Mike Dolt Enterprises
Mike Dolt Enterprises
User: beanland, Location: Vancouver, WA
Asked on: Wednesday, April 27, 2011 at 12:41 pm EDT
Answered on: Tuesday, May 14, 2024 at 4:41 am EDT
Question: 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?
Determining the Branch Circuit Size
According to 250.122(B), the "size" of the branch circuit is based on the largest conductor size used in the circuit. In this case, the largest conductor size is #1 aluminum, which has a current-carrying capacity of 115A. Therefore, the branch circuit size is
115A.
Required EGC Size
250.122(B) requires the EGC to be sized based on the conductor size. Since the branch circuit size is 115A, the EGC must be sized for a 115A conductor. Table 250.122 indicates that a
#6 copper EGC is required for a 115A conductor.
Compliance with 250.122(A)
250.122(A) states that the EGC shall not be smaller than the phase conductor. Since the phase conductor is #1 aluminum (115A), the EGC must be equal to or larger than #1 aluminum. Therefore, the #6 copper EGC meets this requirement.
Allowance for Engineering Analysis:
Engineering analysis can be used as a substitute for 250.122(B) sizing EGC under certain conditions. However, the analysis must demonstrate that the EGC will provide adequate protection against electrical faults. This requires an assessment of the protective device clearing time, fault current, and wire damage curve for the specific circuit.In this case, engineering analysis might be considered to determine if a smaller EGC (e.g., #12 copper) would be sufficient for the circuit. However, it is important to note that the analysis must be documented and approved by the authority having jurisdiction. Flag for review