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-   -   What size cable? (http://www.diyaudio.com/forums/subwoofers/98551-what-size-cable.html)

Cloth Ears 21st March 2007 02:43 AM

What size cable?
 
Gents,

I've got a 1200W amp that's going to be feeding an 11ohm load. Is there a formula that will tell me what the minimum wire size I should be using to ensure no over-heating/melting etc?

MJL21193 21st March 2007 03:21 AM

Think of the wire on a hairdryer. That consumes 1200 watts and has an internal resistance of about 11 ohms...

Cloth Ears 21st March 2007 04:07 AM

D'uh! Why didnae I thinka that? I'll probably go a little thicker, as my wire is encased in something a little softer (and probably with a lower melting point).

Just checking as I didn't want to end up with a little red line between the amp and the speakers.

http://i.slickdeals.net/images/smilies2/tomato.gif

Cal Weldon 21st March 2007 05:27 PM

I think the length of the run determines your wire guage. Up to 20 feet should be OK with 14ga.

Remember big drivers use big electrons to drive them so use big wire. ;)

Cloth Ears 21st March 2007 11:51 PM

OK. I've done some figuring, and I've already got enough good cable sitting around. But, is there any way to work out heat dissipation from cables?

From my figures (and these would only be if I went insane and turned the thig up all the way), and a deal of help from the guys at the12volt.com, I've worked out that I'd have:
  • 1200 Watts
  • into 11 ohms
  • running 10.5 amps
  • at 115 volts
Now, using Nelson Pass' 1980 cable article, that shows 18ga zip as having a resistance of .014 ohms/foot, I've worked out (using the figures above) that it would need to dissipate 1.5 Watts/foot.

Does anyone have an idea of what sort of temperature change this causes for the run of cable (or do we then get into surface areas of cable and such)?

And Cal, don't those big electrons go by another name once they reach a certain size (big cojones)?

Cal Weldon 22nd March 2007 12:11 AM

I use 16 ga inside the box, 14 ga for runs up to 20 feet and 10 ga for when I take the speakers next door.

The hair dryer probably uses 16 - 18 ga and would warm slightly so why not up it to 14 or 12 and be done with it? How long is the run?

I think the really big ones are called jumbo electrons.

And don't forget to elevate your amp above the speakers so they flow easier. ;)

454Casull 22nd March 2007 01:27 AM

http://www.nortechsys.com/intercon_tech_current.html

FastEddy 22nd March 2007 02:07 AM

Cal: " ... I use 16 ga inside the box, 14 ga for runs up to 20 feet and 10 ga for when I take the speakers next door. The hair dryer probably uses 16 - 18 ga and would warm slightly so why not up it to 14 or 12 and be done with it? ..."

What he said ... stranded, of course. (FYI: 1200 watts into 11 ohms is enough voltage to exceed commercial building codes in the USA for running outside of EMT pipe ... down under, burn baby burn that ol' dance hall down.)

The "golden ear" types will suggest teflon insulation ... so I look for THHN (the stuff with the shiney skin).

Cloth Ears 22nd March 2007 02:19 AM

Quote:

Originally posted by Cal Weldon
And don't forget to elevate your amp above the speakers so they flow easier. ;)
No problem. I'm looking at mounting it vertically, so that the jumbo electrons flow out the back of the amplifier at the bottom and straight into the subwoofer box. It's going to be mounted rather like a child in a back-pack - but hopefully no squealing or funny smells;)

Quote:

Originally posted by 454Casull
http://www.nortechsys.com/intercon_tech_current.html
Excellent. From this, if I'm running 10amps though 20awg, the wire will heat up to 80 degrees Celcius. So, by using 10awg, I would be barely above ambient temperature.

Now, gents (and ladies), I'm not planning on running that sort of current, although after I've finished EQ, I might end up with 5amps (not sure yet). So hopefully, I won't be burning anything down.

Fasteddy, this is a commercially available, plug in the wall (ie. not 3-phase) amplifier - the Behringer EP2500. And it's also available in the US of A, so hopefully it self-protects before any fires start. It does have plastic covers for the speaker binding posts, to prevent shocks, and they recommend no exposed wiring. I expect that if I follow recommendations then I will probably be safe:)

Did you mean TSWTSS?

FastEddy 22nd March 2007 02:28 AM

" ... From this, if I'm running 10amps though 20awg, the wire will heat up to 80 degrees Celcius. So, by using 10awg, I would be barely above ambient temperature. ..."

This will make a nice hand warmer, too. You want to take those charts with a grain of salt. They were designed as absolute minimums for inexpensive construction, not optimosed for audio. There is a serious school of audiophile thought that believes that bigger speaker wires are better ... and even I can tell the difference, if the difference is great enough or the speaker run is long enough. A compromise would be to follow Cal's advise = #12 AWG stranded.

And it is really not the heat so much as the downstream resistance of a long run to the speakers. Fortunately your case is a very short run, so #14 will probably be plenty. Especially if none of them jumbos have to run up hill ... :smash:

Also, at least here in the states, the costs between ten feet of #12 and ten feet of #14 or even #18 is about a wash ...


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