It's a dark and stormy night. You flick on the hall
light, plug in the coffee maker and crank up the
portable electric heater. You're starting to feel
comfy, when you hear a faint, yet ominous, click—and everything goes black. It's not a cat burglar or a
poltergeist playing tricks with your electrical system.
It's an overloaded circuit being protected by a tripped
circuit breaker. Kinda spooky and mysterious, eh?
Not if you know a few simple things.
A properly functioning circuit.
Figure A: A Properly Functioning 15-AMP Circuit
This circuit has wires and a circuit breaker that can
easily carry the amperage required by the devices
What's a circuit? What's an overload?
When electricity enters your home, it goes to a circuit
breaker box (or fuse box in older homes), where it's
divided into a number of circuits. Each circuit is protected
by a breaker or fuse. Bedrooms, living rooms and
family rooms where only lights, alarm clocks and other
small electrical items are usually used are normally on
15-amp circuits. Kitchens, laundry rooms, bathrooms
and dining rooms—places where you're more likely to
use toasters, irons, hair dryers and other big-watt
items—are usually served by heavier-duty, 20-amp
circuits. Major appliances like 5,000-watt electric water heaters and 10,000-watt electric ranges demand so
much electricity that they take their own 30- to 50-amp
dedicated circuit (See Fig. D in “Additional Information” below), protected by big, “double
The circuit breaker, the wire and even the wire insulation
are all designed to work as a system—and that system
has limits. Try to push more current through a circuit
than it's designed for and things start happening
(Fig. B). Wires heat up under the burden of carrying the
excess current. When this happens, the insulation
around the wire can degrade or even melt. When insulation
melts, current is no longer confined within the
wire. That's when fires start. Luckily, the circuit breaker
senses the excess current and “trips” to stop the flow of
On the night the lights went out at your house, you
were fine with only the lights
and coffee maker operating.
The real trouble
began when you
plugged in that
Figure B: An Overloaded Circuit
This circuit has too many energy-demanding devices on it
and is trying to carry more amperage than it's designed for.
Things begin to heat up. Luckily the circuit breaker senses this,
trips and “breaks”
One simple equation helps you determine if a circuit is overloaded
To start solving the problem, we need to know one simple “rule of thumb” formula. This formula will help us determine if all the
electrical stuff on a particular circuit is overloading it. This formula
also helps define some everyday terms and how they relate
to one another. After all, light bulbs and space heaters are labeled
in watts; tools and circuit breakers in amps; and our household
electrical system in volts: How do they all fit together?
The simple formula (Fig. C) tells us how: Watts
divided by voltage equals amps. The other equations
shown are just other ways of saying the same thing.
Voltage can most simply be described as the pressure under
which electricity—a chain of electrons—moves. Most household
current is pushed at 120 volts, though current to large electrical
appliances is pushed at the higher pressure of 240 volts.
Amps (or amperes) is a measurement of the number of electrons
the voltage pushes past a given point in
Watts is a unit of measurement for electrical power. It indicates
how many electrons were pushed through an electric gadget to
make it work. It's what the electric company bills you for.
Figure C: The Basic Formula
Use these simple equations to convert different measurements of electricity.
Calculating why that breaker tripped
The circuit and circuit breaker that
you tripped have a capacity of 15
amps, or 1,800 watts (15 amps x 120
volts = 1,800 watts). The lights drew
360 watts, or a measly 3 amps (360
watts divided by 120 volts = 3 amps)—well within the capacity of your
15-amp system. The 800-watt coffee
maker (divided by 120 volts) drew
6.6 amps, substantially more power
than the lights, but their combined
9.6-amp draw is still within the
limits of the 15-amp circuit.
But when you plugged in the
1,200-watt space heater, the 10 amps
it required, plus the draw of the
other two devices, pulled 19.6 amps
through a 15-amp system (Fig. B).
