Overload an electrical outlet?

[Guv Bob]

"Dallas" <Cybnorm@spam_me_not.Hotmail.Com> wrote in message news:u_6dneVYTrXviszNnZ2dnUVZ_t6dnZ2d@earthlink.com...

I've been putting the final touches on a new computer system with three
monitors and a 750 watt power supply. The printer is 600 watts, the
monitors are 170 watts, I have a TV on the other side of the wall
that's 250 watts. add it all up to a usual load of about 15.3 amps. If
I turned on lights, amplifiers I can run the total up to 30.1 amps.

The circuit is 15 amps. :- (

And. I'd like to run all the computer equipment through one outlet so I
can unplug everything when the thunderstorms come.

So, I have a lot of questions:
1) First, why did the computer act like it tripped an overload in its
power supply, most of the load was outside of the computer? Why didn't
the residential breaker trip instead?

2) Can a breaker cut the load for a second without mechanically
flipping to the off position?

3) I seem to remember some people just replacing the 15 amp with a 20
amp breaker when things get tight, is that a dangerous/stupid idea?

4) Does anyone see a problem running about 13 amps through one
household outlet in a tree of several power strips?

Should I increase my fire insurance coverage? :- )


--
Dallas

Howdy doody Dallas. As a firm believer in M/F equal opportunity, I would recommend hiring a female electrician. Make it clear to her that she is expected to wear a regulation Nomex mini skirt and high heels (and that's all), is expected to climb up on the desk and check each the light receptacle voltage. And while she's up there, you can check her receptacle, too.

And you're welcome.

Bob

 

[josephkk]

On Wed, 12 Sep 2012 17:25:03 -0700, dplatt@radagast.org (Dave Platt)
wrote:

In article <u_6dneVYTrXviszNnZ2dnUVZ_t6dnZ2d@earthlink.com>,
Dallas <Cybnorm@spam_me_not.Hotmail.Com> wrote:

So, I have a lot of questions:
1) First, why did the computer act like it tripped an overload in its
power supply, most of the load was outside of the computer?

At a guess, the external load may have dragged down the voltage at the
outlet far enough that the computer power supply's "line undervoltage"
circuit kicked in, and shut down the computer cleanly.

Somehow that doesn't sound reasonable.

Why didn’t
the residential breaker trip instead?

Residential circuit breakers are typically of the "thermal" type.
They require some amount of time to react to an overload... the higher
the degree of overload, the slower they trip.

Ummn, i think you meant: The higher the overload the faster they trip.
Every breaker has what is called an (i^2)*t curve. At threshold tripping
current it may take 4 hours to trip, at 10 X tripping current it must trip
within one second.

This allows a circuit to have a momentary over-current (e.g. a
refrigerator drawing a surge of current when it first starts up)
without repeatedly tripping the breaker.

"Magnetic" circuit breakers will trip very rapidly in the face of even
a short overload... but I believe these are rarely found in
residential applications.

2) Can a breaker cut the load for a second without mechanically
flipping to the off position?

Typically, no.

Well, if the circuit breaker trips it must move to the "tripped" position
and stay there until reset.

3) I seem to remember some people just replacing the 15 amp with a 20
amp breaker when things get tight, is that a dangerous/stupid idea?

It is quite possibly both dangerous and stupid, and likely illegal.
The circuit breaker is supposed to be selected to be below the upper
limit of what the circuit wiring and outlets can handle safely.

It is actually all three. If there is a fire and the insurance company
investigator finds such a circuit they won't pay a dime.

If you draw more current than the wiring and outlets are designed to
deliver, you can suffer excessive voltage drop in the wires (lights
dim, motors slow down, and devices with "low line voltage" shutdown
circuits will shut themselves off. The wires can overhead. Also (and
perhaps more of a problem), the outlets themselves can overheat.

Now, it is *possible* that the wiring and outlets in your house are
actually rated for higher amperage (or that the outlets could be
upgraded for this). If so, it *might* be possible for a licensed
electrician to confirm this, and then swap out the lower-rated outlets
and breaker to increase the branch current level. I don't think it's
terribly likely, though.

Very much agreed.

4) Does anyone see a problem running about 13 amps through one
household outlet in a tree of several power strips?

This is probably in the grey zone. A lot of power strips are rather
cheap, and their internal wiring and contacts are designed for
"convenience" loads rather than high-current applications. If you
pull lots of amps through several of them together, you'll probably be
making the voltage-drop and heating problems worse.

I understand that many areas' electrical codes specifically forbid
"daisy chaining" power strips.

Also true.

Should I increase my fire insurance coverage? :- )

I suggest that you revise your expectations, instead... limit your
circuit use and loads to what they are presently rated for.

Agreed.

 

[josephkk]

On Thu, 13 Sep 2012 13:45:55 -0700, dplatt@radagast.org (Dave Platt)
wrote:

In article <0tb458tm14juranb417mkb30p5a2sakslc@4ax.com>,
PlainBill@yawhoo.com> wrote:

A load that dropped the voltage by more than 10% would trip the breaker
first.

A demonstration of arrogance and stupicity in a single sentence -
quite typical for you.

