[time-nuts] Thunderbolt power supply open heart surgery

Lux, James P james.p.lux at jpl.nasa.gov
Tue Feb 17 14:14:28 UTC 2009

On 2/17/09 2:38 AM, "Neville Michie" <namichie at gmail.com> wrote:

> Hi Jim,
> you may have a problem similar to mine.
> In Sydney, with a nominal supply voltage of 240 Volts,
> the supply at my place is above 250 volts. The supply authority
> says that is in the limits of their specification.

Typically, limits are +/- 5% or +/- 10%.  Either way, 250 is within 5% of

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 do not know why they run it so high, maybe they think they will
> sell more power.
> The problem is that many appliances have a 240 or 250 maximum, dual
> power supplies for 110 V
> switch to take 220 V.

That's actually pretty unlikely. 110V might be what it says on the label,
but in reality, nominal line voltage in the US is 120V, at least at the
meter. There's a difference in the "service voltage" and the "utilization
voltage" in most cases, for instance, 230V electric motors are operated on a
240V service.

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