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Re: [TowerTalk] Shack wiring

To: <>, "Pete Smith" <>
Subject: Re: [TowerTalk] Shack wiring
From: "Jim Lux" <>
Date: Mon, 24 Jan 2005 07:17:23 -0800
List-post: <>
----- Original Message -----
From: "Pete Smith" <>
To: <>
Sent: Monday, January 24, 2005 6:19 AM
Subject: [TowerTalk] Shack wiring

> I'm hoping that history has added the subject of shack wiring, both RF and
> AC, to the range for Towertalk -- if others disagree, I apologize.
> My entire shack is on a single 110V circuit (separate 220 for the
> amplifier).  Recently I have begun to get low voltage warnings (104.5
> volts) from my UPS in the morning, and so have put a digital voltmeter on
> the circuit.  With no load except for the computers (2), the UPS and the
> table light, voltage right now is 108.7.  My two radios (on receive) cause
> the voltage to drop about .4 volts.  When the voltage is running low, I
> typically see 104.7 or 104.8 without the radios, so adding them to the
> sets off the UPS.
> Two questions, I guess -- is the voltage drop with loading I describe
> roughly what you'd expect?

Voltages seem a bit low...
Generally, you design for no more than 2% voltage drop at full load on the
branch circuit.
for 120V, 2% is 2.4V.

 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.

Spec?  It's whatever is in the tariff your utility has filed with your local
regulatory agency.  Probably 120V +/- 5%, though.  And, that would be at the
service entrance.  Allow the 2% drop and you get down to the 117VAC you see
a lot.  There's also usually a provision for the utility to "brown out" to
reduce load and/or that voltages can drop lower in exceptional
circumstances.  The trend now, though, is for "rotating blackouts" rather
than "brown outs" because brown outs are really, really hard on motorized
things like refrigerators and air conditioners, where the efficiency drops
dramatically as voltage is reduced (because IR losses go up).

There's also a difference between what the tolerance for  your service
entrance (5%), and what equipment is supposed to be designed to work over
(10% or more).

Here's what PG&E has to say about it:

105V, at your outlet, is quite low as a practical matter.



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