The modern home/workplace uses lots of cables - electrical appliance cables, phone cables, audio cables, USB cables and so on. Sometimes, one has a cable that is much longer than than needed - say a 3 meter cable, when an appliance is less than a meter from a socket. This excess cable looks untidy and in no time it’ll get entangled up with itself. Several cables intermingled will, in time, produce something resembling the Gordian knot.
Previously, I’ve sometimes wound up long leads in a coil and then tied them with a rubber band or string to keep them organized and tidy. However, I thought I heard or read that this wasn’t a good thing to do with a cable that will have a live current going through it. Perhaps something to do with the current being closely parallel with itself if coiled up.
Is this correct? What about for different cable types (electrical/phone/audio), regarding interference or suchlike? Also, if it’s not good practice to coil live cables, then what would be best practise, rather than leaving them in a big tangle on the floor?
Coiling audio, phone charging, or USB cables shouldn’t be an issue as long as they aren’t coiled tight enough to damage the cable itself.
AC power cords could be an issue depending g on the power being used and the density of the coil. Loose coils allowing airflow for cooling are no problem. On the other hand, a 100 foot extension cord tightly wound on a reel and carrying full rated current can get hot enough to catch fire. I’ve seen displays of such at a local firehouse.
I think the ideal practice, obviously, is to spend a bit and replace the 3 meter cable with a 1 meter cable.
But often that’s not practical, for budgetary or other reasons. Your real question is what’s wrong with the long ones. If you’re talking about electrical power cables, then my understanding is that the main danger is overheating if the appliance is high-powered, such as an air conditioner or toaster oven. But as far as interference, I say let experience guide you; if the devices in the area have have a clear output (no static, etc) then don’t worry about it.
All the above is if the cables are coiled up. If they are not coiled, then the main danger is of tripping over them, which can lead to pulling an appliance of the table, and similar disasters.
Coaxial cable should not be bent into too small a circle, because the interior conductor can get too close to the outer shield. In severe cases they can actually touch, but even if they don’t touch there may be a change in impedance at the point where they are not concentric, which can affect the performance of the cable. It doesn’t matter if the cable is live or not, this can be a permanent physical damage to the cable.
So the issue is purely based around generation of heat (rather than something electrical)?
The instructions for my vacuum cleaner, which has a coiled retractable cable, says to always pull the cable out to a marked red line (which is pretty close to being the fully-extended cable). I presume, since vacuum cleaners are energy-hungry, that this would be to prevent a risk of fire from over-heating of coiled cable inside the appliance?
Yes. It is about heat generation when being used for high current draw applications. You keep hearing people talk about it being due to magnetic or inductive effects, but because the cable carries both legs of the current this is not an issue.
Copper is expensive so cords are designed with a minimum amount, and are only rated for full current use assuming adequate cooling. Coiling them up violates this assumption.
Thanks for the replies. It sounds as though loosely coiling non-high power cables should be okay then.
I see it’s also possible to buy stretchable coiled power cables (although they use a helical coil structure with length, rather than the lasso-like coil of my cable-tidying).
Quite correct: you’re creating an electromagnet, which means heat. However, this is only really a problem for cables that carry a significant current, like mains cables. If you look at a reel of mains cable, you’ll likely see a notice that when used it should be fully unwound.
Note that coiled up cables get hotter than straight cables not so much because everything is closer together, but because coiling intensifies the magnetic fields that all currents generate.
To me, the magnetic field of a coiled cable seems to be much weaker than the magnetic strength of the magnets that I have on my kitchen refrigerator. But the magnets on the refrigerator are not warm at all. Is electromagnetism different than regular magnetism in this regard?
Also to make sure that people pull it carefully and don’t rip the cable out; a partial rip can produce a short circuit.
And yes Keeve, magnetism and electromagnetism are different that way. The first one doesn’t require anything to be moving in order to happen; the second one does (there is no electromagnetism without electrical current).
The thing about pulling all the cable out of a vacuum cleaner is really an ass-covering exercise. Yes, if you left it running for a long time, there is a possibility that it may overheat. Most of us run vacuums for a few minutes at a time, so it isn’t really that much of a risk. In fact, tripping over the loose cable might actually be a higher risk, but the manufacturers couldn’t be sued for that.
Of course - the usual disclaimers apply.
When it comes to power circuits, larger cross section cables are the rule where the cable is enclosed (as in conduit), than where it is open to the air. For example, 4.00mm cable can carry 36 amps in the open, but only 25 amps in conduit.
Wire dissipates power in the form of heat when current is running through it. Pack enough insulation around the wire and the insulation could overheat.
At max allowable current, a 12 AWG extension cord will dissipate about 1 W of power for each foot of length. For a tightly-coiled cord, this can become a problem for the several feet of wire buried “deepest” within the coil. Every second, each foot of cordage is pumping out 1 J of energy. If that energy cannot be efficiently conducted or convected[sup]a[/sup] out of there, the temperature will rise and rise…
So does the heat come from the moving current, or does it come from the magnetism? Or from some combination of the two? Because if it is coming from the current, then it should not matter whether the cable is coiled or stretched.
This is wrong. If the magnetic field was related to the heat generated, then any two wires that created the same magnetic field would generate the same amount of heat. But it’s pretty easy to set up two current-carrying wires that have the same amount of current (and therefore the same magnetic field) but generate different amounts of heat. Just make sure that the two wires have different resistances, and the power converted to heat will be different.
The heat comes from the moving current. Roughly speaking, electric current happens when current flows from high voltage to low voltage. Voltage is proportional to potential energy for a given charge; so as a charge goes from high voltage to low voltage, this potential energy is converted into heat. As I noted above, the magnetic field has nothing to do with it.
It’s from the moving current. Coiling the cable is still bad, because you are packing a long length of cable into a small volume, and not allowing air flow to cool the inner part of the coiled cable.
This is wrong, as MikeS notes. Among the reasons why it is wrong, the cable consists of two current-carrying conductors, carrying current in opposite directions. Their two magnetic fields roughly cancel each other out. This is the reason why normal clamp-on ammeters don’t work unless you dig out the hot or neutral alone.
The two conductors aren’t exactly coincident, so some field still leaks out. You can get more expensive and less accurate clamp meters that work on that. In any case, that’s not the relevant effect here. The concern is just that the same amount of heat gets generated in a smaller space.
We have a Miele canister vacuum with a self-retracting cord that coils up inside the vacuum canister. After vacuuming the house, the cord is rather warm to the touch; it’s not hard to imagine that if a significant portion of this cord were kept coiled inside the canister, and the vacuum were run for a very long time, a lot of heat would build up in there without being dissipated to the environment (as it is when the cord is out in the open). If the ohmic heating in the cord is X watts per meter, then the heat dumped into the cord coil is XYZ, where Y is the length of cord that remains coiled in the canister during use, and Z is the amount of time the vacuum is turned on.
If the cord were coiled up out in the open, this wouldn’t be nearly as big an issue. Also, if the cord were bigger, there’d be less heat generated per unit length of cord. Stationary appliances are typically fitted with a heavy enough cord such that heat buildup isn’t a problem, and you can safely coil the cord up and tuck it out of sight behind the appliance.
