Wednesday, August 6, 2014

Tech Tip: 25 Volt & 70V Cabling Requirements

By Jim McGuinness, Product Support Specialist

We get many calls about distributed audio and the connections associated with those systems.  Sometimes, the applications are new systems and sometimes they are replacement systems.  For new systems it might be a bit easier, since the entire system will be installed fresh and you should have control over most, if not all of the variables.  However, in a replacement system, you don’t always have control over some of the makeup of the system, namely- the wiring, in many cases.  This could be due to cable access and being able to rerun cable, or simply that replacing wiring is labor intensive and it would push the project beyond the budgetary limits set. Therefore, going with existing wiring is often the decision.  You could just have a client that doesn’t want his building knocked apart for wiring if at all possible, as he’s more interested in just replacing the head-end equipment.
 

Applications for distributed audio installations are all over the place these days.  Typical examples are schools, hospitals, nursing homes, hotels, malls, really…you name it.  When we look at these applications, we have to look at the length of cable versus the farthest distance in the system, as well as the power/current being pushed through the system.  This is a very important consideration on new or replacement systems, but especially replacement systems where cabling will be reused.  70V systems are much more forgiving with respect to cable gauge versus a 25V system.  We usually recommend 70V applications where applicable.  In some cases, it is not possible based on local code.  A couple of examples might be in hospitals or schools.  Often they are 25V systems, not 70V systems.
 

Now there’s one other factor that also has to be considered when replacing an existing system where cabling will be reused.  Was the cabling adequate for the system from the get go?  Meaning- was the proper gauge of wire used for the wattage load of the system?  This should always be checked.  What if the wattage of the system changes to a higher value?  That’s another wiring consideration.  If you are replacing a system that was originally a 25V speaker system with a 70V speaker system, then generally speaking, you should be ok.  However, going the other way around is a major concern.

Included in this article is a cable guide chart for both 25V and 70V systems.  The yellow highlighted example in the chart shows a 100 watt typical load for both a 25V and 70V system based on keeping about a 225 foot run of cable for both systems.  For a small store, this might be a pretty typical scenario.  The important point is to look at the wire gauge difference for these two systems.  For 70V you can use a relatively light, 18 gauge wire. However, for the same system in 25V, a heavy 12 gauge wire will be needed to get the same job done.  Signal loss was also considered when this chart was made, meaning that the resistance of the wire itself has to be considered as part of the added load to the amplifier.

In the second orange circled example, we looked at it a different way.  We looked at a 70V system where the impedance and wire gauge will be the constants, and using those constants to find the approximate point on the 25V system chart.  Now the distance went up a bit, but the real key here is the resulting power load limit.  Only a 15 watt system can be installed using the same gauge of wire.  In this example that’s a power factor of about 7 to 1!
 

So, it’s really important to look at these simple things to get a system to perform properly for years to come and not have any issues with the system performance.  Failed systems yield unhappy customers and we all know what that means.

Happy connecting………