Have you asked the equipment supplier? The mechanical connections between different equipment can dictate very small movement tolerances.

If all it has between equipment on different foundations is cables, then it could be a significant amount, that is not a "visible" settlement. However if its bus bars or some other solid connection then you would need to ask the sparkies what they could tolerate, but 10mm maybe?

The TX will have limits on levelness but maybe not global settlement.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

Quote (Phatt)

I'm designing foundations for a transformer, breaker, disconnect switch, control building, and other substation equipment.

These type of equipments are not that sensitive to have any worst kind of effects in case your foundations are within the allowable limits.

If you are worried or your substation Electrical engineer would discard your design when are having settlement of about 25-50mm depend on your type of foundation, it can cause some electrical field problem, then you should ask and discuss with the electrical department, but mostly that would not be a problem.

However another point is that, these type of supporting structure of substation equipment are usually not depedent on each other in a very restricted way, just cables are passed from equipment to another, excluding Gantry having Tension wire from Substation to Transmission line towers.

Quote (Phatt)

The foundation that I am concerned about is the transformer foundation. Its size is about 8m x 5m x 0.6m and its loading is about 45 kN/m^2(kPa) or 0.94 ksf.
Where can I find an allowable settlement for those foundations?

Follow your Geotechnical allowable settlement criteria.

Time is far more precious than you can imagine.

8*5m with 45kPa, you are talking 100mm plus of settlement so likely too much