Bonding Jumpers: Types, Sizing & NEC Code Rules (2026)

If you’ve ever had an inspector flag a grounding and bonding issue — or stumbled on an exam question about NEC Article 250 — there’s a good chance a bonding jumper was at the center of it. These conductors are short in length but critical in function. Get the type wrong, size it incorrectly, or place it in the wrong location, and you have a system that won’t safely clear a ground fault.

This guide covers all four NEC-defined bonding jumper types, the exact sizing rules using NEC Table 250.102(C)(1), and the real-world installation requirements that inspectors look for on every service and separately derived system.

Here’s what we cover:

• The 4 bonding jumper types and what each one connects
• Sizing rules from NEC 250.28 and Table 250.102(C)(1)
• Where the main bonding jumper is — and is not — permitted
• Common code violations and how to avoid them

Bonding Jumper Quick Reference (NEC 2023)

All four bonding jumper types are defined in NEC Article 100. Here is a summary:

Table 1: NEC Bonding Jumper Types — Definitions & Sizing Reference (NEC Article 100 & 250, 2023 Edition)

Type	What It Connects	Where Installed	Sizing Table
Main Bonding Jumper (MBJ)	Grounded conductor (neutral) to the EGC and service enclosure	Inside the service disconnect enclosure only	NEC Table 250.102(C)(1)
System Bonding Jumper (SBJ)	Grounded conductor to the EGC and enclosure at a separately derived system	At the source of a separately derived system (e.g., transformer secondary or first disconnect)	NEC Table 250.102(C)(1)
Supply-Side Bonding Jumper (SSBJ)	Metal parts and enclosures to the grounded conductor, on the supply side of an OCPD	Supply side of service OCPD or separately derived system OCPD	NEC Table 250.102(C)(1)
Equipment Bonding Jumper (EBJ)	Two or more portions of the equipment grounding conductor (EGC)	Load side — across expansion fittings, FMC, switchboard open bottoms	NEC Table 250.122

Key rule: The MBJ, SBJ, and SSBJ are sized from NEC Table 250.102(C)(1) because they carry fault current without an upstream OCPD to limit it. The EBJ is sized from NEC Table 250.122 because it is on the load side of an OCPD.

The Four Bonding Jumper Types Explained

1. Main Bonding Jumper (MBJ) — NEC 250.24(C) and 250.28

The main bonding jumper connects the equipment grounding conductor(s) and service enclosure to the grounded conductor (neutral) at the service equipment. Without it, a ground fault downstream has no low-impedance return path to the source, so the OCPD cannot operate quickly to clear the fault.

NEC 250.24(C) requires that for a grounded AC system, an unspliced main bonding jumper connect the EGC(s) and service enclosure to the grounded conductor within the service equipment enclosure.

NEC 250.28 specifies material and form requirements:

• Must be copper or other corrosion-resistant material
• Acceptable forms: wire, bus, screw, or similar suitable conductor
• If a screw type, it must be identified by a green finish (NEC 250.28(B))

Where it belongs — and where it does not: The MBJ is installed at the service equipment only. It must not be installed at any sub-panel or distribution panel on the load side of the service. Installing it there connects neutral to ground at a downstream point, which creates a parallel neutral return path. Normal load current will flow on the EGC and metal raceways — a direct violation of NEC 250.6(A) (prevention of objectionable current) and one of the most commonly cited grounding errors.

2. System Bonding Jumper (SBJ) — NEC 250.30(A)(1)

The system bonding jumper performs the same function as the MBJ, but at a separately derived system — a source of power that has no direct electrical connection to the conductors of any other system (NEC Article 100 definition). Common examples include the secondary winding of a step-down transformer and a generator with an ungrounded output winding.

NEC 250.30(A)(1) requires a system bonding jumper at the source of each separately derived system. It connects the grounded circuit conductor to the equipment grounding conductor and the enclosure.

