70V audio wiring size

[markebenson]

I have lots of Belden 8762 20awg on hand rated for 300v.

Any issues using this for 70v speakers?

The max length will be 100ft, branching off to160w of speakers on that run.

Thx

 

[tom baker]

Depends.
Where is the cable being installed, plenum, in a wall, conduit?
And is the cable listed for that enviroment?

 

[zbang]

The speakers themselves won't care, but 20g seems light for the application- for 160 watts you probably want to up-size to 16g (or use 2-3 home-runs of 20g). There are calculators and tables for this online.

(And what Tom said, too.)

 

[LarryFine]

You're talking a max of (160/70) 2.2 amps, so you should be okay.

If you're still concerned, make the single home run a gauge or two heavier.

 

[Fred B]

Not sure were this is going, I've work for a speaker mfg we built systems for up to 12000 watt speakers huge massive units. Where did you get 70V ac on speaker? At most we used 16G and gold plated tips. Most were 24 or 28G. Amperage and wattage on a sound system is a whole different thing than with electrical, even though they use similar terminology. I would have no qualms of grabbing 600amp with both hands from a sound system, would never dream if it on an AC system. Done it. (Sound system not AC)
Now if there was a short from the amp, that creates a different story.

 

[hbiss]

Well, lets see because it's not that simple.

First off, this is no different than any other Art. 725 application. What does the label on the back of the amp say about wiring? Most will say CL2 wiring permitted. 8762 is listed as CM so it can be used in CL2 applications. But if the amp label does not indicate CL2 then you can't use it.

Second, a 70.7 volt system will rarely get to 70 volts. The voltage will depend on the audio content which varies constantly as will the current. This all has been taken into consideration if the amp is listed as a CL2 power source allowing you to use CL2 wiring. Larger amps may require Class 1 wiring and will either say so or not say anything.

Thirdly, 8762 is shielded. It's generally NOT recommended to use shielded cable for speaker lines. So on that count I would not say that you should use it.

-Hal

 

[grich]

...8762 is shielded. It's generally NOT recommended to use shielded cable for speaker lines. So on that count I would not say that you should use it.

-Hal

I wouldn't use shielded in this application either. Forty years ago, I saw a poorly-designed amp's output transistors fail after using 10 feet of Belden 8451 as speaker wire. Guessing the extra capacitance added by the shield created a weird load impedance. Haven't done it since.

 

[drcampbell]

... shielded ... speaker wire ... extra capacitance added by the shield ...

Isn't the capacitance inside most speaker cabinets (in their crossover networks) several orders of magnitude greater than anything the cable might contribute? Aren't amplifiers designed around that?

 

[texie]

Done my fair share of 70 volt audio over the years. While we never used shielded cable, it never occurred to me that his could be an issue. If that is the case wouldn't running this in EMT, as would be the case in some commercial job specs, create the same issue?

 

[Russs57]

Shielded cable isn't/shouldn't be a problem although it is seldom needed. There are certain applications, like when you talk back through the speaker (speaker becomes a microphone) where it may be needed.

Belden 1307A is what I see used a lot. Do everyone a favor and don't do it all in one run. Makes trouble shooting a pain. Give each "row" a home run back to the amp.

Figure most speakers will get tapped around 2 watts....so are you using 80 speakers? Sorta having a hard time seeing 160 watt within 100 feet. Systems I work with would easily be ten to 20 times that length of wire.

 

[Speedskater]

Isn't the capacitance inside most speaker cabinets (in their crossover networks) several orders of magnitude greater than anything the cable might contribute?

The crossover capacitors are in series with the speaker drivers. The cable capacitance is in parallel with the amplifier output.

Aren't amplifiers designed around that?

This problem first appeared in the early 80's, with strange hi-fi speaker cables. Modern amplifier designs shouldn't have this problem.
Yet a pro audio installer (large arenas & stadiums) notes that even some big pro amps (not 70V) still have this problem.

 

[grich]

Done my fair share of 70 volt audio over the years. While we never used shielded cable, it never occurred to me that his could be an issue. If that is the case wouldn't running this in EMT, as would be the case in some commercial job specs, create the same issue?

