Bob,
I've not found a requirement yet.
Just a manufacturer's suggestion, probably not enforceable. Kinda like the red heads [Linked Image]

Try page 50&51, or screen 27 of 33 at www.afcweb.com/pdfs/afcpocketguide.pdf
(This site has a lot of good reference material, just in case there's something else that we never heard of [Linked Image] )

It absolutely has been asked by inspectors in more than a couple of jurisdictions around here.
I don't believe that they mean 4/0 MC.

Seems funny that you'd walk all the way to the truck for a red-head, but not 3ft for your RotoSplit. [Linked Image]

I'll keep looking for some requirement...S