Size sub panel for each apartment in a 3 family house
In a 3 family house there are three main panels in the basement, one for each apartment. Some circuits are taken out of those panels, but there is also a sub panel inside of each apartment that the rest of the circuits are taken out of.
Is there any calculation that needs to be done to determine the size of that sub panel?
Right now the panels are fed with a 12-2 (that's right, single leg) and there has never been a problem.
The panels need to be upgraded, would it be alright to refeed them with 10-3, which should be more than enough ampacity for them? Or should it be bumped up to 8-3 just to be safe?