a customer asks me to defend the need for GFCIs (ground-fault circuit
interrupters), I recount the old movie scene where the radio falls
— or is thrown — into a water-filled bathtub, swiftly
electrocuting the unfortunate bather. I then explain that if the
radio had been plugged into a GFCI receptacle, the bather would
still be alive. This leads us to the ultimate purpose of GFCIs —
the protection of life.
GFCIs work? Absolutely. These devices have saved countless lives
and provide much needed protection for both the tradesman and homeowner.
If you are ever unlucky enough to receive an electrical shock, but
lucky enough to have a GFCI in the line, it will feel like you're
being stuck with a needle, then the GFCI will trip and open the
circuit, stopping the current.
ordinary 125-volt residential circuits using NM (non-metallic sheath)
wire, the amperage leaving the panel, usually through a black wire,
must equal the amperage returning to the panel through a neutral,
or white, wire.
continually monitors the amount of current going to the load and
compares it to that coming back. As long as the two are equal, the
electricity is doing its work properly. However, if some of the
electrons are missing and the current coming back from the load
is less than that going to it, the GFCI will trip the circuit. The
logic of GFCI design is that if the current is not coming back via
the wiring, it must be going somewhere else. Often this "somewhere
else" is to earth (ground) through a person holding a tool
an example from my own experience. I was using a drill that was
plugged into an extension cord that, in turn, was plugged into a
GFCI receptacle in my garage. The drill was old and the shell made
out of solid metal. While I was using it, one of the wires inside
the drill shorted to the metal case, which made it electrically
hot. Since electricity can cause muscles to contract, my hand tightened
around the metal handle so that I could not release it. The current
was now leaving the service panel, traveling through the black wire
of the house wiring to the GFCI, then through the extension cord
into the drill. From the drill, the current was flowing through
me to ground. This was a classic ground fault: The electrical short
within the drill caused the current to pass through me to ground,
rather than flowing back to the service panel via the white wire.
The GFCI detected this imbalance and opened the circuit immediately,
saving my life. My only discomfort was the pin prick feeling.
response. The time it takes for a GFCI to open a circuit will vary
from manufacturer to manufacturer, but it should be no more than
V30 of a second to comply with UL standards. The actual amount of
current imbalance that the GFCI must detect before it trips is four
to six milliamps (thousandths of an amp), also a UL standard. Theoretically,
the average person can tolerate four to six milliamps of current
for 1/30 of a second before his or her heart goes into fibrillation.
(Fibrillation means that the heart goes out of sync; the result
can be death.) With GFCI protection, you may still get a shock,
but its duration will be limited to V30 of a second.
doesn't the circuit breaker trip? Most circuit breakers controlling
general purpose receptacles will not trip until at least 15 or 20
amps of current flow has been exceeded. This amount of current is
normally fatal. In order to protect against fatal shocks, you need
a device on line, like the GFCI, that will trip before the circuit
breaker can trip.
sense. Just because you are plugged into a GFCI doesn't mean that
you can cast all common sense to the wind. You can still die if
your body — your heart in particular — is placed between
the incoming black wire and the outgoing white wire. In this case,
your body is in series with the electrical current, just like a
light bulb. As long as your body isn't grounded, you are no different
to the GFCI than a normal working load. If you get caught in this
situation, the GFCI will not trip because there is no current leakage
to ground to create an imbalance...and you could be killed.
residences, GFCIs come in two types (Figure 16). One type looks
like a receptacle. It has a test button on it and sometimes a light.
The second type looks like a 15- or 20-amp circuit breaker with
a test button on it. In both designs, the purpose of the test button
is, when pressed, to place a current imbalance on the circuit. The
GFCI should then trip if it is working properly.
breaker GFCIs. Use a GFCI circuit breaker only if all receptacles
on the circuit require ground-fault protection. It fits into the
service panel like a standard breaker but wires a little differently.
Circuit breaker GFCIs have two main disadvantages: They cost more
than receptacle GFCIs and are somewhat inconvenient. Because they're
located in the service panel, the homeowner has to walk to the panel
each time the GFCI trips the circuit.
GFCIs are fed from the service panel through a standard circuit
breaker. The GFCI receptacle is then placed at the point of use
so that when it trips, the homeowner can immediately reset it without
leaving the room.
using GFCI receptacles wherever possible inside the house, both
for cost and convenience. However, the cost can escalate far above
the cost of a circuit breaker GFCI if you install them at several
locations on a single circuit.
power outdoor receptacles, however, I definitely recommend using
a GFCI circuit breaker. Experience has shown that GFCI receptacles
can have a short life span when located outside, even in watertight
boxes. The boxes and lids may be watertight but they are not vapor
tight. The water vapor seems to shorten the life of the electronics
Wiring of GFICs
GFCI receptacle may be wired incorrectly by a homeowner or novice
electrician. GFCI receptacles have a "line," or input,
side and a "load," or output, side. The line side must
be connected to the wiring that originates at the service panel.
