The Code tends to allow for diversity as the connected load increases. Feeders and Service conductors have more items connected than branch circuits. The Exception to section 210.19 (A) (3) does recognize that in the case of household electric ranges the neutral conductor seldom handles more than 25 A (refer to the explanatory print in the handbook)
The dwelling type Electric dryer on the other hand is typically a smaller branch circuit (30A) and the neutral is always current carrying as long as the drum turns (motor). There is significantly less capacity in the circuit to allow derating the grounded circuit conductor. Also;
210.19 Conductors — Minimum Ampacity and Size.
(A) Branch Circuits Not More Than 600 Volts.
(1) General. Branch-circuit conductors shall have an ampacity not less than the maximum load to be served. NEC Ó
Granted the dryer is not a general branch circuit but I believe the general intent of the Code is to not allow derating on branch circuits. Hope this helps.
Charlie
