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From: europa@avnet.co.uk
Date: 2 May 1997 06:30:50 +0200
Subject: Returned mail:
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Subject: Returned mail: Too many hops 26 (25 max): from
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554 Too many hops 26 (25 max): from <europa@avnet.co.uk> via
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From: europa@avnet.co.uk
(8.7.5/CF5.38H)
(8.6.8.1/CF5.26R)
Date: 2 May 1997 05:58:23 +0200
Subject: Re: wire sizin
Mail*Link(r) SMTP Re: wire sizing
owner-europa using -f
Date: 01 May 97 23:42:36 EDT
From: "Robert L. Nuckolls, III" <72770.552@compuserve.com>
Subject: Re: wire sizing
/Now, here's a brain teaser for you. Why does the
/path length for 5% drop get longer as the wire diameter
/increases???? You guys who have read the book stand
/by here . . . let's see if anyone can deduce the reason
/from what's been published above. Hint: What is the
/mechanism by which a warm object sheds heat energy into
/the surrounding environment?
I've had a few folk nail it but most have offered
possiblities wrapped around better ability to reject
heat because of larger surface . . . actually the
REVERSE is true . . . . Let's look at the wire
table again
/ AWG Ohms/ 35C Rise 10C Rise Max Path
/ No. KFeet Amps Amps for .7 volt Loss
/ at 35C rating.
/
/ 2 .156 100 54 45 Ft
/ 4 .249 72 40 39 Ft
/ 6 .395 54 30 32 Ft
/ 8 .628 40 20 27 Ft
/ 10 .999 30 15 23 Ft
/ 12 1.59 20 12.5 22 Ft
/ 14 2.53 15 10 18 Ft
/ 16 4.01 12.5 7 14 Ft
/ 18 6.39 10 5 11 Ft
/ 20 10.2 7 10 Ft
/ 22 16.1 5 8 Ft
Remember from my earlier post, I said that every 3
steps in AWG number is a 1/2 or 2x factor in diameter.
So, lets compare ratings for 10 and 16AWG wires.
If 10AWG wire has 4 times the cross sectional area
of 16AWG due to twice doubling, you would expect it's
current rating to go up by 4 times too. But no, it's
30/12.5 or only about 2.4 times ? ? ? ? ?
While the cross section went up by a factor of four,
the actual diameter would have gone up by only 2 times.
Recall that area goes up and resistance goes down
by the SQUARE of diameter, outside area (and ability
to reject heat) goes up only DIRECTLY with the
diameter. So, larger wires have LESS outside surface
with respect to their increase in cross section which
prevents you from using all of the wire's apparent
ability to carry more current.
However, since voltage drop is a DIRECT function
of cross sectional area, the derating for limited
heat rejection works to our advantage with respect
to losses . . . and we can run longer path lengths
and stay inside the nominal 5% loss allowance.
It's a subtle geometry problem . . . .
Bob . . .
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