Speaker Cable Gauge vs. Length of Run

[adquock]

I'm new to this forum so I'm not sure how to do a thorough search of previous discussions so please bear with me as I post this.

After getting my cab checked out, the amp tech pointed out a few problems with my proposed rig:

a. My 2x12 cab runs at 10ohms due to one of the speakers being a little weak
b. I ordered a 12 gauge speaker cable at 100 feet to shoot my cab into a closet.

The amp tech that checked out my cab said that I would need at least a 10, better an 8, gauge speaker cable for that purpose. Is that really necessary? Would I need a gauge that hefty because of the condition of my cab? I just got a new amp and would hate to mess it up in any way. I've spoken to some other guys who I see as decently knowledgeable and they didn't take the gauge issue seriously at all. So what's my best bet here? I'm not bent on keeping the 100ft cable but it would save me a lot of time/trouble since I already ordered it. Please advise.

[Enzo]

I have some concerns here:

a. speakers do age and get "weak". They are mechanical things, and the motion wears on them. The paper in the cone can soften. The flexible surrounding material can wear out and lose its stiffness. Not only that, the permanent magnet they rely on can lose its magnetic "charge" over time, it gets weak. The current through the voice coil then has less to push against. Speakers can also fail. The voice coil can warp causing it to rub in the gap. Windings on the coil can "delaminate" meaning come loose from the form, and those can scratch and rub. Plus the speaker can just crap out entirely.

But the voice coil is just a piece of wire wrapped around the center core. SO as it ages, the wire really doesn't change its resistance. SO I have no idea what 10 ohms because it is weak might mean. You have a 2x12. Almost all speakers fall into the 4 ohm, 8 ohm, or 16 ohm category. They can be wired in series or parallel. The DC resistance - what you can measure with a meter - is typically a little lower than the impedance rating. A 4 ohm speaker usually measures 3 ohms or so, an 8 ohm speaker typically 6 or 7 ohms, and a 16 ohm speaker usually checks in at something like 12-14 ohms. But those are rough averages. I have to think your guy measured the resistance of the cab and got 10 ohms. If I saw that, I'd assume we had a nominally 16 ohm cabinet. It would probably be two 8 ohm speakers wired in series.

Suppliers just love to sell you as much cable as they can. 8 gauge speaker wire? Welding cable? My opinion is that is just overkill. Frankly 12ga is too. MY HOUSE is wired with 12ga wire. I can plug 1500 watt space heaters into the wall outlets, and that 12ga wire does just fine. I have no need for 8ga. Remember, if an amp puts out 100 watts, that is the same amount of power that a 100 watt light bulb in a table lamp uses. Would you use 8ga wire for a table lamp?

presumably you want the speaker in a closet so you can crank the AMP all the way up for power tube overdrive or something like that. You clearly are not looking to see how LOUD you can get, or you wouldn't put it in the closet. SO the point is you don't need to sweat the ohm or so that the cable represents. You just want the amp cranked. If a few watts get eaten in the cable, so what?


On the other hand, it never hurts to have a 12ga 100 foot speaker cord handy. That is a really long way though.

[Steve Conner]

12AWG copper wire measures about 1.6 ohms per 1000 feet, so your 100 foot cord is a total of 0.32 ohms. The amp won't notice the extra.

If my cabinet measured a different resistance to what I expected, I'd be worried that one of the speakers was blown, not weak. Or maybe you have two speakers of different impedances, like a 4 ohm and an 8 ohm in series, because one got blown in the past and replaced with the wrong part.

[Joe L]

+1 to what Steve and ENZO said.

The direct answer to your question is plain old 18 gauge will work fine. Your main concern is keep the wire safe from shorts or opens in your routing of the cable. An open will kill a tube amp and a short will kill everything else.

I have 25 ft. of 16 gauge on the subwoofer of my surround system with a Peavey CS 800 pushing 400 watts. That is a much worse case than yours and mine sounds fantastic and has been for 10 years.

[Leo_Gnardo]

Add another +1. I've worked with lots of PA, thousands of watts, hundreds of feet, and never used anything over 12 gauge. Of course shorter speaker runs are preferable but if you need 100 ft to reach your "isolation chamber" then you need it.

There are x-pensive snooty garden-hose size "audiophile" cables for those with $10,000 & up hi fi systems, and as far as I'm concerned all they do is feed the "Emperor's new clothes" legend. I spent all this money - so I'm convinced it sounds better - whether it really does or not.

