70V audio wiring size

[GoldDigger]

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

Sounds like a life safety voice announcement system for a large building.
I don't know if they also include DC supervision.
Except for that they are probably normal 70V line systems.

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[gadfly56]

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

This is the Siemens voice notification system, part of their fire alarm/mass notification offering. It operates at a constant voltage of 70.7 volts. It can be used for paging and background music, if desired. The speakers are supervised.

 

[GoldDigger]

Some people have a wrong impression about what a 70 volt line system is.
It refers to the fact that the line voltage when the amplifier is driven to maximum output will be 70 Volts regardless of the connected speaker load as long as it is within the total wattage rating of the system.
Typically standard 8 Ohm speakers are connected to the line using transformers whose taps are labelled according to the power supplied to the speaker with a 70V input. The system designer chooses the tap for each speaker depending on the sound output needed at that location. It will seldom match the maximum power handling capacity of the speaker.
To simplify the wiring requirements an alternate design with a 20V nominal line voltage is sometimes used. It may need larger gauge wire compared to a 70V system, but has a much lower touch voltage hazard.
If the amplifier power and speaker placement are conservative the maximum line voltage in use may never actually reach 70V.

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[LarryFine]

The constant-voltage system also means you don't have to take speaker impedances into consideration.

 

[James L]

I'm curipus how many conductors to run for a home audio 70v speaker w/ 8ohm option. I got a cut sheet (which I lost), and it showed numerous binding posts...
Red + black - 8ohm
I think the others were...
5v
20v
30v
50v
70v
Something like that for sure.

I was looking at some stuff online, looks like it can be paired with a wall mount volume control, which I am not installing.

I'm only pre-wiring, and the owner doesn't know what system he's installing. I'm just running cables near the panel.

I thought about 14/4 in-wall speaker cable. Does the 70v need multiple conductors?

 

[LarryFine]

The only time I use 4-conductor cable for speakers is for home runs, like to stereo volume controls and switchers.

 

[James L]

The only time I use 4-conductor cable for speakers is for home runs, like to stereo volume controls and switchers.

They're gonna put whatever system by the breaker panel. I'm thinking 4-conductor just in case they use speakers with dual 8ohm voice coil or if a 70v system needs multiple conductors.

Honestly, I never even heard of 70v home audio until 2 days ago. He has yet to decide on a system, and the speakers are 70v or 8ohm (the latter I'm very familiar with)

Each speaker will have a home run

 

[zbang]

I've got you beat by two days, I'd never heard of 70v for a home audio system although I suppose if you were doing a distributed system.... And "home audio 70v speaker w/ 8ohm option" doesn't make sense unless that means a 70.7v line transformer with an "8 ohm" tap, but it would still have a power selector tap (like https://www.parts-express.com/70v-15w-speaker-line-matching-transformer--300-039).

In the context of PA and low-level distributed systems, IME it's either 25v, 70v, (or occasionally 115 and 140.4v on a few McIntosh amps). And those were always two wires per channel.

Note that a fire alarm system should not be discussed as if it was a generic PA.

 

[LarryFine]

Each speaker will have a home run

Then you're golden. All you need is two conductors.

 

[James L]

Then you're golden. All you need is two conductors.

Thanks a bunch.

 

[James L]

.... And "home audio 70v speaker w/ 8ohm option" doesn't make sense unless that means a 70.7v line transformer with an "8 ohm" tap, but it would still have a power selector tap....

I can't really conceive the concept, either. But maybe after I see some components I'll have a better idea

 

[hbiss]

'm curipus how many conductors to run for a home audio 70v speaker w/ 8ohm option. I got a cut sheet (which I lost), and it showed numerous binding posts...
Red + black - 8ohm
I think the others were...
5v
20v
30v
50v
70v
Something like that for sure.

Sorta kinda like that.

Many 70 volt speakers have a 70 volt option as well as a direct 8 ohm option. They will have a rotary switch with which you select the wattage you desire for 70 volt operation as well as an 8 ohm position where the input is connected directly to the speaker and bypasses the line transformer. So those numbers would correspond to watts not volts when fed from a 70 volt sysyem.

