Conductors in Parallel is covered in 310.10(H). Often when working with higher ampacities, we find it to be cumbersome and expensive to keep increasing the size of the wire and conduit. The solution is often to use multiple runs that are connected to a common location on each end. Notice in the 2011 edition of the code that much of this section is highlighted gray, indicating new or revised text. The change here was that the previous code stated that you were permitted to parallel conductors 1/0 AWG and larger; however, it didn’t specifically prohibit you from paralleling smaller conductors, which was the intent and the way it was enforced in all my years of enforcement. However, that’s not what the actual language said, and ambiguous language cancause enforcement issues; therefore, in the 2011 code it was made clear that you are only allowed to parallel conductors 1/0 AWG and larger.