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How
much electricity can you get from an outlet or
circuit?
I've heard that it can be a
potential hazard to plug a power stip directly into another
power strip and that you should rather plug the power strip
directly into a wall outlet "only", it this true? Along
those same lines, I also heard that you should only plug
"one" item into an extension cord even if they have several
plugs available. What are the facts?
--
Cincy Ward, Berkeley, CA, Feb. 2005
For some reason, UL (the group which evaluates
the safety of electrical products) says that you
shouldn't daisy-chain multiple power strips, and that you
shouldn't plug a power strip into an extension cord. Any
instructions that came with your power strip would
probably tell you the same thing.
That said, I can't see what the problem would be in
doing so, as long as you don't plug in so much stuff that
you exceed the capacity of the circuit (see the next
question), and as long as the power strips have nice
thick cords to handle all the electricity you're pulling.
Seems to me that several power strips chained together
would be no different than using a single thick orange
extension cord.
But the officials and manufacturers recommend
otherwise, so if you want to daisy-chain multiple power
strips together, you do so at your own risk.
Regarding a three-outlet extension cord, why would
they put multiple outlets on the cord if you weren't
supposed to use them? As long as the total of the devices
you're plugging in doesn't exceed the wattage or amperage
of the cord (look for the label on the cord) or the
circuit the cord is plugged into, you should be fine.
How many
watts can a standard outlet deliver before it's overloaded?
I ask this because occasionally when I'm using a lot of
electronic appliances, electricity shuts off in parts of my
home. I have to switch the fuse in order to restore power.
What am I doing wrong?
-- Mark Lee, Mar.
2003
Good question! First you need to understand that
each circuit usually supplies power to several outlets
and lights. For example, Circuit A might supply power to
the four outlets in the master bedroom plus the ceiling
light, Circuit B might supply all power to the bathroom,
etc. Each circuit is controlled by a breaker or a fuse.
So you don't really overload an individual outlet, you
overload a whole circuit.
You can't tell which circuit an outlet's on just by
looking at it. The only way to tell is to plug something
in, turn it on, and keep turning off breakers (or
removing fuses) until the appliance turns off. You can
make a circuit map of all outlets and lights in your home
this way. Once you know which outlets are on the circuit
that's being overloaded (and which are not), you can plug
some of the offending appliances into outlets on
different circuits. That way the overloaded circuit won't
have to try to supply so much power.
Also, if there are any lights on the overloaded
circuit, replace them with compact fluorescent bulbs,
which use 75% less energy than normal bulbs.
So, to rephrase your question, how many watts can a
circuit deliver before it's overloaded? It depends
on the circuit. Most modern residential circuit breakers
handle 15 or 20 amps (1800 or 2400 watts), and the
breaker is labeled 15 or 20. That's the total amount of
electricity that can be supplied to ALL loads on the
circuit, which usually includes a few outlets and lights.
Add up all the wattage or amps on all the lights and
devices you want to run off the circuit at the same time
and make sure it's less than what your breaker allows.
(If a device is labeled with both input and output power,
count the input.)
I don't know how much electricity is supplied by an
old fuse circuit, but it's worth looking at it to see if
it's labeled. Clearly you're overloading it if you keep
blowing fuses.
How much electricity can I draw
from an individual outlet?
-- Mr.
Curious
First, please read the previous answer
(above) or you may misinterpret the following answer.
Most outlets are rated to handle 15 amps (1800
watts), so you could draw that much from a single outlet
if there was nothing else running on the same
circuit. But there probably is something
running on the same circuit, which is why you should see
the previous answer above.
We have
about 120 servers running in a computer data center. The
specs on these say that the power supplies are
"Auto-switching 100/240V AC power". Now, if I'm reading your
site right, then the most these should draw would be 2 amps
--however, we have had five plugged into a 15-amp power
strip and the strip has tripped! My question is, how
can these computers be drawing (as they must be) more than 3
Amps each?
--Jessica
Palmer, April
2004
Okay, first of all, the 100/240V label means
that the server can handle any kind of electricity from
100V to 240V, so it will work with the voltage in any
country. (US/Japan is 100-120V, most others are 220V).
But your question wasn't about foreign use, so now that
we've taken care of the 100/240V label let's move on.
Second, I don't see where you're getting that your
servers draw a maximum of two amps. That's unrelated to
the 100-240V label. If the maximum number of amps aren't
listed, then the number of watts will be, and you divide
the number of watts by the number of volts (120V, for the
U.S.) to get the number of amps.
So let's say that one of those two things is the case
and you know your servers should be drawing no more than
10 amps, so why is it tripping your 15-amp power strip?
There are two possible answers:
The first possibility is that you have a faulty power
strip. Try another one.
The second possibility is that when the final server
or two is switched on the brief power surge when the
equipment is turned on is enough to exceed the 15-amp
rating of the strip. The surge you get when you turn on
equipment is so brief and so small that you'll never see
its effect on your electric bill, but sometimes it's
enough to trip a power strip or circuit breaker.
If swapping out power strips doesn't work, I suggest
getting a cheap watt-meter and
measuring how much electricity each server is using.
Either way, I'd be interested in hearing what you
ultimately discover.
I'm trying to determine how many
amps I'm putting on a circuit so I don't overload it, but
I'm having a hard time understanding the labels. For
instance, my DSL modem adapter says "INPUT: 120V 60Hz 30W"
and "OUTPUT: 12VAC 1.67A" I understand how to convert
watts to amps [Watts / Voltage = Amps], so it looks
like in this case the input (30 watts or .25 amps) is less
than output (200.4 watts or 1.67 amps). But your site
says that input is always higher than output. What am I
missing? -- David
Hylton, Oct.
2003
What you're missing is that the input is 120
volts but the output is only 12 volts. Electricity from
the wall is AC, and is 120 volts. The adapter changes
that to low-voltage DC, usually 3, 6, 9, or 12 volts. So
the output is 12V x 1.67A = 20W, which is less than the
30W input. Output is always less than input, because the
conversion process is inefficient.
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