At 08:09 AM 9/2/2005, Joe Giacobello wrote:
>I am getting ready to install a second LM-470 and I have to run
>hardline, 115 VAC, rotor cable and Ameritron remote switch cable from
>the shack to the tower. My guess is the total run will be around 500
>feet. Since I know a lot of you have already installed runs of this
>length, I have the following questions:
>
> 1. Do I have to worry about voltage drop for the 115 VAC on a run of
> that length? The AC is needed for raising and lowering tower.
Yes. I assume you have a fairly good sized motor there? You want to see a
minimum of 110V and preferably 115V *at the motor*.
The "code" requires that the voltage drop be less than 2% from the panel to
the load (so, for a nominal service at 120V, you'd get down to about 117V..
a drop of 2.4V ).
In reality, the service voltage can be lower than the nominal 120V (at the
panel). Depending on the utility, it might be as much as 5% lower (i.e.
114V)... So, if you're at the "low line" condition, and you add in the 2.4V
drop for the 2% nominal, you're down to about 110-112 V, which is getting
towards the low end for the motor.
One thing to watch out for is if you've already got some drop in the feeder
to whereever you're starting from.
500 ft is a pretty long run, and you've got two basic approaches.
One is to put in a big, mondo cable so the series R is low enough to keep
the drop reasonable. If it's a 1 HP unit, it will draw on the order of
10Amps (starting current is what counts, and that could be 3x the running
current). Simple calculations give you a max total resistance of 2.4
ohms. For 1000 ft of wire (that's 500 ft out and 500ft back). AWG10 is 1
ohm/thousand feet, so that would probably meet the requirement. Maybe even
AWG 12.
The other approach is to use a voltage boost/buck transformer at the far end.
http://www.grainger.com/Grainger/productdetail.jsp?xi=xi&ItemId=1611603427&ccitem=
This looks like a moderate sized filament transformer, with a tapped
secondary (typically something like 0-12-24 V or 0-18-36V with 120V or 240V
on the primary). The secondary rating has to be bigger than your load
current (plus some margin). You hook the primary across the line, and the
secondary in series with the load (phased to either raise or lower the
voltage as appropriate).
In your case, it's a 10Amp load, so the transformer would be rated at a
couple hundred VA, which isn't all that huge.
So, for instance, you run AWG14 wire or something out there (2.5
ohms/thousand feet) and your line voltage is abnormally low (say, 115V), so
with the 10 amp load, you'd normally be sitting at 115-2.5*10 = 90
volts. With that 90V on the primary of the transformer, you'd get
90/120*36V out or 27V. You connect it up so that the 90V is bumped up by
the 27 of the transformer, which gets you to 117V. Note that under noload
conditions, the voltage will be pretty high.. 120V+36 = 156V. Make sure
your surge suppression can take it!
Here's the WWGrainger catalog page: (Page 332 in catalog #396)
http://www.grainger.com/Grainger/wwg/viewCatalogPDF.shtml?browserCompatable=true&adobeCompatable=true&CatPage=332
These puppies run about $100 brand new. If you happen to have a suitable
transformer around, and an enclosure, then have at it.
> 2. Do I have to worry about voltage drop for the rotor voltage?
> Should I be using heavy gauge rotor cable?
> 3. I will only need two positions on the Ameritron switch. Can I get
> 10 conductor cable suitable for both the rotor (8) and the switch (2)?
> 4. There has been a lot of discussion of running cables in conduit
> over the years. What is the consensus opinion on the best way to
> do this? My recollection is that some provision has to be made to
> allow for the evaporation of condensation. Is it permissible to
> run all the cables above in a single conduit?
>
>I know there has been discussion of some of these topics before but
>searching the archives is a pretty tedious task. If someone can provide
>the information in a nutshell, I'd be most appreciative.
>
>
>73, Joe
>
>_______________________________________________
>
>See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
>Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with
>any questions and ask for Sherman, W2FLA.
>
>_______________________________________________
>TowerTalk mailing list
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_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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