Film-Tech Forum ARCHIVE: Speaker wire gauge question

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Posted by Tom Wienholt (Member # 1510) on 04-11-2010, 08:40 PM:

I usually use 10 gauge wire for Subs. I am upgrading an auditorium that previously had no subs but has one extra pair of 12 gauge wire running from the booth to the stage. The auditorium is 55 feet long so the wire run is about 75 feet. I will be using two 18" subs hooked in parallel. Can I get away with using the one pair of 12 gauge wire? Or should I go ahead and run a new line?

Posted by Mike Blakesley (Member # 26) on 04-12-2010, 10:57 AM:

We have dual 18" subs and all our wires are 12 gauge (as recommended by Mark G.) and we have had no problems. We have one set of 12 gauge running both of the subs.

Posted by Louis Bornwasser (Member # 3063) on 04-12-2010, 02:26 PM:

The bigger the better. If you run at low volume, it may be OK.

At high level, you could be drawing signifiant current on the speaker wiring. I would view 2 18" on one pair of #12 as a bit skinny. Typically, I use #10 and put one 8 ohm, 18" on 2 wires, the other one on another pair. The issue here is damping factor; how the speaker talks back to the amplifier. Safety/fire are not a concern. Louis

Posted by Robert Minichino (Member # 3495) on 04-12-2010, 03:09 PM:

A higher damping factor just causes the speaker's effective Q to rise a little, resulting a slightly raised response around resonance. This is a minimum-phase phenomenon, so it can be corrected with equalization.

Posted by John Hawkinson (Member # 1135) on 04-12-2010, 06:35 PM:

Isn't this a bit silly? Tom has #12, he should try it and see if it works effectively for him.

Does anyone think differently?

--jhawk

Posted by Sean McKinnon (Member # 612) on 04-12-2010, 07:22 PM:

If it doesnt work out for him then maybe he could use the 12 gauge wire to pull in the new 10 gauge through the conduit.

Posted by Steve Guttag (Member # 268) on 04-12-2010, 08:49 PM:

I'm a bit more pragmatic about it.

How long is the room...lets presume 50-feet, for now though that may be a bit long given the estimated 75-foot cable run.

But given a 50-foot room, one needs over 2000-watts to drive a double 18" subwoofer (typical) to the required level for digital (113dB). Doing the math, one finds that while the subwoofer is being driven to such a power, into 4-Ohms (presuming the 18" drivers are wired paralleled), you need 22.69A on that #12awg cable during that moment in time. Bzzzt...it is a bad design.

Now, it can be argued a great many things. The 22.69 is a VERY rare occurrence as most sub tracks do not hit the max and when they do, it is not sustained. The next question is, is the volume running at 7.0 on a properly calibrated system? Odds again are probably no. Lastly, is the amplifer in this system actually capable of the required 2060-watts and if the answer to that is "yes" are the subs capable of handling that power...the answer is likely "no."

Now, if you are not running digital audio (film or DCinema), then things get much easier and 12awg is fine for the current load.

For digital, I would say that 10awg should be the minimum in this set up or two pairs of 12awg.

-Steve

Posted by Marco Giustini (Member # 4544) on 04-13-2010, 04:46 PM:

I'm with Steve.

It is hard to say "let's try and see if there is any problem" since it may work for a while and then fail. Or it may work without breaking but without the best result that sometimes is not easy to identify by ear.

Someone could say "if I do not notice by ear, why bother?". I would say "since we are doing it, let's do it properly".

Cheers
Marco

Posted by Tom Wienholt (Member # 1510) on 04-13-2010, 10:37 PM:

Marco,

I would agree...I'd rather do it right the first time. Thanks everyone for your suggestions.