Retractable Reel Cord and Heat

[sacherjj]

I went with a 12g for extra safety, but I found that it won't retract fully. I had to cut off about 6 feet to allow it to fully retract without supervision. I only use it for one hand tool (jig saw, router, etc.) so I don't worry about fully uncoiling it. I use a good 12 G 20 feet cord for any of the serious tools.

[big tim]

An experiment

I thought I might as well do an experiment myself. I know it wasn't very scientific but it might prove the point.
I used an electric fan heater, at it's maximum setting of 13Amp(approx 1500W), as a load. The cord 12AWG and 80ft long(160ft of conductor). At that current draw the voltage drop across the cord measured 3.2Volt. Theoretically that should have been 3.25Volt but lets not quibble about 0.05Volt. The wattage to be dissipated by the cord would than be 13 x 3.2 = 41.6Watt.
I let the heater run for 30 minutes. The cord was initially cold to the touch because is came out of a cold closet. After 30 minutes it felt slightly warmer, which I'm sure would be because it was starting to warm up to room temperature. The picture below shows the set-up( I know doesn't look very scientific).


I then covered the cord, see pic below.


After another 30 minutes(total one hour now) the temperature of the coil was slightly warm to the touch, nothing too dramatic though. I then covered the coil with an old drawer to basically enclose it.
The picture blow shows that.


I kept the drawer on the coil for an additional 30 minutes. The coil definitely felt warm to the touch, I then laid a thermometer on top of the coil and put the drawer back on top, waited for about 15 minutes, took the drawer off and read the temperature, it was 84F.
This was after 1hr 45 min. of a continuous 13Amp load.
I'll let you all draw your own conclusion.
Nevertheless always be vigilant.

Tim

[LCHIEN]

Thx fellas. The physics make sense to me but I also wondered if it was one of those urband legend type things.

Loring, I will start thinking of my cord reel as a storage device only.

Good for you, Be safe.

There are a number of factors involved.
When considering general rules engineers and corporations look at the worst case. Because while each user is a single case, and not likely to have a bad experience, 100,000 users with a fire liklihood of .001%
is too high.
The difference between a DIYer and a engineer is that the engineer has to build it a minimum cost yet make all 100,000 work while a DIYer only has to make it work once.

Yes, many engineering limits are conservative but we have to look at those .001% chances for catastrophic failure some of which may result in property loss or death.

They will tell you that those cases/reels are made for storage and that's because they want no ones to catch on fire.
I'll bet someone has overloaded a cord, had it wound up and had the case melt and possibly the insulation short.

BIG TIM, that's not a real bad experiment, you forgot a few things.
FIrst and most important, in any thermal experiment you must Always measure the ambient temperature before and after (because ambients change B&A). Now your final temperature has meaning, otherwise it does not. Heat flow always is based on differential. So was your shop at 50F, 60F, 70 F or 80F? Each starting pooint would make a 10F change in the end point. Its always differential.

Another thing you of course omit is that wire resistance has a positive tempco (temperature coefficient). That means the resistance is a function of temperature and in this case it gets more reisistive (more ohms) when it gets hotter. Its about 4% per degree C for copper.

If you start at 110F instead of 70F thats about 20C or 8% more resistance and 8% more Temperature rise.
Being positive, as it heats up, it causes the resistance to go up, causing in turn more heating. Normally this limits but in certain conditions it can actaully run away.

Finally you measured the end temperature as the outside of the insulation or the temperature insde the box. There will be a temperature gradient dependent upon the materials. The wire being the hottest, the insulation the next, the air lower and the box the lowest. So the temperatures are much hotter at the wire, the winding was not overly tight (by your picture allowing air to circulate and cool the insulation).

It is what it is but there are circumstances where it will be much worse.
Its partly a matter of statisitcs but its also dependent upon the care with which you use it. You want to be on the conservative side.

The NEC (national electric code) is very anal about dressing of wires and wire sizes and this is all so the wires won't overheat. They are conservative becasue this is the basis for wiring in 100 million homes and 20 million businesses and for them even .00001% is too risky.

[Kristofor]

I see where you're coming from, and that's what I was getting at about empirical evidence.

