[time-nuts] Thunderbolt power supply open heart surgery

Hal Murray hmurray at megapathdsl.net
Tue Feb 17 17:00:33 UTC 2009

> As another matter, at light loads in your house/neighborhood, the
> voltages will rise, since the distribution voltage is usually set up
> so that at nominal load, it's correct, and that allows for some IR
> drop in the lines.

I got interested in this area a while ago.  PG&E replaced the transformer on 
the pole right outside my house.  It seems as though my light bulbs don't 
last as long as they used to, but I don't have any real data to back that up.

One day, I got lucky and found their web page:
That refers to
  NMEA/ANSI: American National Standard C84.1  They want $60, but PG&E 
provides enough info to read between some of the lines.

Page 4 says:

If the nominal voltage is 120, PG&E tries for a min of 114.  They have a max 
of 120 for Class A or 126 for class B.  I think that is at the entrance to 
your house.

Page 6 says:

You should expect a min of 110 and a max of 125.  I think that means that C84 
allows 4 volts drop in your house.  The next page says 10% drop (108 V) on 
circuits not supplying lighting loads.

There is lots of weasel wording.

Page 15 has a note about harmful wave forms.  I assume that covers switching 
power supplies.  Page 20 says street lights have to meet 90% power factor.

Several years ago, when I inquired about how to monitor line voltage, 
somebody suggested using an APC UPS.  Many thanks.  That's what I've been 

The voltage in my house usually runs between 120 and 125.  I occasionally see 
it go above.  I'd have to dig some to find a sample.  I also see various 
glitches where it drops below 110.  Most of them are short enough so that my 
PCs that are not on the UPS ride through it.

These are my opinions, not necessarily my employer's.  I hate spam.

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