
>might be what one can expect stranded wire (which has less copper
>cross-section) versus solid-core wire.

Also maybe different DC vs AC, 12v vs 120, wire in free air vs in a
bundle, type of insulation, different applications, different codes.


>But the writer talked about how current is carried on the surface of the
>wire and he right -- at high frequencies, I've never heard anyone describe
>a "skin effect" at DC, so I am not certain whether that writer knows his
>onions or not.

No skin effect at DC.  It starts to have a perceptible though slight
effect at audio frequencies.  Hams working on 6-meters (~55 Mhz) or
higher often use silver-plated copper tubing cause they can get a lot
of surface that way.


>If in doubt, when looking at two ampacity charts, use the one that is more
>conservative. Your chart shows 8 gauge capable of carrying 150A for "short
>runs" at 12v and 46A at 110V, and the difference must be due either to
>worrying about heating effects

Heating effects is what ampacity is all about -- that's why it's more
or less length-independent but changes when you change insulation or
start bundling wires together.


  --
David Beierl - Providence RI USA -- http://pws.prserv.net/synergy/Vanagon/
'89 Po' White Star "Scamp"