Electrical wiring: the "last inch"

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Metspitzer wrote:

Hmm, You are very funny. He was right.
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On 8/30/2009 1:58 PM Doug Miller spake thus:

Changes current, too. E.g., a current-limiting resistor in series with a LED, sized so that the LED won't draw excess current and burn itself out.
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Found--the gene that causes belief in genetic determinism

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Wrong.
If you want to limit the current passing through an LED, you put a resistor in *parallel* with it, not series.
Total current in the circuit remains the same.
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On Mon, 31 Aug 2009 02:47:34 GMT, snipped-for-privacy@milmac.com (Doug Miller) wrote:

I guess you didn't try that.
The resistor in parallel wastes power, and will have no effect on current through the LED (other than if it creates excessive voltage drop, a really inefficient way to dim the LED).
With no resistor in series, current (from a constant voltage source greater than about 2V) will approach infinity, until the LED is destroyed. I've seen that happen.

Not when you add series resistance. LEDs regulate the voltage across them, unless the current is excessive. Since that voltage is around 2V, a 120V source would require a really big series resistor. To avoid that, 120V would usually be applied to a series string of multiple LEDs (in series with one smaller resistor).
A string of 50 LEDs (common in holiday lights) using 20mA from a 120V source would require a resistor of 1000 ohms (that's assuming 2V voltage drop on a LED). There would be 2V across each LED and 20V across the resistor. Use multiple strings in parallel for more light. Current is 20mA and power dissipated by the resistor is .4W.
Changing that resistor to 2000 ohms will change the current to 10mA.
Changing it to 500 ohms will change the current to 40mA. The resistor will need to handle .8W. You may need a bigger resistor.
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On Mon, 31 Aug 2009 02:47:34 GMT, snipped-for-privacy@milmac.com (Doug Miller) wrote:

Nope. And an LED is a "spacial case" too, because it is NOT a resistance. It is a fixed voltage drop.

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David Nebenzahl wrote:

Hi, If you are not joking trying to be funny. Your basic knowledge is LACKing. Please be quiet if you don't know about some thing.
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On Sun, 30 Aug 2009 18:25:36 -0700, David Nebenzahl

I had one do that once. It was a surprisingly loud noise for something that little. Half the plastic case disappeared.
BTW, one time I saw a working LED with no apparent series resistor. It was in a little flashlight powered by 2 button cells (CR2032 IIRC). This seeming impossibility seemed to make use of the series resistance of the battery, which couldn't supply too much current to this LED.
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On Sun, 30 Aug 2009 20:58:38 GMT, snipped-for-privacy@milmac.com (Doug Miller) wrote:

This is true if the circuit is powered by a constant-current source, something unusual.
There may be some confusion here between actual current and current CAPACITY (of a conductor).
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On Sun, 30 Aug 2009 11:50:05 GMT, snipped-for-privacy@milmac.com (Doug Miller) wrote:

It may not affect current, but it sure as heck affects power. The current stays the same, being a series circuit, but the voltage is shared between the wire and the device. - hense power to the device is reduced.
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On Sun, 30 Aug 2009 11:50:05 GMT, snipped-for-privacy@milmac.com (Doug Miller) wrote:

I just rethought my answer and I was wrong. It not only drops the voltage but it DOES drop the current, because the total resistance of the circuit is higher. With a fixed supply voltage the current WILL drop. Combined with the voltage drop it is a double whammy on the power.
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How practial is it, to only plug in 14 gage cords into 14 gage branch circuits? Not very. What's worse, is trying to find a 100 watt outlet, to power your table lamp.
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David Nebenzahl wrote:

Hmm, If only that cord has a load carrying 20 whole amps. I hope you still remember Ohm's law in hi school physics class. I left hi school in 1960.
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On Sat, 29 Aug 2009 15:07:40 -0700, David Nebenzahl

No, the ampacity of cables varies with length, becuase the resistance per foot causes more voltage drop on a long cord than on a short one.

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wrote:

Wrong. The current through a cable is constant no matter what the length as is the maximum current capacity of a wire.
Resistance and voltage drop are a separate matter.
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wrote:

Seems to me there are two pertinent factors.
One is voltage drop which is an IR drop and really only affects the operation of the device at the far end.
Second is the power dissipation in the wire that is the I(squared)R value. That dissipation is spread linearly along the conductor and gives the rise in temperature that is relevant to ignition.
Charlie
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wrote:

Nonsense.
Voltage and current are not the same.
Conductor ampacity is dependent on: - the conductor material (Cu vs Al) - the insulation material - the conductor diameter and NOT on the conductor length. If you think rated ampacity is in any way dependent upon conductor length, then please cite the section of either the CEC or the NEC which says so. I'm not going to hold my breath.
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certinally a 18 gauge light cord on a 20 amp circuit is a fire hazard.
the plugs of all such cords should be required to have a built in fuse.
I once had a customer using a 18 gauge ungrounded light extension cord on a 18 amp grounded machine that tended to burn wires off inside the unit.
I wrote it up as a safety hazard, warning them buy a AC extension
next time I went they had improved things by stapling the extension cord to a carpeted wall:( wire got very hot showed signgs of melting.....
I fixed the machine then went shopping in the mall, and bought a AC extension cord. the customer got mad when I cut their junk cord in pieces and gave them free the air conditioner cord. told them the new extension cord was far cheaper than even the paperwork for the insurance claim when they burned down south hills village.
the customer was pissed and never called me again. frankly i didnt care.
have you noticed christmas lights now have fuses in each plug? that should be required for all plugs
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David Nebenzahl wrote:

Agreed. That was probably the nuttiest comment in this thread.

As others have said, length is a consideration only to limit voltage drop. Length has no relation to the current carrying capacity of the wire.

The code is rather pragmatic. #18 fixture wires on a 20A circuit are not a problem because the lamp socket can not have a bulb over the wire rating. (Someone could screw in an adapter-to-plug and run a space heater - the rules of natural selection would then apply.)
Apparently there are not major problems with overloading #18 extension cords on 20A circuits - dead bodies are very effective at promoting code changes, probably also effective at UL. If I remember right, a consideration was that the available fault (short circuit) current at the end of the #18 cord is high enough so a short will give "instantaneous" operation of the circuit breaker/fuse (not time delay). (That is not likely true for Christmas tree lights.) AFCI breakers may also help (but the selling point was cords that had been abused, like walking on them).
You might be upset with motor circuits, that can have a circuit breaker significantly larger than the wire ampacity. Welder circuits even bigger difference.
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snipped-for-privacy@but.us.chickens says...

They specifically allow the smaller gauge wire within a fixture, and reconcile it by permitting the fixture manufacturer to act as the "Authority Having Jurisdiction" and testing their own fixtures. The fixtures must remain an "end device", and cannot have anything wired in series with them. The short length of smaller wire is proper for the limited current draw.
I believe the code used to allow 14 gauge "drops" in 20 amp circuits, which were runs from switches to lights with the similar logic that a light would never draw more than 15 amps.
--
Dennis


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