As Don said, we cannot know what the minimum for the particular situation in question with the information given.
But, with that said, the minimum per his reference of 250.24(C) > 250.66 shows that it won't be smaller than #8 copper.
Of course some people might claim that 250.24(C)(1) would not apply to SE cable, due to the subsection's title, but I wouldn't even bring that up. :angel:
Nope, you're right, never have. In fact don't see all that much copper SE around here, just occasionally, and only when you shop around. Maybe open wiring on insulators then would be a better example of a method that may technically allow smaller than a #8 neutral with #4 ungrounded conductors for a service, but I don't imagine that is the intent. Nor have I ever looked into it before I brought it up tonight.As far as SE cable goes, have you ever seen cable with 4 AWG ungrounded conductors that also has less than an 8 AWG grounded conductor?
I would think it may be available, but would have to be custom ordered, and unless you are going to order a large quantity, will probably cost more than whatever is commonly stocked.