I've got lots of computer things, lights, and other gadgets plugged
into a couple of power bars. For instance, we have three power outlets
in the computer room -- each outlet has two heavy-duty power bars
plugged in, and those power bars are full with power-consuming
gadgets.

I'm worried that I may be overloading the circuit (the actual house
circuit, not the power bar, as I know it's got its own circuit breaker
built in).  So far, the house circuit breaker hasn't snapped off,
but... my question:

How do I calculate whether I am getting close to overloading a power
bar and the circuit?  For instance, I just bought a power bar that has
an "electrical rating of 1875 watts/125V/15A". Does this mean I just
estimate how many volts/watts each device takes?

Answers need not refer to surge protection, as I'm not concerned about
infrequent power anomolies, just overloading from regular use.

hi jhabley-ga,

Your question is an important one for anyone concerned about
maintaining a safe electrical environment.

In the situation you described, you have powerbars plugged directly
into each of the six outlets located in your computer room. Under this
configuration, it is impossible to overload the circuit in your home
unless a powerbar is malfunctioning or damaged. The reason for this is
that each powerbar is equipped with a circuit breaker, as you
identified in your question. The purpose of placing a circuit breaker
within the powerbar is so that if the devices plugged into the
powerbar want too much electricity, the powerbar will temporarily shut
itself down (ie. "cut the circuit").

It is important to note that a powerbar is not merely an extension
cord with multiple available inputs, but actually a "moderator" of
energy being passed through to the devices that attach to it. The
meaning of the electrical rating on the powerbar is equivalent to the
statement "A maximum of 1875 watts/125V/15A will be allowed to pass
through the powerbar at any given time. If all requesting devices
cannot function at this level, the circuit in the powerbar will be
reset". Of course, this says nothing about the potentially limited
availability of the electricity or a means of providing it. The
powerbar is considered a "safe" device in that it is able to gauge the
amount of capacity in the house circuit. Therefore, any change in
required electricity is verified as possible before being passed to
the device itself.

An insurance company has put together some information about
preventing home circuit overloading and the blowing of fuses. The
following is perhaps most pertinent to the question:

"The power bar, also known as a power strip or multi-outlet strip, is
a bar with several outlets for plugs, providing a safer alternative to
extension cords for electrical appliances or equipment.

The power bar's main advantage over extension cords is that it
protects you against power line surges. These surges are momentary
over-voltages -- up to several thousand volts, lasting just a few
millionths of a second. Power surges can cause severe damage to your
electrical equipment, and be extremely harmful to computer equipment.

A power bar with a circuit breaker, however, shuts off the power in
the event of a surge or a brief line overload. Afterwards, you simply
press the re-set button on the bar.

Make sure that your bar has a breaker and re-set button Without those
two items the bar is really just another extension cord, and offers no
protection from power surges or overloads.

Some power bars are intended for general use, and others for
appliances and computers. Read the packaging carefully."

http://www.dinan.com/ins_tips/showtips.cfm?Cat=PROP&title=PropertyTips

If you have any problems understanding the content above please post a
clarification and I will respond to your request in a timely manner.

Cheers!

answerguru-ga

Outstanding and quick response. Thanks! I didn't realize the power bar
would also moderate the power consumption.

The power bars will not "moderate" power consumption. The bar's
circuit breaker will trip if the consumption exceeds the rating of the
power bar. That protects the power bar - it does NOT protect your
house wiring.

Now, you have 3 outlets, with 2 power bars on each outlet, and the
power bars are rated at 1875 watts/125V/15A. If both power bars on an
outlet were fully loaded that would be 30A on one outlet, which is
probably excessive. The circuit breaker in your house wiring panel
should provide protection for the overall wiring, but you may be
overloading an individual outlet.

>Does this mean I just estimate how many volts/watts each device
takes?

Yes, watts or amps. Volts will remain constant at your mains supply.
Most domestic computer accessories (scanners, speakers, small
printers, etc - almost everything that uses 'wall wart' power
supplies) are very low power and can be ignored. PCs, monitors and
large printers should have their wattage reckoned up (as should
heaters and any other heavy loads) and take care not to exceed the
individual outlet's rating. The rating of your house wiring and
outlets will vary according to your local electrical regulations and
practice.

Owain

It is also prudent not to daisy chain any power strips (power bars).
connecting one power strip to another one is a sure way to overload
the circuit. unless you are running things like vaccums, heaters,
refrigerators etc on the same circuit, you will probably be ok running
one power strip full of computer stuff per actual outlet you have, in
newer buildings.

Well, I think the answer, instead of being five stars, was no good at
all, because it didn't provide any answer to the actual question:

How can one calculate when one is close to overloading the house and
powerbar circuits?

You calculate this using the formula P=UI, i.e. power equals voltage
times current. In the power bar originally quoted this formula is used
with 1875 W = 125 V * 15 A. House fuses at least here in Europe are
rated according to the current they can pass through; 10A, 16A, 20A
and so on.

To calculate how close one is to overloading, 
1) Write down the maximum wattages of all the equipment in the
circuit.
2) Add them together. 
3) Divide the sum with the voltage you are using (125 V in your case)
to get the maximum current consumption.
4) Compare with the current rating of the fuse. 

If the maximum current consumption exceeds the fuses rating, you are
in danger of overloading the circuit. Also, if the maximum consumption
is close to the the fuse rating there is danger of blowing the fuse in
case the wattages on the equipment are not accurate.

This same calculation applies both to the house and powerbar fuses.
However, you just need to add the wattages of the equipment and
compare in the case the fuse capacity is already written in watts as
in the case of your powerbar.

Best wishes,

Petrus Pennanen 
Helsinki, Finland

first you need to determine what outlets are on which circuits. they
may all be the same or could be different and some may even have
outlets in other rooms of the house or serve lights. the easiest way
to trace these circuits is to check if all outlets are on and then
shut of circuit breakers one at a time and see what goes off. then use
an amp probe or an amp meter at your panel to measure the amount of
current the circuit is actually using with all devices running, a
charting or recording meter would show usage over time. Or use on
infrared camera to see the temp rise in the conductor where it is
visible near the panel. do not open your panel unless you are a
qualified electrician or very comfortable working around electrical
circuits. Also, imho you should rerate the origonal answer as a zero
stars as it was wrong.

You can find equipment that might be helpful if you search for
"submeter branch circuits"