Wiring Size for Speakers

[johnnymk]

I was at Sears Hardware yesterday and bought 50 feet of flat wire for 99 cents. It was in the clearance section and I figured, ah, what the heck.
It's rated at 300 ohms. I am trying to figure the maximum wattage this thin wire can handle by using the following formula:

W=I X V ; V= I X R ; therefore W= I X I / R

Since the output voltage is 120 volts and the resistance is 300 ohms, then W= 120 X 120 / 300, which calculates to 48 Watts.

Anything wrong with my assumptions?

[coleslaw]

Originally posted by johnnymk
W=I X V ; V= I X R ; therefore W= I X I / R

Since the output voltage is 120 volts and the resistance is 300 ohms, then W= 120 X 120 / 300, which calculates to 48 Watts.

Anything wrong with my assumptions?

You're using the voltage in your calculation of power instead of the current.

I'm not so sure if you are approaching the calculation the right way, though.

[johnnymk]

Sorry, I wrote the wrong letters down. It should have been
W=V X V / R. In any case, the final calculation is correct.

[i6s1]

There are a couple of flaws with your reasoning. Although you are correct, putting 120V into 300ohms does consume 48 watts. I don't think that the wire is really 300 ohms though, that seems pretty high.

Are you going to plug this into the 120V wall outlet? Don't. It will melt and burn your house down.

Are you going to use this for speakers? Don't. At 6 ohms per foot(?), a 1 foot section would turn your 4 ohm speakers into 16 ohms seen by the amp. You will no longer have your load matching your output, and your music will be very quiet and very crappy.

This wire is not intended for either of the uses that you seem to be getting at. In the title you say speakers, but you use 120v, which is of course wall outlet voltage.

[johnnymk]

Sorry for not checking your response earlier. So 120 volys is not the correct figure to use for speakers? I was wondering about that. What voltage should be used?
I wonder what this wire is intended for? You know, maybe it's for TV antennae.

[i6s1]

The voltage range depends on how loud your speakers are at a specific moment in time. The voltage varies from -lots to +lots. And by lots we're getting into the hundreds of volts, maybe thousands for the very high power speakers.

I don't think that this wire would be used for antenna either. Unless I'm mistaken, TV antenna has a characteristic impeedance of 50 ohms. (Or was it 75?) Adding another 300 would take that to 350, and you would be throwing away 6/7th of your incoming signal.

The only use for this wire is low frequency, low current, low power applications or where signal preservation is not important.

But again I have to say that I'm not really sure the wire is 300 ohms. That may be 300 ohms per thousand feet, or something like that.

[johnnymk]

Maybe it's for a home security system? They also had 100 feet of the same wire and it said 600 ohms. I think tomorrow I am just going to ask them what it is intended for.

[i6s1]

Home security could work, the signals would just be high or low, wasting power isn't important, and it doesn't need high voltage/current. Could be for a doorbell, or a garage door opener. That sort of thing.

[i6s1]

Oh, there is a flaw in the equation you used. If all you hooked up to your voltage source was the wire, everything would be right. But if you hooked it up to something, like a light bulb, your calculations are not right anymore.


Lets say you plug in your Doohickey, which has a resistance of 300 ohms. That 300, plus the 300 from the wire, create a total resistance of 600 ohms. So you have to redo the calculation with 600 ohms. In this case, they're equal, so you would split the power 50/50 to find out the power used in each one. In this case you would have 24 watts used. The wire would use 12, the Doohickey would use 12.

[InfiniteNothing]

I just took a electro mechanics class so in theory I should be able to answer this (assuming summer hasn't killed all my brain cells). Lets see. This 300 number is not quite OHMs that of course depends on how much wire you use so it's most likely a OHMs/100meters or something to that effect. To calculate the amount of wattage that you can "pump" through this wire before you burn the house down we take the lenght of wire and multiply it by the 300 factor. Since the resistors are not in parallel (I'm assuming) we can add the resistance to those of your speaker (probably 8 OHMs). Then we can calculate the energy that goes into heating the wire. Then once we have the heat in units of Joules we can calculate the temperature of the plastic insulator using the specific heat of the material (we can look this up in my CRC hand book) Then we can see if it combusts at this temperature (once again with the CRC). Please PM me or email me and we can chat back and forth to figure this all out (in the name of pure fun of course).
And a note about TVs. I think (not sure) that the antenas expect 300 OHMs and the adaptor (with the screws and the coaxle male) at the end converts this to 75 OHMs. But I'm almost sure this is unrelated to the wire problem.