The SBJ may be installed at one of two locations:

1. At the source — e.g., at the transformer secondary terminals. This is the most common installation.
2. At the first disconnecting means — only when the grounded conductor is carried from the source to that disconnect, per the Exception to NEC 250.30(A)(1).

You must choose one location. Installing system bonding at both the transformer and the downstream panel creates the same objectionable current problem as a double-bonded sub-panel.

3. Supply-Side Bonding Jumper (SSBJ) — NEC 250.102(C)

The supply-side bonding jumper ensures electrical continuity between metal raceways, enclosures, and the grounded conductor on the supply side of a service OCPD or separately derived system OCPD. Because these conductors are not protected by an upstream overcurrent device, the SSBJ is sized from NEC Table 250.102(C)(1) — not from Table 250.122.

A typical field example: service entrance conduit running from the utility meter to the main panel. That conduit has no upstream OCPD protecting it, so a supply-side bonding jumper is required to maintain electrical continuity across all fittings, couplings, and enclosures in that run.

4. Equipment Bonding Jumper (EBJ) — NEC 250.102(D) and (E)

The equipment bonding jumper is a load-side conductor. It connects two or more portions of the EGC where mechanical continuity might otherwise be interrupted. Common applications:

• Across conduit expansion fittings — required by NEC 250.98
• Alongside flexible metal conduit (FMC) where FMC does not qualify as an EGC for the circuit involved — see NEC 250.118(5)
• Between raceway sections in switchboards and switchgear with open bottoms

Because the EBJ is on the load side of an OCPD, it is sized from NEC Table 250.122 based on the overcurrent device rating — not the conductor size.

External EBJ length limit: Per NEC 250.102(E), when an equipment bonding jumper is installed outside the raceway (external), it must not exceed 6 feet in length and must be routed with the raceway it bonds. Keeping it to 6 ft with the raceway limits the impedance increase to an insignificant level.

How to Size a Bonding Jumper — NEC Table 250.102(C)(1)

The MBJ, SBJ, and SSBJ are all sized the same way. The process:

1. Determine the size of the largest ungrounded (phase) conductor supplying the system.
2. Look up that conductor size in NEC Table 250.102(C)(1) to find the minimum bonding jumper size.

When parallel conductors are used, calculate the total equivalent circular mil area of all phase conductors per phase, then look up that total in the table.

Sizing Example — Single Service (200A Residential)

Table 2: MBJ Sizing Example — 200A Service, Copper Conductors (NEC Table 250.102(C)(1), 2023)

Step	Value
Largest ungrounded conductor	2/0 AWG copper
Table 250.102(C)(1) lookup	2/0 AWG phase conductor
Minimum MBJ size	4 AWG copper

Sizing Example — Parallel Service Conductors

Scenario: A 2,000A service uses 5 sets of 400 kcmil copper per phase. What is the minimum MBJ size?

Table 3: MBJ Sizing — Parallel Conductors, 12.5% Rule (NEC Table 250.102(C)(1), Note 1, 2023)

Step	Calculation
Total area per phase	5 × 400 kcmil = 2,000 kcmil
12.5% rule (Note 1, Table 250.102(C)(1))	2,000 kcmil × 12.5% = 250 kcmil
Minimum MBJ size	250 kcmil copper

Important: The main bonding jumper must be a single, unspliced conductor — it cannot be run as parallel conductors. This is confirmed by NEC 250.28(A) which states the MBJ and SBJ shall be a wire, bus, screw, or similar suitable conductor (singular). This point is frequently tested on journeyman and master electrician exams.

Sizing the Equipment Bonding Jumper (EBJ)

The EBJ uses NEC Table 250.122. Find the ampere rating of the OCPD protecting the circuit, then select the minimum EGC size from that table — your EBJ must be at least that size.

