Interpretation of 250.30 B 1


Having a discussion at work about sizing of a GEC for an SDS (150KVA xfmr w/ sec. parallel 250kcmil alu feeding a 400A panel in existing WH with a GES). Anyone on here can confirm my understanding of the wording of 250.30(B)1 for sizing a GEC conductor which states "sized in accordance 250.66 of the largest ungrounded conductors or SET of derived ungrounded conductors " which means adding up one of the "phase sets (2-250kcmil in this case)" = 500kcmil hence the GEC size tying to bldg steel will be #2 copper. Thank you


Staff member
Yes, for parallel conductors you add up the sizes to get to the size of a single conductor and then use T250.66.