[TowerTalk] RMC-1000 remote - what type of cable to run?

[Jim Lux]

On 11/6/14, 11:55 AM, Howard W6HDG wrote:
> Remi just revised his response to me after 5 minutes :)
>
> He wrote, "I stand corrected, the cable do carry 120 volts, and should
> be #20 7 or 8 conductors. It only carries the signals for the relays, so
> no high amperage. And there is no voltage when the control box is turned
> off after extending or retracting."
>
> I then emailed him again and double checked that it could run in the
> same conduit with coax and rotor cables and he replied,  "Yes, as I
> said, it would be off while transmissions/radio operation ".
>
> So there is my answer....heavy duty rotor cable should do it and no need
> for a separate conduit.
>

Unless you want to be code compliant..

Can't run "line voltage" in the same conduit as "low voltage".

it's not a matter of whether it's live or not at any particular time, 
it's whether a short could occur between line voltage and other things.

The rules on "low voltage, limited energy" wiring (e.g. doorbells) are 
such that there's no fire hazard if the low voltage wiring happens to 
short to something (or itself).  Your typical doorbell transformer has a 
very high series inductance, so the output current is limited.

12V landscape lighting is sort of in another class: it's not limited 
energy (hundreds of watts in many systems), but it's low voltage.


So the concern addressed in the code is that your 120V line could short 
to a low voltage circuit which in turn is grounded or otherwise would 
carry a large enough fault current to cause a fire.

It *probably* is the case that the rotor control box has a fuse or other 
overcurrent protection in the line voltage going to the rotor, in which 
case the fire hazard is mitigated.

Then, you're left with the other reason for the "no line voltage and low 
voltage in the same conduit or box" rule: electrical shock.

a GFI could mitigate that hazard. As could an isolation transformer.
Or, a 120V:12V or 24V transformer at each end. (another form of 
isolation transformer).

I've used the 24V transformer thing when I needed 120V power at a 
distant location and where it had to be low voltage on the way for 
safety.  120:24 into the wires, then 24:120V at the load end (actually, 
I had taps on both ends to allow adjusting for voltage drop, because the 
wires were VERY long and had substantial voltage drop).

Then, there's the hazard of inadvertent contact between a power line 
(external to your house) and your antenna system.  That's why there's 
the rules about bonding, metallic jackets, etc.


In general, once you get past very simple systems, the whole "what 
circuits can share an enclosure or raceway" gets complex.

Hence, the general advice:  put line voltage in its own space, and 
everything else somewhere else.




(Jim W9YC will weigh in with all the OTHER reasons you want to be 
careful, in terms of EMI/EMC.. about which the NEC cares little)