# The Naked Scientists Forum

### Author Topic: Is it true that the the length of wires can affect the brightness of a bulb?  (Read 6296 times)

#### The Scientist

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##### Is it true that the the length of wires can affect the brightness of a bulb?
« on: 14/07/2010 14:42:40 »
Assuming in a closed circuit, will the length of wires affect the brightness of a bulb? Please answer as detailed as possible. Thank you!

#### syhprum

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##### Is it true that the the length of wires can affect the brightness of a bulb?
« Reply #1 on: 14/07/2010 15:00:06 »
It all depends on how much current the bulb is drawing, a normal mains voltage lamp drawing say .25 Amps would need a very long length of domestic grade cable to cause any appreciable dimming where as a 12 volt 400 watt projector lamp could well be dimmed by a few meters.

#### tommya300

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##### Is it true that the the length of wires can affect the brightness of a bulb?
« Reply #2 on: 15/07/2010 03:30:36 »
[Rho (L)]/A = resistivity in ohms and ohms law
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The resistivity constant of the wire material, the length of the wire with respect to the cross sectional area in mills of the wire will give you the resistance of the single length of the wire. (don't forget there  is feed and the return wire to complete the circuit.)
Knowing the Wattage of the bulb and the voltage you can derive the resistance of the bulb. Ideally you would like to have the max voltage drop across the bulb. Adapting the voltage divider laws you can figure the better diameter wire for the length of the wire.
There should be tables somewhere in electrical contractors manuals Rule of thumb type information.
.
http://en.wikipedia.org/wiki/Electrical_resistance
« Last Edit: 15/07/2010 03:36:27 by tommya300 »

#### syhprum

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##### Is it true that the the length of wires can affect the brightness of a bulb?
« Reply #3 on: 15/07/2010 06:49:49 »
Lamps in no way present a constant Resistance it varies by a factor of at least 10 to 1 depending whether they are hot of cold also the light output varies as the fourth power of the voltage so that a small drop in voltage causes a considerable drop in light output.

#### tommya300

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##### Is it true that the the length of wires can affect the brightness of a bulb?
« Reply #4 on: 15/07/2010 10:09:06 »
.
Let's say the length of wire in question is an extention core.
Ignoring the LOAD variable resistive values, no disrespect intended, there is a max rating that should be the point of focus and take into concern.
You can not go wrong looking at it things way, because you will be covering the array of minor values, but there is the term, "over kill" which can eat at the pocketbook.
Here is a circuit! It contains a power source, a load and conduit.
This conduit, (wire), like all things, has resistance and it is associated to the material of the conduit, the length of the conduit and the area of the cross section of the conduit.
If the conduit has a small diameter (small cross section) relative to the length, it imposes a high impedance relative to a short length conduit with a larger cross section.

It is possible to make the length of wire, (extention cord) with respect to a littler cross section, to long, that at the end where the load would be, the voltage drop will affect the load's performance.
The Form I presented previously, helps determine this impedance to the engineering of the circuit.
So do not be concerned with the medial values always determine the maximum values and add a 25% to the final values for good measure to cover the initial current spike.

« Last Edit: 15/07/2010 10:20:06 by tommya300 »

#### Geezer

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##### Is it true that the the length of wires can affect the brightness of a bulb?
« Reply #5 on: 15/07/2010 17:24:40 »

So do not be concerned with the medial values always determine the maximum values and add a 25% to the final values for good measure to cover the initial current spike.

As Syhprum points out, light bulbs can have very little resistance.

If you base the conductor requirements of a circuit that is supplying power to a filament light bulb on the maximum values, you'll end up with extremely expensive wiring.

#### tommya300

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##### Is it true that the the length of wires can affect the brightness of a bulb?
« Reply #6 on: 16/07/2010 10:48:46 »

So do not be concerned with the medial values always determine the maximum values and add a 25% to the final values for good measure to cover the initial current spike.
As Syhprum points out, light bulbs can have very little resistance.

