In a certain stereo system, each speaker has a resistance of 4 Ω. The system is rated at 60 W in each channel. Each speaker circuit includes a fuse rated at a maximum current of 4 A. Is this system adequately protected against overload?

Answer 1

What current would flow in the speaker if it were using 60W?

P = I^2*R
I^2 = P/R 60/4 = 15 A
Imax would be less than 4 Amps.

If the fuse does not blow until current reaches 4A, the speaker is NOT adequately protected.

Answer 2

Did you forget to square root 15? Because I^2 = 15, so I = sqrt 15 = 3.87. If that is the answer, then the speaker IS adequately protected.