by Ron Duffy
The Capacitance Connection
Use a Common Tool in a New Way
Several weeks ago, I was installing some low voltage
yard lights. After placing the wires where I wanted
them, I applied power and one of the strings did not
work. I went through the normal troubleshooting of
measuring the voltages and resistance. It became quite
easy to tell that I had an open somewhere between the
transformer and the first light in the string, but where?
I remembered using a multimeter with capacitance capability
just days earlier to solve a problem on our manufacturing line.
I had used it to detect the capacitance difference between a
16 gauge power cord and an 18 gauge power cord. It only
took seconds to recall that the capacitance of the wire is
directly proportional to the length, among other things. I had
a roll of the yard light wire from the store that had not been
opened. It listed 50 feet on the label. I put my multimeter in
the capacitance mode, zeroed out the leads, and clipped
them on to one end of the wire. The multimeter read 1.05
nanofarads. I calculated the capacitance to be 21 picofarads
per foot. I then measured the open wire I had disconnected
from the transformer. It measured 158 picofarads. That calculated
to be seven feet, six inches. With the use of a tape measure, I
found a cut in the wire at seven feet and six inches!
This sparked my curiosity. I took a trip to the hardware
Cat 3 phone
Cat 3 phone,
16 gage sp.
store and measured a new role of Romex. A 250 foot roll
of 14-2 yielded 5. 26 nanofarads — 21 picofarads per foot.
The hardware store worker marveled when I told him what
I was doing and was amazed when I measured a partial roll
and calculated that he had 91 feet left in the roll!
I wondered how good this was. I made some other measurements and calculated the resolution. The capacitance and
range of the multimeter determines the resolution. My results
are shown in Table 1. The last three coaxial cables are from
a cable chart. The resolution becomes worse on longer wires
because the multimeter changes ranges to accommodate the
higher capacitance (note the readings on 1,000 foot reels).
Of course, this is still not bad — 54 inches out of 4,500 feet.
I know the next time I have an open wire in a wall or an
area that is hard to examine, I will grab my multimeter. Before
I make an installation, I will use my multimeter to insure that
there is enough wire on the spool before I start. NV