When is a Heavy Copper PCB actually necessary?

I'm looking into PCB design for higher-current applications and wanted to get some practical opinions on when designers decide that standard 1 oz or 2 oz copper is no longer sufficient.

Heavy copper PCBs seem useful when a board has to handle high current, higher power density, or significant thermal loads. Increasing the copper thickness can reduce resistance and voltage drop and can also provide better current-carrying capability.

But I'm curious about how people approach the decision in an actual design.

Do you normally start by calculating the required trace width and temperature rise and then increase the copper weight if necessary? Or are there situations where you choose 3 oz, 4 oz, or heavier copper from the beginning because of the application?

There also seem to be other factors to consider besides copper thickness, such as:

• Trace width and available routing area

• Temperature rise

• Heat spreading through copper planes

• Number of PCB layers carrying the current

• Via current capacity

• Manufacturing limitations

• Cost and availability of heavy-copper fabrication

I'm particularly interested in the trade-off between simply making traces wider and increasing the copper weight. At what point does heavier copper become the more practical solution?

If you have designed high-current boards, what copper thickness have you found to be a practical starting point, and what ultimately determined your choice?