Vic:
To do this empirically, you can find tables of DC resistance of wire @75
degrees C per 1000 feet in the National Electrical Code (Chapter 9, Table 8
1990 NEC for example). Here are some numbers for your scenario (assuming 16
feet of copper wire):
#12 is 7.77 ohms per 1000 feet: 16*7.77/1000 = .124 ohms * 120 amps = 15
volts .... real bad! -number provided for fun :-)
#4 is 0.308 ohms per 1000 feet: 16*0.308/1000 = 0.0049 ohms * 120 amps =
591 mV
#2 is 0.194 (blah blah...) = 372 mV
#00 is 0.0967 = 185 mV
etc....
Again, the values given are for wire @ 75C (assumes temperature rise and
ambient). You probably won't be subject to too much rise unless you're
going to be running RTTY or you use something much smaller than #4 AWG. By
the way, aluminum conductors are typically 50% higher in resistance
compared to the same length of copper conductors so be careful if you
decide to use aluminum range/dryer/service entry cable! You'd need a
minimum of #2 if you do use aluminum.
Running real big wire doesn't buy you much past #4 copper in my opinion.
The NEC requirements for wire size take the typical length of branch
circuits in to account when they specify the wire size for a given branch
circuit breaker trip current. It is still up to the designer/installer to
assure that the voltage drop of a given circuit is within acceptable norms
(3-5% maximum on a branch circuit). Bottom line is they won't let you go
smaller than what their tables say for a given circuit, but you often have
to go bigger if the circuit is long.
Hope this helps,
Bruce R. Knox W8GN
RF Applications, Inc.
-----Original Message-----
From: Vic Rosenthal [SMTP:rakefet@rakefet.com]
Sent: Wednesday, February 02, 2000 22:50
To: Amps reflector
Subject: [AMPS] Wire size
I'm going to power a solid state amp with about 120 amps of 13vdc. I'll
need
about 6-8 feet of wire to get from the PS to the amp. What wire size
should I
use?
73,
Vic, K2VCO
Fresno CA
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