[TowerTalk] CAT 5 as Rotator cable, sprinkler wire

[Jim Lux]

At 05:05 PM 6/18/2004 -0400, Pete Smith wrote:
>At 03:55 PM 6/18/2004, wa3gin wrote:
>>Some day you should go measure the voltage at the rotor and compare it to 
>>the voltage at the control box.  The results might lead you to upgrade 
>>the guage of
>>wire you're using.  Publish the results for the list.
>
>
>Anyone who is interested in this and hasn't already done so should take a 
>look at the copper wire table in any ARRL Handbook.  It gives ohms/100 
>feet for all AWG sizes, which makes it very easy to figure out whether you 
>have enough copper for the motor leads.
>
>
>73, Pete N4ZR


Don't even need the handbook:

AWG 10 = 1 ohm/1000 ft  (0.1 inch in diameter)
every 3 gauges is half the area, so twice the resistance:

AWG 16 is therefore 4 ohms/1000 ft.

every 6 gauges is half the diameter, so AWG 16 is 50 mils in diameter.


A more practical question to be answered here is "how much voltage 
drop/power dissipation is acceptable".  I suspect that applying a rule like 
"no more than 2% drop" (as used in AC wiring) is overly conservative.

Say the motor runs at 24V and draws a couple amps.  Say you're a real cheap 
guy, and want to use some of that AWG 22 wire you have lying around for the 
200 foot run.  400 ft of AWG 22 will have a resistance (@ 16 ohms/1000 ft) 
of 6.4 ohms.  You'll drop about half the voltage in the wire, but, I'll bet 
the motor still turns, just slower. (The brake, if there is one, might not 
disengage, though.)

The large voltage drop might also cause problems if there's some sensing 
circuit (i.e. a pot) that shares a wire coming back.

You could boost the voltage at the sending end to 36 volts to make up for 
it too (although, you have to watch out, because if the load varies (which 
it does), then the current varies, changing the voltage drop. At light 
loads, you might wind up with too much voltage at the far end.

Then, there's also the "insulation melting" problem.  2 amps * 6.4 ohms is 
about 26 Watts, disspated over the 200 foot length of your cable, or about 
0.13 W/ft.  Probably not going to melt the insulation, but the wire might 
get warm.

Just as a data point, I've noticed cheap 16 AWG extension cords getting 
warm at loads of around 10 Amps.  That's 0.4W/ft. (and, ahem, the voltage 
drop in a 100 ft cord is 8 Volts, well exceeding the 2% guideline)