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Joined: Jun 2001
Posts: 141
amp-man Offline OP
Hey there,

Just thought you'd enjoy hearing about a trouble call I went on this weekend...

A client called me Friday afternoon, frantic. Their computer/cash register in one location keeps crashing, and could it be the circuit that it's powered by? BTW, I installed that circuit a couple of months ago. They need to get the PC working ASAP. If it's not fixable, could I recommend a UPS/power conditioner unit?

Background--the customer is setting up a retail operation in converted industrial space. I installed a 10kVA dry xformer to take 480 to 240/120, and bonded neutral/center tap of xfrmer to earth ground. Installed a loadcenter and about 150' of EMT. Ran two 20A circuits, one for computer and one for convenience outlets. Separate neutrals. Ran isolated ground for computer ckt.

I get on site and see that there's an extension cord plugged into the outlet I installed. It's a nice heavy cord, looks like 12 gage, and it disappears under the bottom shelf of the counter on which the computer is sitting. The PC is about 10 feet from the outlet. A surge protector/power strip is plugged into the far end of the extension cord, and the PC and some other stuff is plugged into the outlet strip. The power strip looks pretty beat up.

I whip out the trusty Fluke T-5 to check voltage. A-OK at the outlet, 119V, ditto at the strip. I jiggle the plugs and the power cords and flip the power strip switch thinking that maybe it's an intermittant-- bad plug or dirty switch. Everything seems OK--there's no loss of power/dropouts that I can see on the tester.

The client says he's had a problem with the computer since he set it up at that location. Maybe the power is dirty; we're in an industrial area and I'm wondering what's going on in the factory next door, which is fed from the same customer transformer and main switchgear.

I get the scopemeter (Fluke 123) and check power at the outlet. Perfect sinewave, clean as a whistle. I go to the power strip and get a "fat" sine wave--the trace is a sine wave, but kind of thick and fuzzy. Hmmm, what the heck is this?

I unplug the power strip and insert the probes into the end of the extension cord. Same "fat" sinewave. I'm thinking it's something to do with the extension cord, and maybe a new cord will solve the problem.

So I unplug the cord from the outlet and figure I'll look it over just to see what I can see. I'll bet that some of you know exactly what the problem was!

The extention cord was a nice one--12 gage, romex brand, AND IT'S 100 FEET LONG! All coiled up under the bottom shelf of the big choke!

I suggested that the client get power strip with a 10' cord.

I'm always amazed at how little people know about electricity. Alomg these same lines, a friend of mine was a sparky at a nearby AF base said that some maintenance men once needed a long extension cord so some Einstein decided that a 250' coil of romex would work great. They put cord caps on the ends of a 250' roll and used (uncoiled) about half of the roll to get where they needed to go. They then started using the big drill motor or whatever they were using.

Imagine their suprise when the coil of romex burst into flame. The coil was acting as a nice choke and the heat generated led to a short circuit, with all the attendant fireworks.

AT any rate, I wish they were all this easy!


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Joined: Oct 2000
Posts: 5,392
Imagine their suprise when the coil of romex burst into flame
LOL!Cliff, that usually makes a 'statement'.

BTW, i read an article, some time ago, that actually recommended coiling extention cords serving sensitive equipment. I believe that it was refered to as a 'Belden Wrap', i wish i could pick it out in this mountain of trade mags.......

Joined: Nov 2001
Posts: 745
In your case, it's easy if you have good troubleshooting skills, as well as the proper testing equipment at hand. Excellent call!

Mike (mamills)

Joined: Nov 2000
Posts: 2,148
I didn't think that "excessive" heat was produced by a coil of wire unless there was some ferrous metal inside the coil. This was the cause of the McCormick place fire in Jan of '67 or '68. The National Housewares show was there and a large SO cord with a 40 or 50 amp load was in use. It was too long and the excess cord was coiled up and dropped over the 411 box on the end of the cord. The box was heated to the point that packaging material ignited and the place burnt down. It is right on the lake, but the fire department couldn't get water because of frozen hydrants. It went to a 5-11 and 5 special alarms. I don't think Chicago has ever sounded that many alarms for a single fire before or since, with of course the exception of the "Chicago Fire".

Joined: May 2001
Posts: 160
Did the outlet strip with the 10Ft cord fix the problem?
Was the fat part of the 60 Hz sine wave some sort of high frequency interference picked up by the 100Ft cord?If so were you able to locate the source of the interference?
Just curious.


Joined: Aug 2001
Posts: 7,520
A couple of points strike me about excessive heat caused by coiling up an extension cord.

First, both the line and neutral wires are parallel throughout and the current is flowing in opposite directions at any moment in time. The magnetic fields would tend to cancel and the inductance would be very low.

Second, even if that were not the case, remember that inductive current is wattless -- it does not generate any heat. The heat in the cables is a function only of the resistance of the copper and the current, i.e. (I^2)*R.

The only way I can see for the current to increase drastically would be if the inductance introduced happened to match any capacitance on the load to form a tuned-circuit series resonant at 60Hz. And that would need some large values of L and C.

I think it's more a case that an enclosed coiled cord fully loaded just can't dissipate the power caused by normal resistive heating fast enough.

The resonance point may have a bearing on the high-frequency noise, though. The small amount of inductance coupled with stray capacitance could easily set up a tuned circuit, and if it happens to be resonant at a frequency on which a lot of noise is present, you've effectively got a very efficient antenna for that noise.

Joined: Nov 2001
Posts: 280
I just tried an experiment wiht 250 feet of 12-2 still coiled up. Put a 10 amp load on it and the coil began to heat up. So far so good your statements hold up. the voltage drop was about 10 volts.
Everything was as you say, until I put the gauss meter on top of it and it went nutz.
Radially it read about 150mg but when set on top of it about a foot from the top of the coil " out of range"
The heating I assume was due to the lenght of wire in the coil 250 feet at 10 amps, but the size of the magnetic field was unbelievable. The load was strictly resistive
all 100 watt bulbs.
This was not any double blind study just hooking some wire to see what would happen, but the magnetic field surprised me.

Anyone can jump in, because what Paul said makes sense but I am not sure.

Joined: Apr 2002
Posts: 914
Very interesting. I guess with the coil you made an electro-magnet, after all windind wire around something is how you make one.

On Voltage Drop. I went on to a job site one day and I saw one of my helpers using a Hole Hawg that sounded real low and was just barely making it through the wood. He said the drill was bad and it was going to take forever to finish. Then I noticed that he had 2 100 ft 16 gauge cords (left from HVAC crew) pluged into a 50 ft 12 gauge off the temp pole. I had him take off the 200ft of 16 and add 100 ft of 10 and amazingly the Hole Hawg worked perfectly. Imagine that.

Joined: Aug 2001
Posts: 7,520
Interesting experiment.

Thinking about it some more, I guess I would expect some inductance, because the line/neutral fields aren't going to cancel out precisely due to the wires not being perfectly uniform throughout the whole 250' coil.

I suppose it comes down to a question of just how much inductance it is possible to get in this way.

How about trying it again just taking the hot wire through one wire of the Romex and wiring the neutral straight to the lamp?

Joined: Nov 2001
Posts: 280
I will try that and let you know the results... By the way your experiment with the tap-water sounded interesting, but any time water is basic/acidic it will conduct as you mentioned, and I believe highly mineralized water is basic ?


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