canadian_woodworker
Is there a simple way to figure out which is the first recepticle on a
circuit? I have an older house - the two upstairs rooms are on the
same
circuit and are not grouned. Id like to add a GFCI plug on the first
recepticle on that circuit, so every recepticle downstream is
protected.
The brute force method would be to guess which recepticle is the
first,
remove the outgoing wires, and test every other outlet for power -
rinse (hook back up the wires), repeat, until ive found the recepticle
that has power and all the others that dont. 
However this will take
awhile - most arent easily accessible - behind furnitire, beds etc.
I have at my disposal the standard home repair tools - volt meter,
etc.
Nehmo -
I don't know exactly how to do it in practice, but in theory…
1.
Unplug everything on the circuit.
At the circuit breaker box, with the breaker off, connect what would
have been the hot wire of the circuit to the neutral. Leave the neutral
connected normally.
Measure between one slot and the other at each receptacle.
At each receptacle, the measurement of the resistance and inductance
will be different. They will be lowest at the first, and highest at the
last.
14 gauge wire only has a resistance of 2.6 ohms per 1,000 feet, so the
difference in resistance will be difficult to measure. But the
inductance should be substantially different at each receptacle. At
higher frequencies, the measurement should be easy.
Or
2.
Get something that detects AC _current_ in a wire without electrically
connecting to the wire. Perhaps a pick-up coil attached to an amplifier
or perhaps a large coil simply connected to an earphone. I'm not used to
the commercial non-contact detectors, but one of them would work. You
want something that makes a different indication for a current flowing
wire and just a hot wire. After you have your detection tool, experiment
with it. Learn to detect a current-carrying wire.
With the circuit breaker on, plug-in a high-wattage lamp in what you
suspect to be the first receptacle. You should not be able to detect
current moving through the wires at any other receptacle. If you detect
current at a receptacle, it's at a position before the lamp receptacle.
Unplug the lamp and plug it in what you suspect to be the last
receptacle. You should be able to detect current at every receptacle.
Note I'm making a distinction between a receptacle with current going
through the wires connected to it and a receptacle that's just hot. All
of them should be hot. The electromagnetic field will be much stronger
around a current carrying wire.
Or
3.
Get a really high-wattage load, perhaps a big electric heater, something
with a high enough wattage to heat its supply wires detectably - but not
dangerously. Use the same system as the current detector. Plug in the
load at the (believed) last receptacle. Check the earlier receptacles
for warm wires. You get the idea.
4.
Fire the circuit up with DC (use a rectifier by the circuit breaker
box). Put a low-resistance load on the circuit at some receptacle. With
a sensitive voltmeter, measure between a hot slot of one receptacle and
the hot slot of another. The existence and the polarity of this tiny
voltage drop will show the relative position of the receptacles. If you
draw a diagram, you'll understand.
You could do this with the regular AC too, but you wouldn't get the
polarity info. You still could figure out which receptacle is first.
You're testing for a voltage drop across a load, which in this case is
just a piece of wire between receptacles. The voltage drop will not be
much.
[I crossposted]
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(||) Nehmo (||)