The amp that burned up on me was a direct-coupled 10W amp with 8-ohm output impedance. Since the output of a 70-volt amp is usually coming from a transformer, that probably changes the game. Also, the matching impedance of a 70-volt, 100W amp is around 50 ohms.

Our church has 200' of unshielded twisted pair speaker cable in conduit with no issues. I would guess the capacitive effects from the conduit to be less than a shielded cable where the shield is in close proximity to the signal conductors.

 

[GoldDigger]

Isn't the capacitance inside most speaker cabinets (in their crossover networks) several orders of magnitude greater than anything the cable might contribute? Aren't amplifiers designed around that?

The capacitance in a crossover network appears as a series impedance to keep the low frequencies out of the tweeter. A series inductor serves the same purpose to keep the woofer from loading down the high frequencies.
Of much greater concern, for low impedance speakers more than for 70 volt line applications, is the series resistance. That should be much lower than you would choose on the basis of voltage drop alone. A low source impedance with minimal wire impedance provides better control of mechanical resonances in the speaker and enclosure.
Not all speaker designs require this, but it is still a good design goal, especially if you do not know the details of the speaker system.

Sent from my Pixel 4a using Tapatalk

 

[hbiss]

From the Crown DCi Series amp manual:

CAUTION: Never use shielded cable for output wiring.

CAUTION: Never connect the speaker return to the chassis of the amplifier, or damage to the amplifier may result. NOTE: Custom wiring should only be performed by qualified personnel.

Class 2 output wiring is required.

To answer your question about length of runs vs wire gauge:

https://www.lowellmfg.com/wp-content/uploads/What-Gauge-Speaker-Cable-Should-I-Use.pdf

-Hal

 

[zbang]

To answer your question about length of runs vs wire gauge:

https://www.lowellmfg.com/wp-content/uploads/What-Gauge-Speaker-Cable-Should-I-Use.pdf

he he , that's exactly the page I looked at for post #3.

 

[markebenson]

Thanks the chart was exactly what I was looking for.

FYI

In multi conductor runs I find it more efficient to run audio / i-t wire in conduit as required rather than buy plenum rated wire and it gives you a cleaner job.

The shield is irrelevant as I already have 10k feet of this wire, I am not buying it.

I use a DBX driverack to deliver the proper frequencies to their speakers amount the other features.

 

[hbiss]

In multi conductor runs I find it more efficient to run audio / i-t wire in conduit as required rather than buy plenum rated wire and it gives you a cleaner job.

The shield is irrelevant as I already have 10k feet of this wire, I am not buying it.

Don't asume that a cable in metallic conduit and a cable with shielding is the same thing like some here seem to think. Conduit has hardly any effect on the electrical properties of a cable running through it. The shield in a shielded cable adds capacitance between it and the conductors, actually making the cable like coax with a characteristic impedance. It's the LC properties of shielded cable that can cause hypersonic ringing in the outputs of an amp and lead to damage.

-Hal

 

[markebenson]

Don't asume that a cable in metallic conduit and a cable with shielding is the same thing like some here seem to think. Conduit has hardly any effect on the electrical properties of a cable running through it. The shield in a shielded cable adds capacitance between it and the conductors, actually making the cable like coax with a characteristic impedance. It's the LC properties of shielded cable that can cause hypersonic ringing in the outputs of an amp and lead to damage.

-Hal

Thank you for the input. Good to keep in mind.

 

[gadfly56]

Don't asume that a cable in metallic conduit and a cable with shielding is the same thing like some here seem to think. Conduit has hardly any effect on the electrical properties of a cable running through it. The shield in a shielded cable adds capacitance between it and the conductors, actually making the cable like coax with a characteristic impedance. It's the LC properties of shielded cable that can cause hypersonic ringing in the outputs of an amp and lead to damage.

-Hal

That's interesting. Siemens specifically tells you that if you run unshielded speaker cable for their voice systems inside metal conduit that you have to reduce the nominal max distance by 1/2 due to capacitive coupling, or so I recall.

 

[hbiss]

Siemens specifically tells you that if you run unshielded speaker cable for their voice systems inside metal conduit that you have to reduce the nominal max distance by 1/2 due to capacitive coupling, or so I recall.

I have no clue what Siemens is doing or even what the system is. We are talking about normal constant voltage paging systems here.

-Hal