6" Nipples in the 1968

[tom baker]

One of my students recalled there used to a 6" nipple rule(where fill derating is not required)
He was right and its in the 1968 NEC, the next I have is 1978 where its 24".
Any ideas as to why it changed from 6" to 24"?

 

[GeorgeB]

One of my students recalled there used to a 6" nipple rule(where fill derating is not required)
He was right and its in the 1968 NEC, the next I have is 1978 where its 24".
Any ideas as to why it changed from 6" to 24"?

While not of an electrical consideration, I ASSUMED it was to allow residential double panel installations with a stud space between panels and to allow conduit from the top or bottom of a panel through to the attic or basement/crawlspace.