7.3 Main Bonding Jumpers, System Bonding Jumpers & Supply-Side Bonding
Key Takeaways
- The Main Bonding Jumper (MBJ, NEC 250.28) connects the grounded neutral conductor to the equipment enclosure at the service equipment, providing the vital link required to return ground-fault current to the transformer source.
- System Bonding Jumpers (SBJ, NEC 250.30(A)(1)) establish the single neutral-to-ground bond for separately derived systems (transformers/generators) and must be installed at either the source or the first disconnect, but never at both.
- Supply-Side Bonding Jumpers (SSBJ, NEC 250.102(C)) bond metal service raceways and enclosures on the supply side of service overcurrent devices where high available fault currents exist.
- Bonding jumpers are sized from NEC Table 250.102(C)(1) for phase conductors up to 1100 kcmil Copper / 1750 kcmil Aluminum.
- The 12.5% Rule (NEC Table 250.102(C)(1) Note 1 & 250.28(D)(1)) requires that where ungrounded supply conductors exceed 1100 kcmil Cu or 1750 kcmil Al, the bonding jumper cross-sectional area must be at least 12.5% of the total circular mil area of the largest ungrounded phase.
7.3 Main Bonding Jumpers, System Bonding Jumpers & Supply-Side Bonding
In alternating-current electrical installations, the connection that links the grounded system conductor (neutral) to the metal equipment enclosures at the electrical service or separately derived system is the single most critical safety connection in the facility. This connection is made by the Main Bonding Jumper (MBJ), System Bonding Jumper (SBJ), or Supply-Side Bonding Jumper (SSBJ).
Without this vital bridge, a line-to-ground fault on any branch circuit or feeder would leave the metallic conduit and panel enclosure permanently energized at line voltage with zero current flowing back to the transformer to trip the circuit breaker.
+-----------------------------------------------------------------------------+
| BONDING JUMPER CLASSIFICATION & FUNCTIONAL MATRIX |
| |
| [MAIN BONDING JUMPER (MBJ)] [SYSTEM BONDING JUMPER (SBJ)] |
| * NEC 250.24(B) & 250.28 * NEC 250.30(A)(1) |
| * Location: Service Disconnecting Means * Location: Separately Derived |
| * Connects: Grounded Neutral to Service Systems (Transformers/Gens) |
| Enclosure & Ground Bus * Connects: X0 Terminal to Case |
| |
| [SUPPLY-SIDE BONDING JUMPER (SSBJ)] [EQUIPMENT BONDING JUMPER] |
| * NEC 250.102(C) * NEC 250.102(D) & Table 250.122|
| * Location: Supply side of Service OCPD * Location: Load side of OCPD |
| * Bonds: Service raceways, meter bases, * Sized based on rating of |
| CT cabinets, auxiliary gutters circuit breaker / fuse |
+-----------------------------------------------------------------------------+
1. Main Bonding Jumper (MBJ) Requirements (NEC 250.28)
Under NEC 250.24(B) and 250.28, an unspliced main bonding jumper must be installed in each service disconnecting means to connect the equipment grounding conductor(s) and the service-disconnect enclosure to the grounded conductor.
Construction and Material Options (NEC 250.28(A))
The Main Bonding Jumper may consist of:
- A wire conductor (copper or aluminum).
- A busbar (copper or aluminum).
- A screw (such as the standard factory-supplied green bonding screw).
- Other similar suitable conductor.
The Green Main Bonding Screw Rule (NEC 250.28(B))
Where a main bonding jumper is a screw, the screw must be identified with a green finish that is visible with the screw installed. This allows electrical inspectors in Idaho to visually verify the presence of the MBJ without disassembling the panelboard.
+-----------------------------------------------------------------------------+
| SERVICE PANEL MAIN BONDING JUMPER SCHEMATIC |
| |
| +-------------------------------------+ |
| | MAIN SERVICE DISCONNECT PANEL | |
| | | |
| LINE 1 (Hot) ===>| [MAIN BREAKER] | |
| LINE 2 (Hot) ===>| [MAIN BREAKER] | |
| | | |
| NEUTRAL ===>| [NEUTRAL BUS] | |
| (Grounded) | | | |
| | | <--- MAIN BONDING JUMPER | |
| | | (Green Screw, Bus, | |
| | v or Wire Jumper) | |
| | [GROUND BUS / METAL ENCLOSURE] | |
| | | | | |
| +-------+---------------+-------------+ |
| | | |
| v v |
| GEC TO EGC TO |
| EARTH BRANCH CIRCUITS |
+-----------------------------------------------------------------------------+
2. System Bonding Jumper (SBJ) for Separately Derived Systems (NEC 250.30(A)(1))
A Separately Derived System (SDS) is a premises wiring system whose power is derived from a generator, transformer, or converter windings and has no direct electrical connection (including a solidly connected neutral) to supply conductors originating in another system.
