[geeks] Questionable Voltage

[der Mouse]

> My house is pretty old.  Mostly ungrounded outlets, but there are a
> smattering of grounded ones around.

> One of those outlets has a power strip with two computers plugged
> into it.  Both of those computers are acting flaky.  [...]

> Will the breaker on the circuit pop before I pull enough current to
> drop the voltage on the circuit, or might I be pulling more than the
> circuit can take and causing my problems?

Depends on the wiring.  Given that it dates back to before grounded
outlets were required, it quite likley was too weak for what you're
doing even when it was new, and it probably hasn't gotten any better in
the interim.

The voltage drop arises from ohmic losses in the wiring between the
power supply and the load.  Here, we approximate the supply as the main
feed to your breaker box and the load as the thing plugged into the
outlet.  There is actually more wire involved between your breaker box
and the utility company, and in the power cord between the outlet and
the computer, but they are unlikely to be problematic.

The details below are based on my North American experience and will
likely be wrong if you're elsewhere (and might be wrong even if you're
not - I've been known to make mistakes :-/).

In modern wiring, on a 15A circuit (which AIUI is commonest), you'll
see most likely #14 wire used.  This provides low enough resistance
that even at 15A, you don't lose more than a few volts in the wire even
when running just shy of tripping the breaker.  But if your place is
wired with lighter wire, it will take less current draw to produce an
unacceptable voltage drop.  There's also the possibility that there's a
junction somewhere that's corroded or loose or otherwise gone bad
that's dropping a lot all on its own - and this would be slightly
disturbing, because it would be dissipating a corresponding amount of
heat.

I'd suggest finding a voltmeter and measuring the voltage at the outlet
with the computers unplugged.  Then plug them in, turn them on, and
measure again.  If the two readings differ by more than a few volts,
you probably are drawing more than the wiring can handle; if the
"turned on" voltage is less than about 100V, this may well be causing
the trouble you're seeing.

In theory, the breaker should have been chosen to match the wiring.  In
practice, it's reasonably likely that someone just slapped in a (say)
15A breaker without checking that the wiring was up to carrying that
kind of current.

> Might an outlet with a separate ground pin be installed without a
> corresponding upgrade in the wiring?

Yes, but (a) it shouldn't be done and (b) it wouldn't help.

It shouldn't be done because, if the wiring doesn't include a ground
wire, it's dangerous to install an outlet that implies there is a
ground available - a device with a grounded plug is likely to be
designed assuming the ground pin is actually grounded.

It wouldn't help because none of the effects under consideration would
be alleviated by just installing a new outlet (possibly excepting the
case where the contacts in the outlet are dirty or corroded and the
plug is thus not making good contact; if this is the problem, the plug
will be warm to the touch after things avhe been on for a while).

> I hope I don't have to rewire up there soon, but I'm afraid that I
> might have to.  This is in my office, and I don't even have
> everything running yet.  I wanted to eventually have a 220V 20A plug
> up there with a good sized UPS, but running the new home run from up
> there to the basement two floors down will be enough of a pain that I
> might not want to do it myself.

Given the questions you're asking, I offer the thought that you might
actually not be qualified to wire it up yourself.

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