25 volt system

Randy Gartner

Randy Gartner

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Jan 12, 2011
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Is wiring a 25 volt system the same as wiring a 70 volt system,ie: set taps on speakers so that they do not exceed the maximum output of the amplifier?

Also,opinion on NEC and 70 v systems. I see some references saying that 70 volt systems can use class C wiring methods to wire 70volt speaker system. Knowing that the code is updated every 3 years,just wondering if this is reguarding an older code reference and is that still the case today?
 
Re: 25 volt system

Afaik, a 25 volt system would work just like a 70 volt system, just at a lower voltage. Keep in mind the maximum total power available on a 25 volt system is less than that of a 70 volt system. So if you need a lot of speakers at more than a minimal wattage, it isn't going to work for you.
 
Re: 25 volt system

Brian jojade said:
Afaik, a 25 volt system would work just like a 70 volt system, just at a lower voltage. Keep in mind the maximum total power available on a 25 volt system is less than that of a 70 volt system. So if you need a lot of speakers at more than a minimal wattage, it isn't going to work for you.
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A 25V system is pretty much the same as 70V system, just a lower turns ratio on the output transformer. The power available is whatever the amp delivers. A speaker with 70V transformer taps will make proportionately lower power, but fully loaded the amp will make and deliver full power.

Because of the lower voltage there will be higher current and more wire losses, for similar power delivered.

The lower voltage systems are often used in schools for increased safety. Perhaps check for code issues specific to schools if that's where you are working. The insulation rating for 70V "should" be more than adequate at 25V.

(in other words I do not really know).

JR
 
Re: 25 volt system

Randy Gartner said:
Is wiring a 25 volt system the same as wiring a 70 volt system,ie: set taps on speakers so that they do not exceed the maximum output of the amplifier?
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Both are 'constant voltage' systems, there are also 100V and 140V systems, although more common outside the US or for systems with extremely long runs.

Randy Gartner said:
Also,opinion on NEC and 70 v systems. I see some references saying that 70 volt systems can use class C wiring methods to wire 70volt speaker system. Knowing that the code is updated every 3 years,just wondering if this is reguarding an older code reference and is that still the case today?
Click to expand...
Class C would be referring to the specifics of the stranding for the individual conductors. I believe that starting with NEC 2008, when using Class C or Class D conductors for power cables it is required to use connectors and terminations listed for fine stranded conductors. This is intended to prevent or limit damage to the strands in the conductors and cold flow at the terminations.

Or were you actually referencing the wiring Class? Recent model amps should indicate on the rear panel whether the output is considered Class 1, 2 or 3 and most 70V amps would typically be Class 2 or 3.

Or is the issue a combination, such as worrying about the stranding only if it relates to a power, rather than a power limited or low voltage, wiring Class? Maybe someone else knows the answer to that.
 
Re: 25 volt system

The question was about wether the 70 volt system falls under the perview of the NEC. I know generally the NEC considers anything under 50 volts to be low voltage. But didn't know how a 70 volt sound system would be addressed.No local codes address the situation. Most places in this area use the NEC .To the best of my knowldege,PA still did not adopt the 2011 code.We're still using the 08 code.I will dig out my copy of the 08 codebook and see if I can find anything that applies to a 70v sound system.
 
Re: 25 volt system

Don't now about the NEC, but there's always been this reasoning floating around out there among installers:
(stolen from a Rane Note http://www.rane.com/pdf/ranenotes/Constant_Voltage_Audio_Distribution_Systems.pdf)

Voltage Variations -- Make Up Your Mind
The particular number of 70.7 volts originally came about from the second way that constant-voltage distribution reduced costs: Back in the late '40s, UL safety code specified that all voltages above 100 volts peak ("max open-circuit value") created a "shock hazard," and subsequently must be placed in conduit -- expensive -- bad. Therefore working backward from a maximum of 100 volts peak (conduit not required), you get a maximum rms value of 70.7 volts (Vrms = 0.707 Vpeak). [It is common to see/hear/read "70.7 volts" shortened to just "70 volts" -- it's sloppy; it's wrong; but it's common -- accept it.] In Europe, and now in the U.S., 100 volts rms is popular. This allows use of even smaller wire. Some large U.S. installations have used as high as 210 volts rms, with wire runs of over one mile. Remember: the higher the voltage, the lower the current, the smaller the cable, the longer the line. [For the very astute reader: The wire-gauge benefits of a reduction in current exceeds the power loss increases due to the higher impedance caused by the smaller wire, due to the current-squared nature of power.] In some parts of the U.S. safety regulations regarding conduit use became stricter, forcing distributed systems to adopt a 25 volt rms standard. This saves conduit, but adds considerable copper cost (lower voltage = higher current = bigger wire), so its use is restricted to small installations.
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Re: 25 volt system

IMO the more applicable excerpt from that link is.

from rane note said:
In some parts of the U.S., safety regulations regarding conduit use became stricter, forcing distributed systems to adopt a 25 volt rms standard. This saves conduit, but adds considerable copper cost (lower voltage = higher current = bigger wire), so its use is restricted to small installations.
Click to expand...
JR
 
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