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Re: [TowerTalk] Shack wiring

To: "Tower Talk List" <>
Subject: Re: [TowerTalk] Shack wiring
From: "Jim Brown" <>
Reply-to: Jim Brown <>
Date: Mon, 24 Jan 2005 09:18:44 -0600
List-post: <>
On Mon, 24 Jan 2005 09:19:54 -0500, Pete Smith wrote:

>Two questions, I guess -- is the voltage drop with loading I describe above 
>roughly what you'd expect?  If necessary I can dig out the standby power 
>requirements for the two radios.  And second, what is the US spec for line 
>voltage?  The power company is coming out to investigate and I'd like to 
>know where I stand.

>From my own experience, it is ALWAYS worth the trouble from a learning 
perspective to go through the exercise of cranking through the numbers and 
troubleshooting with a meter for problems like these. It's simple Ohm's law -- 
there are copper conductors of known size between your breaker panel and your 
outlet. You try to get a rough idea about how long they are, go to the wire 
tables in the Handbook for the wire resistance per ft, calc the drop from the 
current drawn by your radios, etc. 

Now, there is one other "gotcha" that isn't obvious. I don't remember if I 
covered it in my power/grounding tutorial, but I should add it if I didn't. 
Remember that current is drawn by electronic equipment in short pulses at the 
top of the AC cycle to re-charge filter caps, and that these pulses can be of 
pretty high peak level. That means that the IR drop on the line will be higher 
than the AVERAGE current would predict. Also, only the best voltmeters actually 
read true RMS -- they read peak or average and apply a fudge factor for a sine 
wave. But thanks to the distortion by the loading of these power supplies, 
neither the voltage or the current is a pure sine wave (and the current isn't 
even close).  

What I would do if i were you is look with the voltmeter at a lot of other 
outlets around your house. If you see the voltage at all of them reading low, I 
would first check the CAL of the voltmeter. If that's good, I would then call 
the power company and find out why your line voltage is low. If the line 
is higher at the other outlets, you know you have a voltage problem in your 

The peak current loads of electronic power supplies is a VERY strong argument 
for using #12 for all branch circuits and individual home runs to as many 
outlets as possible. #12 copper has roughly 2/3 the IR drop of #14 (3 wire 
equals one half the resistance). There is some discussion of this in the white 
paper, but I don't remember if I addressed the voltage drop and wire size issue.

The low voltage condition could mean that something has changed (degraded) 
somewhere in the wiring between you and that great generator in the far beyond. 
Maybe it's in your house, maybe it isn't. A CHANGE in your home that produces a 
drop of several volts very likely indicates a fault that could cause a more 
serious problem, like overheating. 

Jim Brown  K9YC


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