This is the only referance i find, others may have more information, what did the project engineer have to say about it.
"The impedance of a circuit in an aluminum raceway or aluminum-sheathed cable will differ from the impedance of the same circuit in a steel raceway or steel-sheathed cable; therefore, separate raceways and cables must have the same physical characteristics. See 300.20 regarding induced currents in metal enclosures or raceways."
