John,
I would agree that a 20 amp OCPD would require a grounded conductor with an ampacity of 20 amps or more.
I would also agree, and I don’t think it will ever be an issue again with NM cables, but I’m wondering if this is why the values for #12 and #14 at 60-degree C were changed in the 2011 NEC.
Otherwise, it seems that since the grounded conductor is not connected to an overcurrent device, it would only have to sized at 100% and 240.4[D] wouldn’t seem to apply. Currently, #14 is rated at 20A at 60-Degree C in Table 310.16 of 2008 NEC.
I do see where 240.23 states that a change in the grounded conductor shall be permitted when a change occurs in the ungrounded conductors are changed, but it doesn’t appear to be required and the change could mean smaller or larger.
