The magnetic field of a long straight wire at the distance d away from the wire is determined by the formula
where I is the current in the wire and mu_0 is the magnetic permeability constant
Plugging in the field of the Earth B = 5.5*10^-5 T and the distance in meters (25 cm = 0.25 m), you get
Pi will cancel and solving for I you can find
`I=(2*5.5*10^(-5)*0.25)/(4*10^(-7))=68.75` Amps - is the maximum current the wire can carry in order not to exceed the field of the Earth at the distance 25 cm away.