It's like a python swallowing a pig;
the system just can't handle the
load. The circuit breaker tolerated
this for a while. But when the excess
current and resultant heat began
deforming the two pieces of metal
inside the breaker, they started
“pulling the trigger.” And when the
metal pieces bent to a certain point,
the trigger snapped two contact
points apart, interrupting the flow
of electricity and shutting down that
circuit. If there's a huge, sudden
draw on a circuit, a little electromagnet
in the circuit breaker can
pull the contact points apart too. If
you have fuses, the excess heat melts
a wire inside the fuse, which in turn
stops the flow of electricity.
If this had been a 20-amp circuit—one with thicker, No. 12 wire that
could carry 2,400 watts—the breaker
wouldn't have tripped. But once
the wire is in the wall and the breaker
is in the breaker box, there's not
much you can do to upgrade an
established circuit. But you do have
Larger appliances, like electric
water heaters, dryers and
stoves, require so much power
that electricity is brought to
them via 240-volt circuits.
That's because the voltage in
240-volt circuits “pushes”
twice as hard. For example, a
6,000-watt electric flugelhorn
on a 120-volt circuit would
require a 50-amp circuit (6,000
watts divided by 120 volts =
50 amps). That would require
mammoth wires. But that
same 6,000-watt flugelhorn on
a 240-volt circuit requires only
a 25-amp circuit (6,000 divided
by 240 = 25) and a smaller
wire and circuit breaker.
Two solutions: The simple and the long term
The simple solution is to plug the
space heater into an outlet on a circuit
that has excess capacity. You
can determine the existing load on
a circuit fairly easily: Click off the
circuit breaker, then flick on light
switches and test outlets to see
which ones no longer function. Then add up the total watt load of
devices on that circuit. This is often
easier said than done. Sometimes a
circuit labeled “bedroom” will
power outlets in the laundry room.
Or the upper and lower outlets of a
duplex receptacle will be on different
circuits. Once you have a circuit
mapped out and the electrical loads
added up, you'll be able to tell if you
can plug more devices into the circuit
without overloading it.
As you add up the electrical loads,
keep in mind that a wire rated at 15
amps can carry 15 amps all day
long. However, 15-amp breakers
and fuses can only carry 12 amps—80 percent of their rating—on a
continuous basis. Continuous basis
is considered to be a circuit loaded
to capacity for three hours or more.
This 80 percent rule applies to all
breakers and fuses.
For more in-depth information
on calculating loads, see Preventing Electrical Overloads.
The best long-term solution is to
install a new dedicated circuit and
outlet for the heater. Most electricians
will suggest a dedicated circuit
for any appliance that will draw
more than half the capacity of a circuit.
Fig. D in “Additional Information” (below) shows the wattage of
appliances that commonly have
dedicated circuits. Anytime you
install a large electrical appliance—whether it's 120 or 240 volts—install it on its own dedicated circuit
with the correct size wire and
As you can see from Fig. E, a
20-amp circuit with thicker, No. 12
wire can carry more current than a
15-amp circuit with No. 14 wire.
When you're wiring or rewiring a
kitchen, laundry room, bathroom
or dining room, the National Electrical Code will require you to
install 20-amp circuits, which can
carry more current. If you use a lot
of power tools, it makes sense to use
20-amp circuits for your garage,
workshop and basement too.
For information on how to wire a new circuit, see How to Connect a New Circuit.
Figure E: Wire Sizes
The larger 12-gauge wire can safely carry more amperage than
the smaller 14-gauge wire—without overheating.
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No tampering allowed
Homeowners who put a “penny in
the fuse box” to prevent fuses from
blowing have short-circuited brains.
Without a fuse to disrupt the flow of
power when too many amps are
pushed through a circuit, wires
overheat, wire insulation melts and
fires break out. And you can't simply
replace a 15-amp breaker with a
20-amp breaker; that's the modern-day
equivalent of putting a penny in
the fuse box. Remember, the circuit
breaker, wire and wire insulation
are all designed and sized to work