Rated Cables
When speaker cable is installed inside the walls, it is important to use cable that is rated for in-wall use. This is often referred to as premise wiring rated. Most speaker cables sold for consumer use are not rated for premise wiring. There are two strong reasons for insisting on premise rated wiring. First, it complies with the National Electric Code. Second, it assures you the materials and the design of the cable meet well established standards for fire safety.
Gauge Selection
Because the impedance of loudspeakers is quite low, (usually 4 or 8 ohms) the resistance of the cable feeding the speakers becomes quite relevant in determining how much power actually reaches the speaker. As an example, a hundred foot run of 16 AWG cable will have a round trip
resistance of 0.8 ohms. If this is used to feed a 4 ohm speaker, approximately 17 percent of the power will be lost to the cable, and 83 percent will reach the speaker. Larger wire will reduce the power loss.
From a practical standpoint, the wire sizes normally used for whole house audio speaker wiring are either 16 AWG or 14 AWG. These sizes provide a good compromise between line loss, cost, and ease of installation. Most of the equipment available for whole house audio systems have connection devices designed for these wire sizes. Going to a non-standard wire size can immensely increase the difficulty of installation.
Power Loss Budget Table
The following speaker wire power loss budget table provides a guideline for selecting the right size speaker wire.
|
Speaker Wire Power Loss Budget Table |
| Select Speaker Impedance and Maximum Acceptable Loss, then look up
maximum cable run distance. |
Speaker Ohms |
db Loss |
Power
Loss |
16 AWG Run
in feet |
14 AWG Run
in feet |
| 4 |
0.5 |
11% |
60 |
100 |
| 4 |
1 |
21% |
130 |
210 |
| 4 |
2 |
37% |
290 |
460 |
| 4 |
3 |
50% |
500 |
790 |
| 8 |
0.5 |
11% |
120 |
190 |
| 8 |
1 |
21% |
260 |
410 |
| 8 |
2 |
37% |
580 |
930 |
| 8 |
3 |
50% |
990 |
1580 |
A 3db loss may seem like a lot, but since the ear has tremendous dynamic range, you probably won't notice that size loss and with the power of the amplifiers typically used, it is easy to overcome a 3db loss. There is another effect that you should consider. Some high end amplifiers have special speaker compensation circuits. Over the full range of frequencies handled by a speaker, it will perform better at some frequencies than others. Some high end amplifiers sense the response characteristics of the speaker and automatically compensate for non-linear responses. This is sometimes called servoing. When the resistance of the speaker wire is a significant part of the speaker impedance seen by the amplifier, it can impact the effectiveness of an amplifiers compensation capabilities. For most amplifiers intended for whole house use, this is of little relevance.
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