The load side must be connected to any downside receptacles that
are to be protected (Figure 17).
a receptacle GFCI is wired incorrectly by pig tailing the downstream
receptacles off the line side. These receptacles are now in parallel
with the GFCI and are not ground-fault protected. Only those receptacles
feeding out of the load side will be protected.
Remember to label any downstream receptacles as ground-fault protected.
Use the stickers supplied with the receptacle expressly for this
purpose. Inspectors often overlook this, but be sure to do it anyway.
Without the label, the homeowner has no way of knowing that a particular
outlet is protected.
It is also possible to wire a circuit-breaker GFCI incorrectly.
However, the incorrect hookup would be immediately apparent if the
"test" button doesn't trip the device. Under test, this
type of device typically places an eight milliamp ground-fault on
test GFCIs (using the test button located on the GFCI) immediately
after installation. If you are at a site where you will be using
a preexisting GFCI to power your tools, always test it first to
verify that the ground-fault protection is still working. It's possible
to obtain 125 volts from a GFCI receptacle without its ground-fault
protection working. Manufacturers normally request monthly testing
tester. Do not test a GFCI by shorting across the hot-to-neutral
slots in the receptacle. This will not test the GFCI and may cause
damage. Three-prong plug-in testers with a push button are specifically
designed for the testing of GFCIs and are commonly available at
most electrical supply houses. This type of tester typically places
a .0068-amp current imbalance on the line to trip the GFCI. All
electricians, contractors, and inspectors should carry and use these
little testers (Figure 18).
can also use a plug-in tester to test a GFCI that has several receptacles
on its load side. First test the actual GFCI receptacle or GFCI
circuit breaker. Then test the most distant receptacle working off
its load side. The GFCI should trip when you push the button on
GFCIs in an ungrounded circuit. Plug-in testers create an actual
fault to the ground wire in a three-wire circuit, causing the GFCI
to trip if it is working properly. However, this can lead to uncertain
test results for a GFCI installed in an ungrounded (two-wire) circuit.
The GFCI may actually work fine, but it will not respond to the
tester since there is no ground wire to short to.
for UL-approved receptacle GFCIs, the test button on the device
itself will still yield an accurate test. This is because the built-in
test device works by taking some of the current from the black wire
on the load side of the GFCI and shunting it back to the white wire
on the line side to unbalance the circuit. This is, in effect, a
ground-fault simulation rather than a true ground fault, but the
imbalance effect is the same.
National Electric Code (NEC) defines where and how GFCIs should
be used. Here are some of the more common regulations affecting
residences. (Unless otherwise stated, "receptacle" refers
to a 125-volt, single-phase, 15-amp or 20-amp standard residential
All countertop receptacles within a 6-foot straight-line distance
from the kitchen sink must have GFCI protection. Since, according
to code, two separate circuits must feed the countertop receptacles,
I normally wire my kitchens with one GFCI circuit to the left of
the sink and one GFCI circuit to the right (assuming the sink is
in the center of the countertop). This normally separates the load
evenly and, if I come back ten years later to troubleshoot a problem,
I know exactly how the circuits are wired. I use GFCI receptacles,
as opposed to circuit breaker GFCIs, since the former can be reset
at the point-of-use in the kitchen. Countertop receptacles beyond
the 6-foot limit, as well as other general use kitchen, dining,
and pantry receptacles, can be wired into the line, or unprotected,
side of the GFCI.
All receptacles installed in bathrooms must have GFCI protection.
I always use receptacle GFCIs for reset convenience.
Every receptacle in a garage must have GFCI protection unless it
is not readily accessible, such as a receptacle located on the ceiling
for a garage door opener, or one serving a plug-in appliance occupying
dedicated space, such as a freezer. Any 230-volt outlet is exempt,
as is the laundry circuit.
All receptacles installed outdoors that are readily accessible and
within 6 feet 6 inches of grade level must have ground-fault protection.
Unfinished basements and crawl-spaces at or below grade level. All
receptacles installed in these locations must have ground-fault
protection, except for:
A single (not duplex or triplex) receptacle supplied by a dedicated
branch circuit for a plug-in appliance such as a freezer or refrigerator
• A laundry circuit
• A single receptacle supplying a permanently installed sump
protection. In most areas of the country, builders are
required to use a GFCI-protected temporary panel. This type of panel
normally protects single-phase, 125-volt, 15-amp and 20-amp receptacle
outlets. If you use a generator of five kilowatts or less, you may
be exempt. Extension cords with built-in GFCI protection are also
available for job-site use and are recommended by OSHA (Figure 19).
Not to Use GFCIs
though it isn't against code, room lights should not be placed on
a GFCI unless there is a specific need for doing so. The reason
is simple: If the GFCI trips, you don't want to be left in the dark
trying to find your way out of the room — especially in the
bathroom, where the floor might be wet and slippery, with many objects
to bump against or trip over.
this by wiring only the receptacles in the room, if you want them
protected, off the load side of the GFCI, but put the lights on
the line side. Also, unless there is a specific reason, don't use
a GFCI for equipment and appliances that cannot go without power
for an extended time, such as a freezer or sump pump, since GFCIs
are sensitive and are subject to nuisance tripping.