Largest breaker.

[Fordean]

Can you legally put a 60 amp breaker in a 100 amp panel. I always went with a 1/3 of the main rule. But I think it just needs not to exceed Main breaker Rating.
And Disregard the 1/3 rule

 

[480sparky]

1/3 rule?

You can put 20 100a breakers in a 100a panel if your load calcs say so.

 

[hillbilly1]

Other than load calcs, the other limiting factor would be the manufacturers labeling, which is usually size, not number.

 

[kwired]

Most 100 amp main breaker panels have same bus as is used in 125 amp panels and the bus is rated to accept 125 amp breakers. Many panels over 125 amps still only have breaker to bus connection rated for 125 amp max, but there are larger breakers that utilize more than one space per pole and are listed for such use.

If you want to limit max breaker to 1/3 the main you can, nothing says you have to. If you do that you still only have 60 amp max in a 200 amp panel or 70 if you round up to next standard size. There are a lot of 100 amp sub feed panels fed from both 100 and 200 amp main breaker panels out there.

 

[Beaches EE]

It's been discussed and sometimes recommended to install conductors, or conduit that will accommodate conductors, that match the maximum rating of a sub panel in case of future load expansion. This is a general statement and of course individual circumstances must be taken into account.

 

[infinity]

Can you legally put a 60 amp breaker in a 100 amp panel. I always went with a 1/3 of the main rule. But I think it just needs not to exceed Main breaker Rating.
And Disregard the 1/3 rule

There is no 1/3 main rule so you're free to install a 60 amp OCPD in a 100 amp panel. Where did you hear about a 1/3 rule?