Bonding Jumper Sizing Calculator

Size Main Bonding Jumpers (MBJ), System Bonding Jumpers (SBJ), and Supply-Side Jumpers (SSBJ) per NEC §250.102 & Table 250.102(C)(1).

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⚡ Test your knowledge
NEC Article 220 was renumbered in the 2026 code cycle. What is the new article number?
  • A. Art. 100
  • B. Art. 120
  • C. Art. 200
  • D. Art. 230
💡 NEC 2026 renumbered Article 220 (Branch-Circuit, Feeder, and Service Calculations) to Article 120.
Energy Management Systems were moved from Article 750 to which article in NEC 2026?
  • A. Art. 100
  • B. Art. 125
  • C. Art. 130
  • D. Art. 150
💡 NEC 2026 moved Energy Management Systems from Article 750 to Article 130.
Which NEC standard size overcurrent device comes after 90A?
  • A. 95A
  • B. 100A
  • C. 110A
  • D. 105A
💡 Per NEC §240.6(A), the standard sizes are …90, 100, 110, 125… There is no 95A standard size.

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How it works & the NEC rules behind it

Bonding jumpers provide the low-impedance electrical path necessary to conduct fault currents back to the source and trip overcurrent protective devices. Under NEC §250.102, bonding jumpers installed on the supply side of service overcurrent devices are sized using Table 250.102(C)(1) based on the size of the largest ungrounded phase conductor.

Types of Bonding Jumpers (NEC Article 250):

  • 1. Main Bonding Jumper (MBJ, §250.28): Connects the grounded circuit conductor (neutral) to the equipment grounding conductor / enclosure at each service disconnect.
  • 2. System Bonding Jumper (SBJ, §250.30(A)(1)): Connects the grounded conductor of a separately derived system (transformer or generator) to the equipment ground.
  • 3. Supply-Side Bonding Jumper (SSBJ, §250.102(C)): Installed on the supply side of service disconnects (such as between CT cabinets, meter sockets, and service panels).
  • 4. The 12.5% Area Rule (§250.102(C)(1) Note 1): Where the largest ungrounded phase conductor (or sum of parallel conductors) exceeds 1,100 kcmil Copper or 1,750 kcmil Aluminum, the bonding jumper must have an area not less than 12.5% of the total phase conductor cross-sectional area.

Formulas & equations

Standard Size = Lookup from NEC Table 250.102(C)(1) [Up to 1100 kcmil Cu] Conductors > 1100 kcmil Cu Area = Total Phase Circular Mils × 0.125 (12.5% Rule) Parallel SSBJ in Each Conduit = Sized based on phase conductors in that conduit

NEC reference table

Largest Phase Conductor (Copper) Largest Phase Conductor (Aluminum) Min Copper Bonding Jumper Min Aluminum Bonding Jumper
#2 AWG or smaller1/0 AWG or smaller#8 AWG Cu#6 AWG Al
#1 or 1/0 AWG2/0 or 3/0 AWG#6 AWG Cu#4 AWG Al
2/0 or 3/0 AWG4/0 AWG or 250 kcmil#4 AWG Cu#2 AWG Al
Over 3/0 through 350 kcmilOver 250 through 500 kcmil#2 AWG Cu#1/0 AWG Al
Over 350 through 600 kcmilOver 500 through 900 kcmil#1/0 AWG Cu#3/0 AWG Al
Over 600 through 1100 kcmilOver 900 through 1750 kcmil#2/0 AWG Cu#4/0 AWG Al
Over 1100 kcmilOver 1750 kcmil12.5% of Phase Area12.5% of Phase Area

*Source: NEC Table 250.102(C)(1) (Grounded Conductor, Main Bonding Jumper, Supply-Side Bonding Jumper).

Worked example, step by step

Example: Sizing Supply-Side Bonding Jumper for 800A Service (2 × 500 kcmil Cu)

Service: 800A Service with two parallel sets of 500 kcmil Copper conductors per phase.

  • 1. Calculate Total Phase Area: 2 × 500,000 CM = 1,000,000 CM (1,000 kcmil).
  • 2. Check 1100 kcmil Threshold: 1,000 kcmil is ≤ 1,100 kcmil → Use Table 250.102(C)(1) directly.
  • 3. Table Lookup: "Over 600 through 1100 kcmil" requires a #2/0 AWG Copper single bonding jumper.
  • 4. If Installed in Parallel Conduits: Based on 500 kcmil in each raceway ("Over 350 through 600 kcmil"), install a #1/0 AWG Copper SSBJ in each individual conduit per §250.102(C)(2).

Frequently asked questions

Table 250.66 sizes Grounding Electrode Conductors (to earth); Table 250.122 sizes Equipment Grounding Conductors on load circuits based on the OCPD rating; Table 250.102(C)(1) sizes Main, System, and Supply-Side Bonding Jumpers on the supply side based on phase conductor size.

For a 2000A service with 6 × 500 kcmil Cu per phase (3,000,000 CM total), 12.5% requires: 3,000,000 × 0.125 = 375,000 CM. You must install at least a standard 400 kcmil Copper bonding jumper or parallel bonding jumpers totaling at least 375 kcmil.

No. This calculator sizes Main, System, and Supply-Side Bonding Jumpers (NEC §250.102) that bond the electrical grounded conductor to equipment. An intersystem bonding bridge (§250.94) is a separate accessible terminal device that bonds other systems — cable TV, telephone, satellite — to the building grounding electrode system; it is not sized from Table 250.102(C)(1).

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