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Author Topic: Voltage drop question  (Read 9456 times)

Jonathan Johnson

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Re: Voltage drop question
« Reply #10 on: April 14, 2017, 08:23:59 pm »

Exactly. However, because the heating effect is uniform across the length of the extension cord (resistor), my little 10 ft piece of test cable should exhibit the same localized temperature rise as a 100 ft cable. So if you take a 100 ft of 16 gauge extension cord and make pass 30 amperes of current, it should heat up similarly to my short short cable with 3 volts passing 30 amperes of current. I've simply created a small sample from the middle of a larger sample.

And if that 100' extension cord is coiled, the heat won't dissipate so it can get hot enough to melt the insulation.
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Ed Hall

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Re: Voltage drop question
« Reply #11 on: April 14, 2017, 11:20:10 pm »

And if that 100' extension cord is coiled, the heat won't dissipate so it can get hot enough to melt the insulation.

Is that why large feeder, 4/0 etc., isn't coiled but excess is laid in a figure 8 or is that done for inductive reasons?

I'm thinking inductive reasons because if 4/0 would get hot it should be larger or multiple runs.
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Kevin Graf

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Re: Voltage drop question
« Reply #12 on: April 15, 2017, 08:48:45 am »

In a cable with a closely spaces pair of wires, the inductance in the send conductor cancels the inductance in the return conductor.
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Jonathan Johnson

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Re: Voltage drop question
« Reply #13 on: April 15, 2017, 12:56:35 pm »

Is that why large feeder, 4/0 etc., isn't coiled but excess is laid in a figure 8 or is that done for inductive reasons?

I'm thinking inductive reasons because if 4/0 would get hot it should be larger or multiple runs.

I always figured it was because laying heavy cables (and hoses) in a figure 8 is a lot easier than the over/under technique.
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Mike Sokol

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Re: Voltage drop question
« Reply #14 on: April 15, 2017, 10:06:17 pm »

Is that why large feeder, 4/0 etc., isn't coiled but excess is laid in a figure 8 or is that done for inductive reasons?

I've run the numbers before, and there's just not enough induction with any possible feeder coil to create heat at 60 Hz. However, as noted above, a bunch of wire colled together closely, especially in a roadcase, will get hot simply due to the trapped heat and lack of circulating air. That's why we always pull all the feeder cables out of the roadcase if it's going to be loaded to near capacity.
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Ivan Beaver

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Re: Voltage drop question
« Reply #15 on: April 16, 2017, 08:53:58 am »

Is that why large feeder, 4/0 etc., isn't coiled but excess is laid in a figure 8 or is that done for inductive reasons?

I'm thinking inductive reasons because if 4/0 would get hot it should be larger or multiple runs.
Yes a coil of cable will help keep the heat in a smaller area, but the inductance is the main reason.

When you coil it up, you form a nice inductor that can produce lots of heat-both to itself and internal objects.

Here is a fun little test.

Take a standard passive loudspeaker apart to where you can get to the xover, it doesn't matter what the model is.  A stronger one is more fun however. :)

Now grab a screw driver and run music through the cabinet.

Stick the screw driver in the middle of one of the coils.  The effect will vary depending on where the coil is in the circuit and what it is used for.

YES, the screwdriver will change the value of the coil it is inserted in, but it should be fine for this little test.

Hopefully you will be able to "feel" the energy inside the coil, because the screwdriver will be bouncing around with the music.

Leave it there a little bit, could be a few seconds or a minute or so.

Now remove the tip and touch it.  BE CAREFUL, it could be REALLY hot.

That should give you an idea of inductive heating.

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Jonathan Johnson

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Re: Voltage drop question
« Reply #16 on: April 16, 2017, 10:49:47 am »

Yes a coil of cable will help keep the heat in a smaller area, but the inductance is the main reason.

And when you lay it in a figure 8, now you have two inductors. Sure, it's less inductance (per coil) and it may not be as even as a round coil (I don't know the effect of a teardrop shaped "coil"), but you haven't solved a whole lot as far as induction. I mean, what's the difference between a 50' feeder round-coiled and a 100' feeder in a figure 8?

Maybe people THINK they are solving an inductance problem, but that just shows a lack of understanding. I'll admit to a lack of understanding myself, but I'm not afraid to challenge "conventional wisdom" when it doesn't make sense to me.
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Kevin Graf

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Re: Voltage drop question
« Reply #17 on: April 16, 2017, 11:47:10 am »

Oh dear, oh dear.
A cord or cable is not an inductor. The field around one conductor is almost completely canceled by the opposite polarity field around the other conductor. 
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Jean-Pierre Coetzee

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Re: Voltage drop question
« Reply #18 on: April 16, 2017, 11:58:52 am »

Oh dear, oh dear.
A cord or cable is not an inductor. The field around one conductor is almost completely canceled by the opposite polarity field around the other conductor.

This, put a clamp meter around a twisted pair cable and you will see what he's talking about. Equal current flows in each conductor just in opposite polarity, if the cable isn't twisted there will be some current due to one conductor being closer than the other but honestly it is negligible.

The real problem with coiling a cable is the lack of air around the cable causing heat to build up. If you coil power cable just derate it by x(you will need to ask the smart people by how much) and you should be just fine, otherwise lay it out straight.

The figure 8 coil has more surface area to dissipate heat from, this is the entire reason.

There is a lot of research done on the position of the ground conductor and inductive leakage between the current carrying conductors and the ground conductor depending on where it is in the cable, this is a bigger concern for sound guys IMHO.

EDIT: Same applies for 3 phase, the 3 phases cancel each other out. You might actually notice some current from triplen harmonics generated by dimmers/switch mode power supplies and such but should still be pretty low.
« Last Edit: April 16, 2017, 12:04:15 pm by Jean-Pierre Coetzee »
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Matthew Knischewsky

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Re: Voltage drop question
« Reply #19 on: April 16, 2017, 01:49:51 pm »

Yes a coil of cable will help keep the heat in a smaller area, but the inductance is the main reason.

When you coil it up, you form a nice inductor that can produce lots of heat-both to itself and internal objects.

Here is a fun little test.

Take a standard passive loudspeaker apart to where you can get to the xover, it doesn't matter what the model is.  A stronger one is more fun however. :)

Now grab a screw driver and run music through the cabinet.

Stick the screw driver in the middle of one of the coils.  The effect will vary depending on where the coil is in the circuit and what it is used for.

YES, the screwdriver will change the value of the coil it is inserted in, but it should be fine for this little test.

Hopefully you will be able to "feel" the energy inside the coil, because the screwdriver will be bouncing around with the music.

Leave it there a little bit, could be a few seconds or a minute or so.

Now remove the tip and touch it.  BE CAREFUL, it could be REALLY hot.

That should give you an idea of inductive heating.
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ProSoundWeb Community

Re: Voltage drop question
« Reply #19 on: April 16, 2017, 01:49:51 pm »


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