If you base the conductor requirements of a circuit that is supplying power to a filament light bulb on the maximum values, you'll end up with extremely expensive wiring.

.
My apologies to you all. Just call me Mr. Magoo?
I have a habit not thinking in terms of unique conditions. When someone says circuit, I thing of in terms of load and power source. Not a specific load and then I go off, and do not read precisely as if I were reading a contract, just react. That can get a bit sticky. Never wanted to step on toes.

When I seen this question all I can think of some several conditions I experienced. "How can you go wrong with experience," I said to myself.
In my mind it pointed in this direction.
A 5500KVA generator running an air conditioner. Tried using a 50 foot 12 awg extention cord it did not work. Directly plugged it in and it worked. Then tried 50 foot 8 awg and it worked.
Yes the 50foot 30awg was 25 dollars, the 50 foot 8 awg came from a 150 foot spool cost was much much more. Upside I also could run an AC welder with it. I needed the wire because where I live and electricity has gone down everytime the water well was needed.

On the upside the subject was covered explicitly.
Sorry Syphprum for being slightly off subject and looking at the question in general terms, I never said you were incorrect, thanks Geezer, for understanding, I did mention in a previous thread, "You can not go wrong looking at it things way, because you will be covering the array of minor values, but there is the term, "over kill" which can eat at the pocketbook."...
.
« Last Edit: 16/07/2010 11:43:57 by tommya300 »

#### syhprum

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##### Is it true that the the length of wires can affect the brightness of a bulb?
« Reply #7 on: 16/07/2010 13:05:11 »
Are you sure your generator is 5,500 KVA that sounds very large, do you perhaps mean 5,500 VA or 5.500 KVA ?
An automobile 5 liter V8 engine running flat out would be needed to provide 250KVA, a gas turbine or ships Diesel would be needed for 5500KVA.
« Last Edit: 16/07/2010 13:15:41 by syhprum »

#### tommya300

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##### Is it true that the the length of wires can affect the brightness of a bulb?
« Reply #8 on: 16/07/2010 13:19:32 »
Are you sure your generator is 5,500 KVA that sounds very large, do you perhaps mean 5,500 VA or 5.500 KVA ?
An automobile 5 liter V8 engine running flat out would be needed to provide 250KVA, a gas turbine or ships Diesel would be needed for 5500KVA.

Thanks it is 5500 watt! 5.5 meg is uterly rediculus! sorry again
Boy, am I having a difficult time! I yiyiyi
« Last Edit: 16/07/2010 14:04:41 by tommya300 »

#### Geezer

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##### Is it true that the the length of wires can affect the brightness of a bulb?
« Reply #9 on: 16/07/2010 18:18:52 »
I wonder how long that extension cord would last if you did deliver 5,500 kVA through it. I imagine the result would be fairly spectacular.

BTW - we get our water from a community well too. The nightmare scenario is a fire breaks out in the trees where the power lines run. This takes out the power lines and now we have no water to fight the fire. So, we now have a generator to keep the well going.

#### tommya300

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• Posts: 655
##### Is it true that the the length of wires can affect the brightness of a bulb?
« Reply #10 on: 16/07/2010 19:07:13 »
I wonder how long that extension cord would last if you did deliver 5,500 kVA through it. I imagine the result would be fairly spectacular.

BTW - we get our water from a community well too. The nightmare scenario is a fire breaks out in the trees where the power lines run. This takes out the power lines and now we have no water to fight the fire. So, we now have a generator to keep the well going.

Yes the 5.5 mega what (watt)? That is an expensive light show, the bear wire and high current would end up making glass in the sand lot.
Yea that's a catch Oh Oh, fire by lightening cascaded by no electricity to the water pump.

Thank you both for your patience.
« Last Edit: 16/07/2010 19:09:54 by tommya300 »

#### The Naked Scientists Forum

##### Is it true that the the length of wires can affect the brightness of a bulb?
« Reply #10 on: 16/07/2010 19:07:13 »