IMO there is no right or wrong way - well I guess if you have frequent tripping of overcurrent protection you must have been wrong.
I would suggest adding up all known fixed loads like permanently installed lighting, fastened in place appliances, etc. then add any known not fastened in place appliances and this is typically the minimum necessary, But type or use of some items could still have some impact on what you may be able to get away with. One family may not load it much, but the next one to move in may have different ways and it will not be enough.
That's what I thought, I can't imagine why the EC that did the electric on the house decided to use a #8 neutral with #6 conductors (it is protected by a 60 amp OCPD). I just didn't want to call him out on it and then find out I was mistaken because of some obscure method for sizing a sub panel neutral. Thank you for the reply.
Given what kwired posted, I still have to question what you thought... and/or continue to think.
First, the only thing affecting the calculated load of the subpanel is loads in the same area served by both subpanel and main panels that are determined by footage. These are typically limited to general lighting and receptacles. Code gives no prescribed way to segregate these, but proportional to connected load (or Code theory such as 180VA per receptacle) would be the most logical.
With that said, there is potentially an easier way to determine whether the neutral is properly sized. The maximum unbalanced neutral current calculated load plus the line-to-line loads cannot exceed the rating of the panel, supply ocpd or ampacity, whichever is least (i.e. weakest link). A quick check can be performed by simply subtracting any line-to-line loads from the weakest link value, and that would be the
maximum required neutral ampacity for that subpanel.
Given subpanel is fed with #6 hots and #8 neutral, there's only a 15A difference in ampacities @ 75?C. If there are any line-to-line loads of 15A or greater, the neutral is properly sized as long as the #6 is properly sized....