My old TS-520S is said to put out 160 watts DC CW and
200 watts PEP SSB .
Whats the minimum dummy load requirement to tune it ?

There are kits for 100 watt dummy loads. Would that be
enough for brief ( ~ 10 sec ) tuning ?

Are there standards as to how the rating of a dummy load
is to be interpreted ?
====================================
Professional dummy loads are normally rated for continuous use .
They often have cooling fins with the actual resistor being of the thin
film type on a heat sinking tab.

I have made my own dummy loads (for HF and lower VHF) from 20 pcs 1KOhm
2watts ,resistor ,so good for 40Watts as is ,but when immersed in motor
oil capable of handling 100+Watts for 10-15 minutes.
I used carbon film resistors ,but metal oxide ones are perhaps better.
The advantage of using many resistors (in parallel) is a very low
inductance.
I fitted the resistors (sandwiched between 2 jam jar lids with leads
just long enough to solder to lids through holes) inside a cylindrical
metal can with a SO239 connector fitted in centre of its lid.
To prevent oil 'swetting' from can I covered the seams with solder on
the inside. Even after many years ,resistance was 50.6 Ohms or about.
A simple but effective homebrew project.

Frank GM0CSZ / KN6WH