NORMALLY a 15 amp outlet is wired with 14 guage copper, but many homes
built in the 60's and 70's used 12 guage aluminum wire. The
calculated voltage drop through a 100 ft run of 14 gauge copper NMC
cable carrying 15 amps would be 7.8 volts, aluminum would be 9.5
volts. HOWEVER, it is unlikely the wire runs directly from the
circuit breaker to the outlet in question, and we can't assume a
straight-line distance. Common wiring practice is to run the wire
back into the basement (or the attic) between outlets. If the attic,
each excursion would add 15 feet to the length. Also, each outlet
represents 2 (if using wire nuts) to 4 additional connections, each
increases the voltage drop slightly.

Also a factor: time. I've just looked at a couple of "current vs.
time-to-trip" curves for circuit breakers, and they seem to share some
common characteristics. Below a certain threshold (4x to 9x the
nominal rating), the breaker has a long (often *very* long) "time to
trip" - at 3x the nominal rating, the time-to-trip on these two is on
the order of 10 seconds. It isn't until you hit a higher-current
threshold that the breaker opens "instantaneously" (tens of
milliseconds or less).

Citations:

http://electrical-engineering-portal.com/time-current-curves

and Google on "circuit breaker trip curve" to find numerous other
example curves of the same general nature.

So, using the figures Bill cited above: if you drew 30 amps through a
100-foot 14-gauge aluminum-wire run, there would be 19 volts of
drop... and in the curves I'm seeing here, it looks as if the breaker
wouldn't open for over 20 seconds. That's more than a 10% drop in
nominal voltage, and this doesn't account for any voltage drop in the
outlet itself.

A current-draw surge of 45 amps would drop around 28 volts, and these
breakers wouldn't open for about 10 seconds (3x rated current draw).
This would put a nominal "120-volt" circuit down right around the
100-volt level.

I can tell you from personal experience, that if I start a 10"
hand-held circular saw (nominal draw about 6 amps I believe), and have
it plugged into a 15-amp circuit which shares an outlet with our den
and computer, the momentary surge of current draw when the motor
starts does *not* trip the breaker, but *does* cause the computer in
the den to abruptly power itself off - just as the original poster's
system did! I've had to learn not to use my power tools out by the
back shed (which has this shared circuit) when my wife is on-line...
the storm of "I lost the email I was writing" is just too painful to
bear

That is almost worth getting a UPS for the computer.

?-)

 

[John G]

josephkk has brought this to us :

On Wed, 12 Sep 2012 17:25:03 -0700, dplatt@radagast.org (Dave Platt)
wrote:

In article <u_6dneVYTrXviszNnZ2dnUVZ_t6dnZ2d@earthlink.com>,
Dallas <Cybnorm@spam_me_not.Hotmail.Com> wrote:

So, I have a lot of questions:
1) First, why did the computer act like it tripped an overload in its
power supply, most of the load was outside of the computer?

At a guess, the external load may have dragged down the voltage at the
outlet far enough that the computer power supply's "line undervoltage"
circuit kicked in, and shut down the computer cleanly.

Somehow that doesn't sound reasonable.

Why didn’t
the residential breaker trip instead?

Residential circuit breakers are typically of the "thermal" type.
They require some amount of time to react to an overload... the higher
the degree of overload, the slower they trip.

Ummn, i think you meant: The higher the overload the faster they trip.
Every breaker has what is called an (i^2)*t curve. At threshold tripping
current it may take 4 hours to trip, at 10 X tripping current it must trip
within one second.

This allows a circuit to have a momentary over-current (e.g. a
refrigerator drawing a surge of current when it first starts up)
without repeatedly tripping the breaker.

"Magnetic" circuit breakers will trip very rapidly in the face of even
a short overload... but I believe these are rarely found in
residential applications.

2) Can a breaker cut the load for a second without mechanically
flipping to the off position?

Typically, no.

Well, if the circuit breaker trips it must move to the "tripped" position
and stay there until reset.

3) I seem to remember some people just replacing the 15 amp with a 20
amp breaker when things get tight, is that a dangerous/stupid idea?

It is quite possibly both dangerous and stupid, and likely illegal.
The circuit breaker is supposed to be selected to be below the upper
limit of what the circuit wiring and outlets can handle safely.

It is actually all three. If there is a fire and the insurance company
investigator finds such a circuit they won't pay a dime.

If you draw more current than the wiring and outlets are designed to
deliver, you can suffer excessive voltage drop in the wires (lights
dim, motors slow down, and devices with "low line voltage" shutdown
circuits will shut themselves off. The wires can overhead. Also (and
perhaps more of a problem), the outlets themselves can overheat.

Now, it is *possible* that the wiring and outlets in your house are
actually rated for higher amperage (or that the outlets could be
upgraded for this). If so, it *might* be possible for a licensed
electrician to confirm this, and then swap out the lower-rated outlets
and breaker to increase the branch current level. I don't think it's
terribly likely, though.

Very much agreed.

4) Does anyone see a problem running about 13 amps through one
household outlet in a tree of several power strips?

This is probably in the grey zone. A lot of power strips are rather
cheap, and their internal wiring and contacts are designed for
"convenience" loads rather than high-current applications. If you
pull lots of amps through several of them together, you'll probably be
making the voltage-drop and heating problems worse.

I understand that many areas' electrical codes specifically forbid
"daisy chaining" power strips.

Also true.
In Australia Power strips are generally fitted with a breaker so the

first strip governs the total load (10 amps)on a daisy chain and so
ensures (goes a long way) no overload occurs.

I think this has been a requirement for may years now.

Should I increase my fire insurance coverage? :- )

I suggest that you revise your expectations, instead... limit your
circuit use and loads to what they are presently rated for.

Agreed.

--
John G