I read that its bad to run speaker wire for more than 5 feet near an AC wire
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Because each AC line has 2 wires carrying the current in the opposite directions the magnetic field from each wire almost cancel each other. The net magnetic field is very small and falls quickly with the distance from the AC line. If you run a speaker wire parallel to an AC line for 30 feet at the separation between them of 3 feet than the induced voltage on you speaker will be about 1 microvolt. Nothing to worry about.
Below is a long a boring derivation of how one can arrive to this number. Read it at your own risk!
For a long AC line a good estimate for induced magnetic field is
B=0.1*I*delta/R^2,
where I is the current in the AC line, delta is the separation between the 2 wires in inches and R is the distance from the line in inches. B is in Gauss, this is a unit for magnetic field measurement.
Let's take an example. I=15A (max for a typical household AC line), delta=3/8", R=36". Then B=0.4 milli Gauss
How large is it? Well, about 1,000 less than the Earth magnetic field. In reality the field will be even less because we assumed the AC line is running flat. If the wires are twisted then field will drop down much quicker with the distance.
How much voltage this field can induce in the speaker wire? Again, the speaker wires run very close to each other, so they form a loop with a small area. Even better if they are twisted.
If, in the worst case scenario the speaker wire run parallel to AC line then the induced voltage can be estimated as
V=B*L*s/3000 where B is our magnetic field in Gauss, L is the distance that the AC line and the speaker wires run parallel to each other (in feet) and s is the separation between the speaker wires (in inches). Frequency 60 Hz is assumed.
Ok, almost over. Take the previously calculated B=0.4 mG , L=30’, s=1/4” then V=1micro volt. Absolutely nothing to worry about.
