If an electric wire is allowed to produce a magnetic field no larger than that of the Earth (0.55 x 10-4 T) at a distance of 25 cm, what is the maximum current the wire can carry?

we are given in the problem the following dimensions or specifications

B = 0.000055 T r = 0.25 m constant mu0 = 4*pi*10-7

The formula that is applicable from physics is

B = mu0*I / (2*pi*r) I = 2*B*pi*r/mu0 I = 68.75 Amperes