This is one of those very common jumper cables with banana plugs. It says 18 AWG in the datasheet (link below), but it can handle up to 15 A? One of the labmates told me it could handle up to 20 A?! How do I judge? https://www.digikey.com/en/products/detail/pomona-electronics/B-12-0/603349
Current ratings for wire are not hard numbers. Copper can handle the current up until it melts or loses structural integrity.
But in practice, the max temperature of the insulation used will limit this combined with the operating environment:
- hot vs cold ambient temperature
- enclosed (i.e. cabinet or conduit) vs open-air vs submerged
- in static air vs moving air
- bundled with other wires or all alone
Or even the acceptable voltage drop for the length of wire used for the current carried which has nothing to do with temperature at all.
Also, some applications do not allow the wire to exceed a certain temperature for safety while exceeding that temperature is not a problem for other applications even though the wire holds up in both cases. A wire too hot to touch might be okay out in the open...but do you really want something that hot buried inside your wall?
For example, wire on a model airplane exposed to the air as the plane flies through the sky can handle a lot more current than the same wire in a cabinet, or in a cabinet and bundled with other wires.
So all wire ampacity tables have some conditions attached to them such as ambient, temperature, environment, insulation type, conductor material, bundling, and allowable temperature. What theses conditions are is not always made explicitly clear but tables are usually associated with a particular usage.
The item in question is an assembly not a wire. So a simple possibility is the connectors are the limiting factor not the wire.
Don't overlook specs can be rather arbitrary. I'm doubt the manufacturer designed the assembly, then carefully perform temperature rise measurements with a dozen thermocouples and precisely determine the current at which the maximum safe temperature was reached. It was probably the other way; the catalogue had 8A and 25A options so marketing wanted a 15A to fill the gap and 20AWG seemed a bit 'light duty'.
Finally this is not safety rated part when international standards have prescribed test methods. So the permitted ambient and temperature rise was selected by the manufacturer.
Current ratings for wires and connectors are all about how much power you want going through it for a given amount of time. Pomona has specified that 18AWG cable can run continuously carrying 15A. You can put more current through it but it might heat up more than you like.
As DKNguyen said other conditions like ambient temperature and airflow affect the final steady state temperature. If you're going to run the wire outside the specified limits you need to take all of that into account to ensure nothing melts.