outlet voltage drop

[Tom Del Rosso]

I have a toaster oven and a microwave in the same outlet. A couple of
window fans are running on the same breaker.

If the toaster oven is on, the voltage at the outlet drops about 4 volts.

If the microwave is at half power (it's a 1300W unit) the drop is about the
same.

Is this typical, or is it a sign of too much resistance somewhere, which
would be getting hot?


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[Michael A. Terrell]

Tom Del Rosso wrote:

I have a toaster oven and a microwave in the same outlet. A couple of
window fans are running on the same breaker.

If the toaster oven is on, the voltage at the outlet drops about 4 volts.

If the microwave is at half power (it's a 1300W unit) the drop is about the
same.

Is this typical, or is it a sign of too much resistance somewhere, which
would be getting hot?

Not enough information. How long is the wire to the breaker, and
what gauge? If the line is at 130 volts, that is a 10 A load. A 4 volt
drop would dissipate 40 Watts. This would indicate .4 ohms in the
wiring. If it was all in one spot, it would be a problem. In a kitchen
that should be a 20A circuit, with 12 AWG wire. That has 1.588 ohms per
1000 feet. Double that for the two wires, for 3.176 ohms per 1000 feet,
or about 125 feet of wire from the breaker box to the outlet. If the
outlet is old or damaged, have someone who knows what they are doing
replace it. There is also some drop in the wiring from the power
transformer, and some sag in the windings.


What is the voltage between the neutral & the ground pins, under
load? If it is half the drop for the load, it should be OK. The ground
doesn't carry any current, and doesn't have a voltage drop. If there is
much variation, check for a loose or open neutral. Overall, that
doesn't sound too bad.


--
It's easy to think outside the box, when you have a cutting torch.

 

[Jeff Liebermann]

On Sat, 6 Aug 2011 15:51:22 -0400, "Tom Del Rosso"
<td_03@verizon.net.invalid> wrote:

I have a toaster oven and a microwave in the same outlet. A couple of
window fans are running on the same breaker.

If the toaster oven is on, the voltage at the outlet drops about 4 volts.

If the microwave is at half power (it's a 1300W unit) the drop is about the
same.

Is this typical, or is it a sign of too much resistance somewhere, which
would be getting hot?

Michael T. has all the right questions. I'll add a bit more by asking
where are you measuring the voltage? Inside the microwave oven, at
the wall outlet, or at the breaker box? Assuming you're in the US, 4v
out of 120v (instead of 240v) is 3.3% which is fairly small.

So, let's do the math. I'll guess you're measuring at the wall outlet
because it's the most convenient. I'll also guess that you have about
50ft of #12 AWG solid wire in the wall to the breaker box. At 0.00187
ohms per ft, that's 0.187 ohms resistance. There's also some
resistance between the breaker box and the transformer on the pole,
but that's much heavier guage wire, so I'll ignore that. However,
I'll round the resistance up to 0.2 ohms so I don't have to find my
calculator.

At 1300 watts and 120v, your microwave oven draws 10.8A. The voltage
drop in the wiring is:
10.8A * 0.2 ohms = 2.2volt
Ok, so it's not 4v, but it's close enough since I ignored outlet
contact resistance, screw terminal resistance, drop across the
breaker, temperature, and made assumptions on the house wiring gauge
and length. Change the numbers to fit your situation and see if it
comes closer to your 4v measurement.

The typical toaster oven burns about 1400 watts, so it's comparable to
the 1300w microwave oven in power line drop.
<http://www.oksolar.com/technical/consumption.html>

The window fan will have an effect on the line drop, but since you
didn't specify whether it was running during your measurement, I can't
guess what the effect might be.


--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

 

[Winston]

Jeff Liebermann wrote:

On Sat, 6 Aug 2011 15:51:22 -0400, "Tom Del Rosso"
td_03@verizon.net.invalid> wrote:

I have a toaster oven and a microwave in the same outlet. A couple of
window fans are running on the same breaker.

If the toaster oven is on, the voltage at the outlet drops about 4 volts.

If the microwave is at half power (it's a 1300W unit) the drop is about the
same.

Is this typical, or is it a sign of too much resistance somewhere, which
would be getting hot?

In addition to what Michael and Jeff said, I
suggest inspecting the outlet and every outlet
on that circuit from the breaker onward.

Please replace any outlet that is installed
using the 'backstab' method:
http://www.rd.com/images/tfhimport/2004/20041101_Dead_Outlet_page005img001_size2.jpg

At the very least, remove the stabbed wires, clean
them and then use the screw terminals instead.

--Winston