Jerramundi
Senior Member
- Location
- Chicago
- Occupation
- Licensed Residential Electrician
I'm sizing a Common GEC for an installation that consists of a single set of overhead service conductors, which drops down to a triple meter bank, and then splits into 3 sets of 100A service entrance conductors that terminate in 3 separate 100A main breaker panels.
In sizing the Common GEC, instinct tells me to take the size of the largest ungrounded conductor in the overhead service conductors and refer to 250.66. Done. Or am I?
Upon searching the code book to verify my instinct, I come upon 250.64(D)(1), which states "...the common grounding electrode conductor shall be sized in accordance with 250.66, based on the sum of the circular mil area of the largest ungrounded conductor(s) of each set of conductors that supplies the disconnecting means."
This tells me to go to Chapter 9 - Table 8 - Conductor Properties and find the Circular Mil Area of a #1 AWG (83690 c-mils) because the service entrance conductors are 100A aluminum, multiple it by 3 (one for each set of conductors)(251,070 c-mils), find the corresponding wire size ( ??? Table 8 ends at 211,000 c-mils), and size my common GEC according to that wire size in 250.66, but I keep ending up with some ridiculous number...
What am I doing wrong?
In sizing the Common GEC, instinct tells me to take the size of the largest ungrounded conductor in the overhead service conductors and refer to 250.66. Done. Or am I?
Upon searching the code book to verify my instinct, I come upon 250.64(D)(1), which states "...the common grounding electrode conductor shall be sized in accordance with 250.66, based on the sum of the circular mil area of the largest ungrounded conductor(s) of each set of conductors that supplies the disconnecting means."
This tells me to go to Chapter 9 - Table 8 - Conductor Properties and find the Circular Mil Area of a #1 AWG (83690 c-mils) because the service entrance conductors are 100A aluminum, multiple it by 3 (one for each set of conductors)(251,070 c-mils), find the corresponding wire size ( ??? Table 8 ends at 211,000 c-mils), and size my common GEC according to that wire size in 250.66, but I keep ending up with some ridiculous number...
What am I doing wrong?
