Transformer Losses

"iron" losses are losses that occur and remain the same whether your transformer is loaded or not. that is why they are called "fixed" losses or "no load" losses. when measured during an "open circuit" test they also include resistive losses on account of the Imag^2xRprimary (very small and negligible compared to losses in the core). mathematically they are calculated and measured with relation to the to the applied primary voltage and hence your reference.

"copper" losses occur and increase with the load on the transformer. that is why they are called "variable" losses or "load" losses. when measured during a "short circuit" test they also include "stray" losses on account of leaked flux (in a well built transformer very small compared to the resistive loss in the windings). mathematically they are calculated and measured with relation to the to the drawn current due to the load and hence your reference.

When you stop at a red light and keep the engine running, it consumes fuel, though in a small quantity. This is the 'minimum entry fee' for driving a car ! Analogous to iron losses.

When the light turns green, you engage gear, press the accelerator (gas pedal for the Americans) and then you consume fuel seriously. (_{There are a at least two Rolls Royces in Bangalore, and i swear, they use up one tankful of my car every revolution of their engine} ) This is analogous to copper losses, wouldn't you say?

Well, the OP asked which causes which loss? Continuing my automotive analogy, i would say "The nut that holds the wheel "

I have a classic car with a giant 429CI V8 engine (approx. 7.0L) that I swear makes an idle sound that seems to say "gallon-o-gas, gallon-o-gas, gallon-o-gas, gallon-o-gas, gallon-o-gas...".

When I was younger gas was cheaper so I didn't care. But back then if I pulled up next to a cop it said "gimme-a-ticket, gimme-a-ticket, gimme-a-ticket, gimme-a-ticket, gimme-a-ticket..."

Hah!

Yes, this Rolls Royce guy kept his engine idling when he pulled up to the pump. The attendant politely told him to shut off the engine, since the feed from the pump could not keep up with the consumption of the idling engine