Aren't you glad you asked your friends at MEF - you saved yourself a week or 2 grocery money.

[bob p]

Resistance of Wire (Wire Resistance Calculator)

[soundguruman]

Big thick wire is a waste of money. 16-18 is more than enough.
The more important thing is to make sure the cable has good solid plugs. Switchcraft is preferable, but watch out for fake switchcraft from China.
(these look just like switchcraft on the outside, inside they are crap)
Avoid "hot wires" cables, the plugs are defective, and can destroy your amplifier.

Open the plug, and see that the internal rivet is rock solid, and that the internal terminals do not move or spin around on the rivet.
Cables with lose rivet / lose terminals are garbage. Check very carefully, and never chance your amp to a garbage cable.

Just because a cable is new, does not mean that it is good.
Just because a cable is "guaranteed," does not pay for a melted output transformer, burned sockets, and blown tubes. But this is exactly what happens when you use "brand new" garbage cables.
*never assume that a salesperson knows anything about speaker cables, or that they are recommending a "good" cable.

[Leo_Gnardo]

Open the plug, and see that the internal rivet is rock solid, and that the internal terminals do not move or spin around on the rivet.
Cables with lose rivet / lose terminals are garbage. Check very carefully, and never chance your amp to a garbage cable.

Just because a cable is new, does not mean that it is good.
Just because a cable is "guaranteed," does not pay for a melted output transformer, burned sockets, and blown tubes. But this is exactly what happens when you use "brand new" garbage cables.
*never assume that a salesperson knows anything about speaker cables, or that they are recommending a "good" cable.

Good advice! On these points, I totally agree.

For the OP, adquock, I'd say no harm in sticking with the 12ga since it's already on the way. If you were to return it & order a thinner gauge, shipping charges will eat you up.

For others who may want to buy 100 ft runs for 100W service, I would go a little heavy and use 16 or 14 ga.

Also those who can solder, make your own. I've seen sloppy soldering & wire trim that threatens to short out even in "trustworthy" and "lifetime guaranteed" cable brands. For those building their own, especially if you are outfitting a backline, wiring up a PA, or a service shop occasionally making up custom length speaker cables, I recommend the 16 ga cable from Conquest. They will sell you whatever length you need. Conquest in Illinois is also an excellent source of authentic "made in the United States of Mexico" Switchcraft connectors. At least they're not Chinese fakers. Note - Conquest "does not sell to individuals, only businesses."

Connectronics also makes excellent "raw" cable but AFAIK sells only in full rolls of 100 meters (@ 330 ft.) SPX 150 is 16 ga equivalent and SPX 250 13 ga. Very tough jacket but also very flexible. Hi quality copper & easy to solder. Last I checked Connectronics has an office/warehouse in Connecticut, how appropriate.

[bob p]

Big thick wire is a waste of money. 16-18 is more than enough.

That's a good rule of thumb -- for low power and short cable runs it doesn't really matter, and lamp cord works fine. but good engineering practice requires that you consider the ampacity rating of an insulated conductor when making a wire choice. In the event that you're using either high current or long runs, or both, then ampacity becomes significant. Any engineering handbook or textbook will have a table of wire ampacity in it. It's such an important consideration that people even make on-line calculators to make the information easy to find. See post # 6.

[Joe L]

Damn!!! Eight replies and not one disagreement! That's some kind of record.

[Steve Conner]

No it isn't!

For once I don't even disagree with Soundguruman. I have seen cheap jack plugs short out and blow up power amps in exactly the way he describes. I've always made my own speaker cables with the best quality Neutrik plugs I could get.

In an ideal world, 1/4" jack plugs would not be used for speaker connections at all, they are a terrible choice for the job.

[Leo_Gnardo]

In an ideal world, 1/4" jack plugs would not be used for speaker connections at all, they are a terrible choice for the job.

Yes it is !

Well they're sort of OK for low power say up to 50 or 100W. Not so good for big PA. A few years back I saw a 1/4" jack on a JBL 18" sub that had charred, and was making the power amp go into protect mode. I measured 1 ohm across the carbonized connector, and it was a switchcraft. So phone jacks in 500 & 1000W service = NOT recommended. As far back as 1975 I saw some touring bands who had their amps outfitted with dual banana jacks, PA as well. Better connection plus choice of polarity. Also a chance of accidentally reversing polarity.

