Saturation puts a practical limit on the maximum magnetic fields achievable in ferromagnetic-core electromagnets and transformers of around 2 T, which puts a limit on the minimum size of their cores. This is one reason why high power motors, generators, and utility transformers are physically large; to conduct the large amounts of magnetic flux necessary for high power production, they must have large magnetic cores. In applications in which the weight of magnetic cores must be kept to a minimum, such as transformers and electric motors in aircraft, a high saturation alloy such as Permendur is often used.