At what point do I need to figure 8 my power cables?

[BlueGreene]

So I recently did a gig where we used one of my 100' extension cords for the job of a 25 footer. Leaving long power lines coiled will generate a magnetic field right? But at what point do I need to unwind it and figure 8 it? I didn't know, so I took the time to unwind it but was that necessary? I was pulling less than 15 amps. Probably little more than 10. But, there was going to be many turns on that coil if I had left it over-undered.

 

Is there a rule of thumb for this kind of stuff?

[wesg]

I'd love to find out, too. I started a fire at work once. I had a brand-new extension cord, and only needed one foot more, so I lazily left it in the package. Then put about 8 or 10 amps through it. It caught fire after 20 minutes, so I unplugged it and threw a pot of coffee on it.

 

I over-under all my overly-long cords now and cross my fingers.

[Craig Vecchione]

Wes, was this a 100-footer too?

 

Typically, it would have to be a tight and uniform coil, as with your still packaged example, to have enough windings in close enough proximity to generate heat. But rather than chance it, I typically just slide the coil into a flatter version...it can still be recoiled (sortof) but won't heat up, at least in my experience.

[wesg]

This wasn't a 100-footer. 25 foot 18 awg if my memory serves me correctly. It was quite a few years ago, but I remember it was orange, and all our long cords are yellow or black.

[agedhorse]

In general, I find that the whole figure eight thing to be way overblown, even at 100 amps 3 phase on #2 cable I haven't found anything more that typical I**2 x R heating effects. I also have not discovered the effects of hum either, with the exception of perhaps a mic next to the coiled distro.

 

Perhaps things might be a little different at 400 amps with 4/0, OR with single conductor strands coiled independently, but in 35 years of doing this I have not been able to duplicate it under normal operating conditions (and I have tried).

 

 

[Craig Vecchione]

This wasn't a 100-footer. 25 foot 18 awg if my memory serves me correctly. It was quite a few years ago, but I remember it was orange, and all our long cords are yellow or black.

 

I wonder if it was a defective cord. I heard about the phenomena ages ago but have never had any heating that I wouldn't attribute to a cable carrying a high current load and all that heat being in a smaller area than if the cable was stretched out.

[agedhorse]

A 25' x 18 gauge cable with 8-10 amps isn't going to cause enough heating to burn. If you work the math out, that's 50 feet of current carrying conductor, 0.319 ohms. Solving for Pd, that would be 10**2 x .319 = 32 watts. Not enough even if you could tightly bundle the wire, to ignite or even melt the typical SJT/SVT type insulating jackets. After 20 minutes, that would have been only 10 watt-hours of energy. My soldering iron is bigger than that, all concentrated into a very small area.

[BillESC]

Back in the mid 70's I think it was my company was providing Super Troupers and operators for a Todd Rundgren concert. A 100' 12/3S cables was used from an outlet maybe 10 feet from the lamp. Half way through the concert the lamp went out and it turned out the cable had melted enough to short. My experience.

[Audiopile]

Back a few decades ago I experienced (my own doing) a 250ft. roll of romex being used as a 50ft. extension cord... the remaining 200ft. coiled in the box... it when up in smoke in a big way... admittedly it was unwittingly likely packing 50 amps through it. (where's that facepalm smiley thing... you need that when you do something stupid and the fire department's deployed... even if it's out in the middle of a hay field) That's the short story. Anyhoo... since then (and well before that for quite awhile)... I've never had a heating problem with some extra power cable coiled, or wadded in a pile (as the case may be with me)... but I keep tabs on it when it is all bunched up... rarely happens anymore though... I've "evolved" to the point of having enough of the right stuff to match the cable to the job... usually within 5ft overage... which is nice.

[agedhorse]

I've owned a few Super's in my time and used to do frame up rebuilds on them in our shop. (We also used to service the Strong 1K cinema lamps which are almost exactly the same, as well as the larger models of carbon arc lamphouses for the cinema industry.)

 

A Super can easily draw a little over 20 amps at 120V, so running the same calculations, R (200ft) = 0.324 ohms, I = 22A, Pd = 157 watts, now this could get pretty warm after a while, problematic with low temp cable.

 

It's the squared term that gets you every time.

[wesg]

Andy - thanks for running the math on that. That incident has bothered me for years. I actually remember the hardware at the end of that cord.... two Sun Microsystems Sunfire v445s, two Sun StorEdge 3510 FC arrays, both fully loaded, and a couple of switches. *downloads data sheets*

 

778 * 2 + 440 * 2 + 200 = 2636W / 120V (power factor=1) = 21A _MAX_

 

Holy cow, I way underestimated the power draw. Is 21A enough to cause grief, or was I dealing with a defective cable?

 

On a related note, it's interesting that the 15A breaker didn't blow -- either the breaker is bad or the equipment wasn't drawing max load. Either way, I need to have a chat with my SE on Monday morning..that should never have been hooked up like that, one of us should have caught that. Better late than never, safety is job one. Good thing the power bars in the data center for the production equipment all have power displays. We never deploy over 44% of circuit capacity, so that we can tolerate a 50% power outage (one UPS leg).

 

Wes

[agedhorse]

At 21 amps, multiply by 1.25 for the minimum circuit ampacity which puts you squarely into 30 amp territory.

 

The 15 amp breaker should have opened after 5 - 10 min at that amount of overload... either the draw spec. is wrong or the breaker is defective, or the breaker is bypassed (I have seen this a fair amount).

[Craig Vecchione]

There are plenty of breakers that fail to open when they should. As they age they can go in either direction...fail to open, or trip far too early. And some aren't used correctly for the type of load they'll encounter, which has its own set of problems.

 

But 21 amps through an 18AWG cordset even stretched arrow straight would likely melt its insulation eventually...it's typically rated for no more than 10 amps @ up to 50 foot lengths.

 

The coiling factor is additionally a problem because it effectively creates a bundle. Code usually requires derating for a number of conductors (over 6?) bundled for more than 2 feet. Most coils you'll find cordsets in will have plenty of bundled length for purposes of this discussion.