Only a matter of time before the nanny state takes phone plugs away from us too. We saw banana connectors disappear from the back of power amps & big bass amps. And now manufacturers have to put metal cages around vacuum tubes, to keep kiddies from grabbing them & getting singed. What happened to the right to live dangerously? Just a little...

[bob p]

In an ideal world, 1/4" jack plugs would not be used for speaker connections at all, they are a terrible choice for the job.

Somewhere around here there's a thread where I mentioned that I like using binding posts with 3/4" banana jack terminals like you'd find in test gear. I like them because they force you to verify your connections at every set-up, and in the field you can repair a broken cable connection with nothing more complicated than a pocket knife. The result? I got spanked because the connections weren't reliable because they weren't soldered. Personally, I think that clamping down on a wire with a rotating bolt or inserting a banana plug can make a pretty darned good connection.

I don't think that 1/4" phone jacks are all THAT bad -- if you use them within the ratings for their applications. For the typical low voltage, low current, old-school guitar or bass cabinet there's not much wrong with them -- especially if you use the right-angle plugs which are a bit more durable than the in-line type and are more resistant to being sheared off at the site of insertion in the event of an accidental collision. Even 40 years ago Leo Fender had thought things through pretty well.

But when it comes to modern high power pro sound reinforcement the choices need to be different. If you've got a kilowatt amplifier driving a remote speaker array, then you've potentially (pun!) got a lot of volts and/or current happening at the same time. You need to use a low-Z connector and a cable that are properly rated for the job, and in that application a 1/4" phone jack and an 18-gauge lamp cord isn't going to cut the mustard.

The rule of thumb that SMG mentioned is a good one, but even good rules of thumb can break down if you try to apply them outside of the realm for which they were created.

[bob p]

Getting back to the OP's question: he never told us the power output of his amplifier, so one part of the equation is missing. We know that he has a ~10R load, which is going to limit the current that his amp is going to have to supply in comparison to something like a 2R or a 4R load. But we don't know if he's using efficient drivers with low power ratings, or modern inefficient drivers that demand a really high powered amp. We don't even know if this is a guitar or a bass rig. If it's a bass rig, then it's possible that the amp/speaker combination could be one of those new-paradigm subwoofer technology setups that uses brute force to pump a lot of power into a very inefficient cabinet.

Now for the math: with 100-feet of 18-gauge cable, the calculator that I linked to up above says that the resistance in the cable is going to be 0.639R. In answering the problem it's worthwhile to think of cable and the cabinet as a voltage divider. We've got 0.639R + 10.0R = 10.639R.

Now compare that to 100-feet of 12, 10 or 8-gauge cable:

18ga: 0.639R + 10R = 10.639R
12ga: 0.159R + 10R = 10.159R
10ga: 0.100R + 10R = 10.100R
08ga: 0.063R + 10R = 10.063R

Notice that with a "high" Z load of ~10R the majority of the power will always be dissipated in the speaker cabinet, and in each case only a small fraction will be dissipated as heat in the cable. But it's important to realize that the power loss in the cable always increases as the amp's output current increases, so it's not a good idea to ignore it in the event that you're dealing with high power levels. If that's the case then a larger size wire is always a good idea. If you're dealing with a low Z load then larger sized wire becomes very important.

For garden-variety low-power use with MI amps and home entertainment systems with reasonable length cable runs I like the low-voltage 12-gauge wire that's used for low-power outdoor lighting. You can probably find it locally in any of the large home improvement stores here in the USA. they sell it both in spools and by the foot.

[adquock]

Thanks for your replies, I read them a while back but couldn't reply until now. Yeah, I'm running a tube amp into my cab and I don't believe it has a 16 ohm out. The cab has two 16 ohm speakers which I guess results in 8 ohms. One of the speakers is "weak" as I've said earlier, my tech tried to explain it to me but the info went over my head.

As for my rig, I'm running a Orange AD30htc into a carvin 2x12 with vintage 30s in them. I'm not sure what the specs are for the amp as I'm very much a novice when it comes to the technology behind gear, which is why I'm here to learn.

Thanks for all the replies, It's been very helpful. I'm looking to probably keep the cable unless I get enough convincing evidence that it would hurt the amp, which is my main concern. I can deal with lo-fi sound loss as long as it doesn't compromise the tone too much.