[geeks] Questionable Voltage

[Sridhar Ayengar]

der Mouse wrote:
>> 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.

That's the answer I was looking for.

> 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.

I'm aware of all of the above.  I just am unable to measure without 
unplugging something, which I can't do right now.

> 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.

The breaker panel is almost new.  The wiring is 80+ years old.  I doubt 
they checked when they were installing it, which was just before I 
bought the house.

I insisted that the squirrel-chewed main electrical feed needed to be 
replaced as a condition of my buying the house.  When the seller had 
that done, she also had the old fuse box replaced with a breaker panel, 
since the power company refused to work on it, the fuse box, that is.

>> 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'm aware of all of this.  I didn't mean that I was going to be 
installing outlets in this fashion.  I meant that the outlets that were 
already installed might have been installed in this fashion.

>> 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.

Oh, I've rewired an entire house before and wired in generator backup 
systems and rotary phase converters before.  It's just that wiring this 
old is outside of my experience.  There's still knob-and-tube in places, 
so it appears I have my work cut out for me.

Peace...  Sridhar