How big is a branch circuit for 250.122(B)

[beanland]

250.122(B) requires that I size the EGC for a circuit based on the conductor size. Since a branch circuit might have conductors of different types and sizes, what is the "size" of this branch circuit: 20A OCPD feeding 50 feet of #12 copper; #12 copper spliced to 150 ft of #1 aluminum; #1 aluminum spliced to 50 feet of #12 copper.

What size EGC is required? 20A requires #12 copper per Table 250.122. But, #1 aluminum is rated 115A so is #6 copper required per Table 250.122? But, #1 is 83,690 cir mils to #12 copper 6530 cir mils. Using 250.122(B) I need to increase the size of the EGC to 83,690 cir mils. But is that copper or aluminum? But, 250.122(A) says that the ECG never has to be bigger than the phase conductor. Does that mean the ECG can be #12 copper or #12 aluminum?

Is engineering analysis (protective device clearing time, fault current, wire damage curve) allowed as a subsitute for 250.122(B) sizing EGC?

 

[charlie b]

I think you treat each section of the circuit separately. And I don't think you can use an engineering evaluation to show that the larger size EGC would not be required for the tripping characteristics of the breaker and the total impedance of the run.