Bonding Jumper Sizing
Sizing for supply-side, main, and system bonding jumpers based on largest ungrounded conductor.
Important Notes
- 1.1. If multiple sets of service-entrance conductors are used, the equivalent size of the largest service-entrance conductor shall be determined by the largest sum of the areas of the corresponding conductors of each set.
- 2.2. See Chapter 9, Table 8 for conductor properties (Circular Mils).
Professional Usage Disclaimer
This data is provided for informational purposes only. Always verify with the latest edition of the National Electrical Code (NFPA 70) and consult with a licensed electrical professional or AHJ (Authority Having Jurisdiction) before performing any electrical work. Zing² is a reference tool, not a substitute for qualified engineering.
Professional Field Notes & Gotchas
- ▸The 12.5% Rule: If your ungrounded conductors exceed 1100 kcmil Copper or 1750 kcmil Aluminum, the bonding jumper must be sized at no less than 12.5% of the area of the largest phase conductor.
- ▸Supply-Side vs Load-Side: This table is for the Supply Side. Load-side bonding jumpers are sized per Table 250.122 based on the OCPD.
- ▸Material Matching: Using Copper bonding jumpers with Aluminum service conductors is common, but ensure the lugs are AL7CU or AL9CU rated and use antioxidant compound.
- ▸Circular Mils Math: When sizing for the 12.5% rule, use Chapter 9, Table 8 to find the exact CM area of your conductors.
NEC 2023 vs 2026: What changed?
NEC 2023 vs 2026: Clarifications were added to the 12.5% calculation methodology to ensure that when phases are shifted to larger kcmil sizes for voltage drop, the bonding jumpers scale appropriately to minimize fault impedance.
Technical FAQs
How to size a supply-side bonding jumper?
Refer to Table 250.102(C)(1). Match your largest ungrounded (phase) conductor to the table to find the required size for the Copper or Aluminum bonding jumper.
What is the 12.5 percent rule in NEC?
For very large services (>1100 kcmil Cu), the bonding jumper cannot be pulled from a table. You must calculate 12.5% of the phase conductor's circular mil area to determine the minimum jumper size.
Does the main bonding jumper go in the panel?
Yes. The main bonding jumper creates the critical link between the grounded (neutral) conductor and the equipment grounding system at the service disconnecting means.