that if you want a really large current, you have to move really large charges, and really large charges repel each other unless they happen to be opposite charges, and if they cancel each other out, you don't get any current.

Jon,

that is wrong. In case of superconductivity, You do not have

any charges not cancelled by opposite charges in the "wire".

There are limits to the superconduction imposed by currents,

but this is due to the magnetic fields of the currents and

some not really understood factors.

For the "classic" superconductors (mercury, lead, Niob, pure

metal elements) the limits are rather low, some Amperes can

be enough to kill the superconduction.

Big currents (thousands of Amperes) are technically possible with

the so called superconductors of "second order", This are

extremely brittle compounds made from Nb and Sn.

These compounds are more ceramic than metallic, the magnetic

fields penetrate this alloys in a very special way.

This kind of superconductors are not covered by BCS theory,

so nobody really knows why they can carry big currents and

magnetic fields. Even less is understood with the high temperature

superconductors, which do not allow high currents (to date)

Georg