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
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
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