And I've never seen one with a bunch of binding posts but I wouldn't put that past Chinese consumer junk.

If you don't know whether the customer is going to use 70 volts or 8 ohms, it would be best to home run from each speaker to the amp location.

-Hal

 

[James L]

Sorta kinda like that.

Many 70 volt speakers have a 70 volt option as well as a direct 8 ohm option. They will have a rotary switch with which you select the wattage you desire for 70 volt operation as well as an 8 ohm position where the input is connected directly to the speaker and bypasses the line transformer. So those numbers would correspond to watts not volts when fed from a 70 volt sysyem.

And I've never seen one with a bunch of binding posts but I wouldn't put that past Chinese consumer junk.

If you don't know whether the customer is going to use 70 volts or 8 ohms, it would be best to home run from each speaker to the amp location.

-Hal

Yeah, I saw a rotary switch in images when I did a Google search.

I just wasn't sure if it worked like a 0-10v dimmer, or more like an automobile windshield wiper motor with seperate wiring for H/M/L speeds.

I get what you're saying about the connections now, and makes sense. Im sure I looked at the diagram wrong when I thought it showed various voltages.

And I always run separate home runs on anything low voltage. The last daisy-chaining I did was phone jacks back in the mid-90s

Thanks for explaining.

 

[hbiss]

I just wasn't sure if it worked like a 0-10v dimmer, or more like an automobile windshield wiper motor with seperate wiring for H/M/L speeds.

The constant voltage system originated in the 30's as a way to do away with calculations. The switch simply selects taps on the primary of the transformer. The secondary of the transformer (8 ohms) goes to the speaker.

On speakers that are not "packaged" in a nice box, such as ceiling speakers, the transformer is right there and you do it the hard way. You get to pick between the black (common) and one of the four or so leads to wire nut your thru wiring to depending on how many watts you want it to be.

-Hal

 

[LarryFine]

The idea is that the total load watts, as determined by the tap selections, not exceed the amp's output power rating.

 

[James L]

The idea is that the total load watts, as determined by the tap selections, not exceed the amp's output power rating.

Interesting.
I spent a long time in car audio, so I think in terms of speaker ohm ratings affecting an amp's output. I'll have to see what type of system he goes with.

Thanks for the help.

 

[hbiss]

The idea is that the total load watts, as determined by the tap selections, not exceed the amp's output power rating.

Actually you want to leave about 10% less than the amps rated power.

... so I think in terms of speaker ohm ratings affecting an amp's output.

That's exactly why the constant voltage system was invented. You don't have to do that, just add up the wattages to make sure that the total doesn't exceed the amps rating and you are golden. Makes it easy to have some speakers louder than others too.

-Hal

 

[zbang]

You don't have to do that, just add up the wattages to make sure that the total doesn't exceed the amps rating and you are golden.

Bit of a problem when you look at an existing system and find 14 speakers at 10 watts on a 100 watt amp ("So that's why it was cutting out all the time!").

 

[LarryFine]

You can use a single 2-wire run to supply many speakers in parallel. The transformer takes care of impedance concerns.

Actually you want to leave about 10% less than the amps rated power.

Well, "not exceed" covers that.

You don't have to do that, just add up the wattages to make sure that the total doesn't exceed the amps rating and you are golden.

Oh, now you say it the same way I did.

 

[hbiss]

Good design practice dictates that you choose an amp with at least 10% more power than you need. But under no circumstances should you exceed the rated power output.

Bit of a problem when you look at an existing system and find 14 speakers at 10 watts on a 100 watt amp ("So that's why it was cutting out all the time!").

Happens all the time. You'll also find regular 8 ohm speakers hung on 70 volt lines which will really drop the impedance. Probably won't cut out since amps are pretty well protected but you won't get the volume you expect.

A non-recommended way to find those illegal speakers without transformers is to disconnect the wiring from the amp and apply 120 volts. Those 8 ohm speaker voice coils will vaporize.

-Hal