However, remember that you're using ~60% of the capacity of the extension cord, in a lose coil, in Toronto, Canada with ambient room temperature of somewhere near 75 degrees...

If someone tries the same thing with a 14AWG cord wound around a central spool, inside an enclosure, in their garage in Texas in July they may be starting from 115 degrees. I'm sure it would melt under extended use (heck we've seen the pictures). Even with a 12AWG the temp alone would increase resistance ~8% from your test (or 27% compared to my garage last week) and that may be enough to start the feedback process to make it overheat.

[edit] The expert beat me to the punch!

[edit2] It's about 0.4% per degree for a temp coefficient for copper, which is how Loring did the math, but not how he typed it out in one spot.

[LCHIEN]

I see where you're coming from, and that's what I was getting at about empirical evidence.

However, remember that you're using ~60% of the capacity of the extension cord, in a lose coil, in Toronto, Canada with ambient room temperature of somewhere near 75 degrees...

If someone tries the same thing with a 14AWG cord wound around a central spool, inside an enclosure, in their garage in Texas in July they may be starting from 115 degrees. I'm sure it would melt under extended use (heck we've seen the pictures). Even with a 12AWG the temp alone would increase resistance ~8% from your test (or 27% compared to my garage last week) and that may be enough to start the feedback process to make it overheat.

[edit] The expert beat me to the punch!

I think you said the same thing in a lot less words.

[big tim]

You're right of course Loring. That's why I said that it wasn't a very scientific experiment and put in the caveat at the end. However the coil was about as tight as you might expect it to be on a reel, I would think. 80ft long almost 1/2" thick cord.
The shop was kept at about 65 degrees. I had to crack open a window because the heater was starting to raise the temp. in the shop.
Now, having said all this, would I run a 1500Watt heater in a coiled up cord or a cord reel? The answer would be NO. That simply wouldn't make sense. Why would you use a cord reel if you have to leave it substantially rolled up. I wouldn't hesitate though to run a sander or something along that line on a cord reel. You cut the current by 50% and the wattage goes down to a quarter.
Nevertheless, as I said before "Always be vigilant"

Tim.

[LCHIEN]

...You cut the current by 50% and the wattage goes down to a quarter.
Nevertheless, as I said before "Always be vigilant"

Tim.

Well, if the voltage drop is under a couple of percent, then cut the current by 50% the wattage will be reduced by close to 49-50% also. not 25%

And, if all we run are heaters, then the power loss in the wire is moot from a performance standpoint, what goes to the heater 100% becomes heat, what gets lost in the wire also 100% becomes heat, its a lossless system.
Of course, you don't really want to think this way, if the side effect is setting your garage on fire.

[big tim]

I don't want to make this in a battle of wits (I don't even know if I have spelled that right) But I said that if you cut the current by 50%, i.e. down to half of what it was before, than the voltage drop across the cord will drop down to half of what it was before, thus the wattage to be dissipated by the cord will drop down to(not by) 1/4 or 25% of what it was before.
Yes, using your cord as a space heater is not a good idea.

Tim

[mikeinkcmo]

I like the KISS method. If it starts to smell, its not well.

[TheRic]

...And, if all we run are heaters, then the power loss in the wire is moot from a performance standpoint, what goes to the heater 100% becomes heat, what gets lost in the wire also 100% becomes heat, its a lossless system.
Of course, you don't really want to think this way, if the side effect is setting your garage on fire.

But wouldn't setting your garage on fire produce more heat??!!

[LCHIEN]

But wouldn't setting your garage on fire produce more heat??!!

yes, that would temporarily increase the efficiency of youir electric heat.

[sacherjj]

For some reason I'm getting an interesting dialog in my head.

Wife: "Honey, the house is on fire! Why are you moving wood? Call the fire department!"

Husband: "Get out of the house and give me just a second to finish this."

Wife: "You aren't going to help by stacking all your wood on the lawn. The house is burning!"

Husband: "But the house is just Pine. This is Wenge, Walnut, Cherry, Oak, Teak, Ebony, and Bubinga. Just shut up and push my BT3100 out here too."