What is behind Art. 310.4

[mjdugan]

I would like to know the technical basis that Code-Making Panel 6 used to determine that ?Aluminum, copper-clad aluminum, or copper conductors of size 1/0 AWG and larger?, ?shall be permitted to be connected in parallel? as stated in Article 310.4 of the NEC. Was the restriction of using only conductors size 1/0 AWG and larger based on concerns regarding a minimum conductor current carrying capability or concerns about the physical conductor size?

While the diameters (excluding insulation) of copper and aluminum conductors of the same gauge are equivalent, the current carrying capabilities of a 1/0 AWG copper and 1/0 AWG aluminum are not equivalent. If the 1/0 requirement is based on concerns regarding a minimum conductor current carrying capability, it would seem logical that Article 310.4 should require aluminum and copper-clad aluminum conductors to be of size 3/0 AWG and larger.

The stated purpose of the Code is: ?the practical safeguarding of persons and property from hazards arising from the use of electricity.? Assuming that paralleled conductors are installed in accordance with all other applicable sections of the Code, is there a technical basis that would explain why paralleling conductors of a size smaller than 1/0 AWG presents more of a ?hazard? than paralleling conductors of size 1/0 AWG or larger?

Thanks for your help.
Mike

 

[iwire]

The short answer is the smaller conductors are more susceptible to overload from differences in length or of differences in impedance from terminals and such.

As far as why the same for CU and AL.......lets not overly complicate the code....much easier to just say 1/0 for all.

Did you read the exceptions that do allow smaller conductors in parallel?