Article 250 is the largest article in the NEC® and contains the following three tables:
The left side of Table 250.102(C)(1) corresponds to the largest size ungrounded conductor installed, and the right side provides the required size of the grounded conductor, main bonding jumper, system bonding jumper, or supply-side bonding jumper.
Note 1 under Table 250.102(C)(1) contains the 12.5% rule to be used when the largest ungrounded conductor or equivalent area for parallel conductors is over 1100 kcmil copper or 1750 kcmil aluminum.
See image for an example of the table.
What minimum size copper main bonding jumper is required at an electrical service supplied by 400 kcmil ungrounded copper service entrance conductors?
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