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Author Topic: Voltage drop question  (Read 9413 times)

Ed Hall

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Re: Voltage drop question
« Reply #20 on: April 16, 2017, 02:49:26 pm »

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.

An extension from a distro, yes. The question I had was about the feeder leading from the service entrance or generator to the distro. Think camlocks. They are single conductor relatively high amperage.

Thanks to all who replied. I have learned a lot here and that why I ask. To learn.
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TJ (Tom) Cornish

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Re: Voltage drop question
« Reply #21 on: April 16, 2017, 05:17:24 pm »

An extension from a distro, yes. The question I had was about the feeder leading from the service entrance or generator to the distro. Think camlocks. They are single conductor relatively high amperage.

Thanks to all who replied. I have learned a lot here and that why I ask. To learn.
The feeder, just like extension cords, has multiple conductors. Unless you make a separate coil for each conductor, the same cancellation occurs.
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Mike Sokol

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Re: Voltage drop question
« Reply #22 on: April 16, 2017, 08:23:10 pm »

I don't have time to look for it right now, but I did do the math on this once, and IIRC unless you coil a single feeder wire tightly around an iron core of some sort, the amount of inductance is way too low for any heating effect at 60 Hz. And anything with a neutral and hot in the same sheath would certainly cancel out most external magnetic fields. It would be pretty simple to set up an experiment to prove this one way or another, but I'm heading to Holland and Paris for a few weeks and there's lots of packing to do tomorrow. I'll pick this thread up once I get back.
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Stephen Swaffer

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Re: Voltage drop question
« Reply #23 on: April 16, 2017, 10:04:06 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.

If you look up "ampacity" in the NEC, you will see a chart with 60 deg C, 75 deg C, and 90 deg C ratings.  Any wire, from 20 awg or smaller to 4/0, or 500 MCM or larger has resistance and will create heat depending on the current flowing through it.  Those ampacities are based on how hot the wire will get under the conditions listed-typically 3 current carrying conductors in a raceway.  If you follow code correctly, you will have to derate the ampacity by the number of conductors in a raceway-by the time you get to 10, the derate is a full 50%.  This is essentially what you are doing when you coil a wire-putting a bunch more wires inthe same space-while a coil is not in a raceway, it still is limited in the amount of heat it can dissipate.
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Jonathan Johnson

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Re: Voltage drop question
« Reply #24 on: April 16, 2017, 10:58:18 pm »

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.

Of course a bundled cable won't exhibit significant inductance.

But what about feeder cables that aren't bundled?
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David Sturzenbecher

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Re: Voltage drop question
« Reply #25 on: April 16, 2017, 11:18:38 pm »

Of course a bundled cable won't exhibit significant inductance.

But what about feeder cables that aren't bundled?

Feeders are still in relatively close proximity to one another.


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Tim McCulloch

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Re: Voltage drop question
« Reply #26 on: April 17, 2017, 03:53:14 am »

Of course a bundled cable won't exhibit significant inductance.

But what about feeder cables that aren't bundled?

And Code requires that feeders of single conductor cables NOT be bundled, presumably because of heat. 
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David Sturzenbecher

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Re: Voltage drop question
« Reply #27 on: April 17, 2017, 06:20:29 am »

And Code requires that feeders of single conductor cables NOT be bundled, presumably because of heat.

What defines a "bundle" here? Every feeder that I have seen over 5ft has been held together with tape every 10ft or so.


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John Roberts {JR}

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Re: Voltage drop question
« Reply #28 on: April 17, 2017, 10:47:57 am »

If you look up "ampacity" in the NEC, you will see a chart with 60 deg C, 75 deg C, and 90 deg C ratings.  Any wire, from 20 awg or smaller to 4/0, or 500 MCM or larger has resistance and will create heat depending on the current flowing through it.  Those ampacities are based on how hot the wire will get under the conditions listed-typically 3 current carrying conductors in a raceway.  If you follow code correctly, you will have to derate the ampacity by the number of conductors in a raceway-by the time you get to 10, the derate is a full 50%.  This is essentially what you are doing when you coil a wire-putting a bunch more wires inthe same space-while a coil is not in a raceway, it still is limited in the amount of heat it can dissipate.
I recently had to macgyver a fuse for the rear channel of my surround system (kids don't try this at home)  :-[. The smallest wire I could find was 30ga (actually one strand from some 22ga stranded) that I soldered to the outside of the open fuse. Doing some WWW research I discovered that my DIY fuse was rated for >10A...  Only slightly better than a penny in the fuse box...

I swapped out a good fuse from one of the other identical amps in case something was actually wrong with that one amp, and put the DIY fuse in one of the other amps that didn't blow a fuse... My new fuses arrived a few days later and my house didn't burn down in the meantime.  ;D

Of course the fuse current for wire (when the metal melts) is probably higher than the rated ampacity which will be derated based on insulation temperature capability, heat dissipation, and other safety factors.

JR
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Mark Wilkinson

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Re: Voltage drop question
« Reply #29 on: April 17, 2017, 12:37:21 pm »

I recently had to macgyver a fuse for the rear channel of my surround system (kids don't try this at home)  :-[. The smallest wire I could find was 30ga (actually one strand from some 22ga stranded) that I soldered to the outside of the open fuse. Doing some WWW research I discovered that my DIY fuse was rated for >10A...  Only slightly better than a penny in the fuse box...

Ok sorry :), but IME no way  30ga carries 10A . 
One of my past lives was elevator adjustor / troubleshooter at large...
Sometimes shorts would appear intermittently, based on momentary circuit logic, that couldn't be metered statically. 
It was easy to chew up a box of buss cartridge fuses trying to hunt the short down.
# of strands of 18ga to temporarily use as substitute during troubleshooting only was pretty common knowledge/practice. 
I don't remember it all so well anymore, but 1 strand carried less than 6A
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Re: Voltage drop question
« Reply #29 on: April 17, 2017, 12:37:21 pm »


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