Single Location Mandate
Under NEC 250.30(A)(1), an unspliced System Bonding Jumper must be installed to connect the grounded conductor (the X0 neutral terminal of a 480V-to-208Y/120V transformer) to the equipment grounding conductor and transformer enclosure.
- Permitted Location: The SBJ must be installed at a single point on the system, which can be either:
- Inside the source enclosure (inside the transformer housing), OR
- Inside the first disconnecting means enclosure (the SDS main distribution panel).
- Prohibition: Installing an SBJ at both the transformer AND the first disconnect is a severe code violation, as it creates a parallel neutral return path producing objectionable circulating current under NEC 250.6.
3. Supply-Side Bonding Jumpers (SSBJ) (NEC 250.102(C))
Metal raceways, metal enclosures, wireways, and meter socket enclosures located on the supply side of the service disconnecting means are subject to extraordinary fault energy. Because there is no service circuit breaker upstream to clear a ground fault, a fault on a service raceway is limited only by utility primary fuses or substation breakers.
Under NEC 250.102(C), Supply-Side Bonding Jumpers must be installed around raceway concentric/eccentric knockouts, meter bases, and current transformer (CT) cabinets to ensure an ultra-low-impedance, heavy-gauge fault path.
+-----------------------------------------------------------------------------+
| SUPPLY-SIDE BONDING JUMPER (SSBJ) INSTALLATION |
| |
| UTILITY TRANSFORMER |
| | |
| v (Service Raceway - Rigid Metal Conduit) |
| +---------------+ |
| | METER BASE | <--- SSBJ bonds meter enclosure to grounded neutral |
| +---------------+ |
| | |
| v (Service Raceway - EMT with Bonding Bushing) |
| +---------------+ |
| | SERVICE PANEL | <--- Bonding bushing linked to Grounded Bus via SSBJ |
| | [MAIN OCPD] | |
| +---------------+ |
+-----------------------------------------------------------------------------+
4. Sizing Bonding Jumpers: NEC Table 250.102(C)(1)
Main Bonding Jumpers, System Bonding Jumpers, and Supply-Side Bonding Jumpers are all sized in accordance with NEC Table 250.102(C)(1).
+-----------------------------------------------------------------------------+
| NEC TABLE 250.102(C)(1) OVERVIEW |
| |
| LARGEST UNGROUNDED CONDUCTOR OR | MINIMUM SIZE OF BONDING JUMPER |
| EQUIVALENT AREA FOR PARALLEL |------------------------------------- |
| COPPER ALUMINUM | COPPER (Cu) | ALUMINUM (Al) |
| --------------------+-------------+-------------------+----------------- |
| 2 AWG or smaller | 1/0 or smaller| 8 AWG | 6 AWG |
| 1 AWG or 1/0 AWG | 2/0 or 3/0 | 6 AWG | 4 AWG |
| 2/0 AWG or 3/0 AWG | 4/0 or 250 | 4 AWG | 2 AWG |
| Over 3/0 - 350 kcmil| Over 250-500| 2 AWG | 1/0 AWG |
| Over 350 - 600 kcmil| Over 500-900| 1/0 AWG | 3/0 AWG |
| Over 600 - 1100 kcmil|Over 900-1750| 2/0 AWG | 4/0 AWG |
| |
| >>> OVER 1,100 kcmil Cu or 1,750 kcmil Al -> APPLY 12.5% RULE (NOTE 1) |
+-----------------------------------------------------------------------------+
5. The 12.5% Rule for Large Conductors (Table 250.102(C)(1) Note 1)
When the total cross-sectional circular mil area of the largest ungrounded phase conductor (or the sum of parallel conductors per phase) exceeds 1,100 kcmil Copper or 1,750 kcmil Aluminum, Table 250.102(C)(1) does not provide a direct wire gauge.
