In early 2012, UL has changed the standard numbers for plastic electrical conduit. Previously UL 651B was the standard for Continuous Length HDPE Conduit. UL 651A was the standard for PVC and HDPE stick conduit. NOW:
1. UL 651 A is for continuous length and stick HDPE Conduit.
2. UL 651 is for PVC Conduit.
No doubt there will be some confusion going forward as some engineers will be quick to change their specifications, while others will not be aware of the recent changes. Blue Diamond is certified to manufacture HDPE Conduit under its UL listing and would be happy to explain the changes to your specifiers. Call George Zagorski, PE at BDI 859-224-0415, or email at: firstname.lastname@example.org