Re: [TowerTalk] Shack wiring

[Jim Lux <jimlux@earthlink.net>]

At 08:06 AM 1/24/2005, Tom Rauch wrote:
> > My entire shack is on a single 110V circuit (separate 220
> for the
> > amplifier).
>
> You mean 120. The USA standard (last time I looked) was
> 120/240.

That's the difference between "service voltage" and "utilization 
voltage".  You'll find, for instance, that there are 240V and 480V three 
phase services, but you'll only be able to buy 230V or 460V three phase 
motors.  The difference is in the "allowance" for voltage drop in power 
distribution from service entrance to point of use (2% for NEC)



> Wow. I would guess a typical HF radio might draw an amp or
> less from the mains on receive. That would mean you have an
> ESR of .2 ohms in the mains. That's pretty bad.

If you use the NEC 2% drop guideline, and assume 120V service, that's 2.4V 
drop.. a 15A circuit could have 0.16 Ohms series resistance.  That's not 
too far from 0.2 Ohms..

Just ballparking here.. AWG14 copper (which is what most houses are wired 
with) is 2.5 ohms/1000 ft.  To get to 0.16 ohms, you only need 64 feet. 
There's two conductors, so if you're more than 32 feet from the panel, 
you've got 0.16 ohms in series (not even counting the resistance of the 
connections, switches, circuit breakers, receptacle/plug combination along 
the way).

Remember that the NEC (which drives most wiring requirements) is really 
only concerned with burning down the house (it IS published by the National 
Fire Protection Association, after all)... As long as the voltage drop 
isn't all in one place, they're not too concerned with "good regulation" at 
the load.  Also bear in mind that the 2% is a guideline...

Good practice on load design is that it should accept 10% variation around 
the nominal input voltage, just for such reasons.

Interestingly, noticeable light blink doesn't take a very big voltage 
change (<0.5 volt, in my fuzzy recollection)



Jim Lux, W6RMK 


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