Instead, Note 1 to Table 250.102(C)(1) and NEC 250.28(D)(1) mandate the 12.5% Rule:
Minimum Bonding Jumper Area (cmil) = Total Phase Circular Mils x 0.125
Conductor Properties Reference Table (NEC Chapter 9, Table 8)
To select the standard wire size corresponding to the calculated circular mil requirement, use NEC Chapter 9, Table 8:
| Conductor Size | Circular Mil Area (cmils) | Conductor Size | Circular Mil Area (cmils) |
|---|---|---|---|
| 1/0 AWG | 105,600 cmils | 350 kcmil | 350,000 cmils |
| 2/0 AWG | 133,100 cmils | 400 kcmil | 400,000 cmils |
| 3/0 AWG | 167,800 cmils | 500 kcmil | 500,000 cmils |
| 4/0 AWG | 211,600 cmils | 600 kcmil | 600,000 cmils |
| 250 kcmil | 250,000 cmils | 750 kcmil | 750,000 cmils |
| 300 kcmil | 300,000 cmils | 1000 kcmil | 1,000,000 cmils |
6. Sizing Bonding Jumpers in Parallel Raceways (NEC 250.102(C)(2))
When supply-side conductors are run in multiple parallel raceways (e.g., three separate conduits feeding a switchboard from a utility transformer):
+-----------------------------------------------------------------------------+
| PARALLEL RACEWAY SUPPLY-SIDE BONDING METHODS (250.102(C)(2)) |
| |
| METHOD A: INDIVIDUAL SSBJ IN EACH RACEWAY |
| - Sized based on largest ungrounded phase conductor in THAT RACEWAY |
| - Minimum size requirement: NEVER smaller than 1/0 AWG Copper |
| (or 3/0 AWG Aluminum) |
| |
| METHOD B: SINGLE COMMON SSBJ ROUTED OUTSIDE RACEWAYS |
| - Sized based on total combined circular mil area of all phases |
| using Table 250.102(C)(1) or the 12.5% Rule |
+-----------------------------------------------------------------------------+
7. Step-by-Step Worked Mathematical Calculations
Worked Example 1: 1,200-Ampere Service with Parallel 500 kcmil Conductors
Scenario: A 1,200A, 480Y/277V service is supplied by three (3) parallel conduits. Each conduit contains one 500 kcmil Copper conductor per phase. Calculate the minimum size required for a single common Main Bonding Jumper (MBJ) under NEC Table 250.102(C)(1).
Step 1: Calculate Total Circular Mil Area for Phase A
- Phase A consists of three parallel 500 kcmil conductors.
- Total Circular Mils = 3 x 500,000 cmils = 1,500,000 cmils (1,500 kcmil).
Step 2: Check Against Table 250.102(C)(1) Limits
- 1,500 kcmil exceeds 1,100 kcmil Copper.
- Therefore, apply Note 1 (The 12.5% Rule):
Step 3: Calculate Required Minimum Circular Mils
- Required cmils = Total Phase cmils x 0.125
- Required cmils = 1,500,000 cmils x 0.125 = 187,500 cmils.
Step 4: Select Standard Conductor from NEC Chapter 9, Table 8
- 3/0 AWG = 167,800 cmils (Too small!)
- 4/0 AWG = 211,600 cmils (Exceeds 187,500 cmils; compliant).
- CONCLUSION: The minimum common Main Bonding Jumper is 4/0 AWG Copper.
Worked Example 2: 1,600-Ampere Service with Parallel 600 kcmil Conductors
Scenario: A 1,600A service is supplied by four (4) parallel conduits, each containing a 600 kcmil Copper conductor per phase. Calculate:
- The minimum size for a single common Main Bonding Jumper (MBJ).
- The minimum size for individual Supply-Side Bonding Jumpers (SSBJ) installed in each raceway.
Part 1: Sizing the Common Main Bonding Jumper (MBJ)
- Total Circular Mils per phase = 4 x 600,000 cmils = 2,400,000 cmils (2,400 kcmil).
- Apply 12.5% Rule: 2,400,000 cmils x 0.125 = 300,000 cmils (300 kcmil).
- Standard size matching 300,000 cmils is 300 kcmil Copper.
- CONCLUSION: Common MBJ = 300 kcmil Copper.
Part 2: Sizing Individual Supply-Side Bonding Jumpers (SSBJ) per Raceway
- Each raceway contains one 600 kcmil Copper conductor per phase.
- Look up 600 kcmil Copper in Table 250.102(C)(1):
* Row: "Over 350 kcmil thru 600 kcmil" -> Requires 1/0 AWG Copper.
- Verify minimum parallel size under NEC 250.102(C)(2):
* Minimum size is 1/0 AWG Copper.
- CONCLUSION: Individual SSBJ in each raceway = 1/0 AWG Copper.
Worked Example 3: Sizing System Bonding Jumper for a 75 kVA Transformer
Scenario: A 480V-to-208Y/120V, 3-phase, 75 kVA dry-type transformer has secondary full-load current of 208A. The secondary conductors are 250 kcmil Copper THHN. Determine the minimum size System Bonding Jumper (SBJ) to be installed in the transformer enclosure.
Step 1: Identify Largest Secondary Conductor
- Largest ungrounded secondary conductor = 250 kcmil Copper.
Step 2: Look up Conductor in Table 250.102(C)(1)
- Range: "Over 3/0 thru 350 kcmil Copper"
- Column: Minimum Copper Bonding Jumper Size = #2 AWG Copper.
Step 3: Conclusion
- The System Bonding Jumper connecting terminal X0 to the transformer frame must be at least #2 AWG Copper.
A 2,000-ampere service is supplied by five parallel sets of 500 kcmil copper conductors per phase (total 2,500 kcmil Cu). Under NEC Table 250.102(C)(1) and the 12.5% Rule, what is the minimum standard size copper Main Bonding Jumper required?
Where a Main Bonding Jumper is a screw under NEC 250.28(B), how must it be identified?
Under NEC 250.30(A)(1), where must the System Bonding Jumper (SBJ) for a separately derived transformer be installed?