# Load of a device in circuit

This question is based on the answer in my previous question: Current vs voltage in high voltage transmission lines

Suppose we have a system that consists of a primary circuit and secondary circuit, with some transformer between them. No devices on the secondary circuit have any additional transformers, there's just one transformer between the primary and secondary circuit.

What exactly does it mean to say that a device on the secondary side can "sink" a large load? I can see it has to do with a large power requirement, but in the simple scenario above, I'm not sure whether it makes sense - is it not enough to simply think of the resistance of the device?

It's also mentioned that if the power load gets too large, the voltage will "sag". What would be exact calculation for why the voltage will sag, and what would be the corresponding situation that would cause this, in my simple scenario that I've described above? Does this assume a specific type of source, or is it true regardless of whether it's a battery or a generator?