# Clamp Meter

Discussion in 'Electronic Basics' started by Binnyrus, Jan 21, 2008.

1. ### BinnyrusGuest

Is it possible to use a clamp meter (Fluke 322) to distinguish between
current carrying wire and a switch leg? I have a space in my home
that I'd like to add a pull chain light fixture to. There are two
wires that run through the space. I am suspicious that one is a switch
leg for a three-way switch . . . but have no idea how to determine
this. Can anyone help me out with how to diagnose the problem?

2. ### BobGuest

Assuming it is mains wiring.

You can put the clamp meter around one of the wires on the AC current
range then switch all the switches in the area. If the wires are
connected
to a load you should see current/no-current on the meter depending
on switch position. Check light bulbs are working if you can't find a

You could also use the fluke 322 on it's voltage range with the probe
You will need to make contact with the copper inside the wires without
getting your hands in contact with the probes.
The results will depend on how the switch is wired and how your mains
distribution is configured. If you measure between local mains earth
and both of the wires you should see a change when you operate the
connected
switch.

Bob

3. ### ehsjrGuest

You can't find the "switched leg" (xxx in the diagram
below) with a clamp on meter. The "switched leg" will
always have current when the light is on, and never
have it when the light is off. The same is true of the
hot leg and the neutral leg - if the light is on, there
is current in the wire. The clamp meter can be used to
determine which wires are the "travellers".

Regarding "switch leg for a three-way switch" :

SW1 SW2
o-------T-------o xxx
AC -Hot-o o------+
o-------T-------o |
[Light]
|
AC -Neutral-------------------------+

Above is typical 3-way wiring. The AC could also come in
on the right, with slightly different wiring. Or the
light could be between the switches. Regardless of how
the wiring is run, the two wires marked T are "travellers"
and are always accompanied by a third wire when properly
installed.

A clamp meter on a traveller will show no current
with the light on with one of the switches in the
up position, and current in that wire when the
same switch is in the down position and the light
is on. The same meter clamped on any wire other
than a traveller, will always show current when the
light is on, regardless of switch position.

This does not apply to your wires - they are not
travellers. Proper wiring would have a third
wire accompanying the travellers. They could be
the switched leg and the neutral or the hot leg
and the neutral, but your clamp meter can't tell
the difference between hot, switched or neutral
leg.

Ed

4. ### Peter BennettGuest

To get a useful reading with a clamp-on ammeter, you must put it
around a single conductor. If you put it around the two-conductor
cable normally used in house wiring, you will read zero regardless of
the current carried by the wires, as the magnetic fields produced by
the supply and return currents will cancel. This will happen with a
branch going to a switch, as well as on a simple feed to an outlet.

Re-reading your post I see that you mention a cable to a three-way
switch - such a cable would likely have three conductors, where a
cable feeding an outlet would only have two (in both cases, plus the
bare ground conductor)