Table 4: Equipment Bonding Jumper Minimum Size — Copper Conductors (NEC Table 250.122, 2023 Edition)

OCPD Rating (Amps)	Min. EBJ Size — Copper
15	14 AWG
20	12 AWG
30	10 AWG
60	10 AWG
100	8 AWG
200	6 AWG
300	4 AWG
400	3 AWG
600	1 AWG
800	1/0 AWG
1,000	2/0 AWG
1,200	3/0 AWG

Bonding Jumpers on the Job Site

Service Panel Installation

Most listed residential and commercial panels ship with a green bonding screw pre-installed. That screw is the main bonding jumper — it connects the neutral bus to the metal enclosure. Your job at installation is to confirm it is installed at the service panel and removed at every sub-panel downstream. Inspectors check this immediately when opening a panel. It is one of the most frequently cited NEC Article 250 violations in residential work.

Transformer Secondary (Separately Derived System)

When installing a 480V–120/208V step-down transformer for a tenant space or building, you must install a system bonding jumper — either at the transformer’s secondary terminals (X0 terminal to enclosure and EGC) or at the first disconnecting means. Do not bond at both locations. Also install a grounding electrode conductor (GEC) from that same bonding point to a grounding electrode, per NEC 250.30(A)(4).

Flexible Metal Conduit (FMC) Connections

Per NEC 250.118(5), FMC qualifies as an EGC — meaning no separate equipment bonding jumper is required — only when all of the following conditions are met:

• The circuit OCPD rating is 20 amperes or less
• The FMC length is 6 feet or less
• The FMC is not installed in a hazardous (classified) location
• The FMC is not subject to physical damage

If any condition is not met, an EBJ is required inside or alongside the FMC. For liquidtight flexible metal conduit (LFMC), NEC 250.118(6) allows LFMC to serve as an EGC without a separate EBJ for circuits up to 60A at 480V or less and lengths of 6 feet or less. Longer runs or larger circuits require an EBJ.

Water Meter and Gas Piping Bonding

Per NEC 250.68(B), the grounding electrode conductor or bonding jumper connection to a metal water pipe must be made on the street side of the water meter whenever possible. If not accessible on the street side, a bonding jumper must bridge the meter to ensure continuity — because some meter designs use non-conductive couplings that interrupt the metal path. Interior metal gas piping must also be bonded per NEC 250.104(B).

Common Code Violations Involving Bonding Jumpers

MBJ Installed at a Sub-Panel — NEC 250.6(A) Violation

Installing a main bonding jumper (neutral-to-ground connection) inside a sub-panel on the load side of the service creates a parallel return path for normal neutral current. That current will flow on the EGC, through metal raceways, structural steel, and metal plumbing. This is a violation of NEC 250.6(A) (objectionable current over grounding conductors), creates a shock hazard on metal surfaces, and may introduce electrical noise into sensitive equipment circuits.

Undersized Bonding Jumper — Fault Clearing Failure

An undersized bonding jumper increases the impedance of the fault current return path. Higher impedance reduces available fault current, which slows OCPD operation. In extreme cases the breaker may not operate quickly enough to prevent the metal enclosure from remaining energized at a dangerous voltage long enough to cause electrocution. NEC 250.4(A)(5) requires the ground fault current path to have sufficiently low impedance to enable prompt operation of the overcurrent protective device.

Missing EBJ on FMC — Compromised EGC Path

A motor connected with FMC exceeding 6 feet, with no equipment bonding jumper and without the circuit meeting the conditions of NEC 250.118(5), has no reliable EGC path. If the motor frame develops a fault, fault current cannot return to the source at sufficient magnitude to operate the OCPD — the frame remains energized until someone or something provides an alternate path.

Double-Bonding a Separately Derived System

Installing the system bonding jumper at both the transformer secondary terminals and the downstream disconnect panel creates objectionable current flow on the EGC of the feeder between those two points — the same failure mode as a double-bonded sub-panel on a service system. The SBJ must be installed at one location only, as stated in NEC 250.30(A)(1).

Conclusion

Bonding jumpers are what make the grounding system functional when a fault occurs. Three rules to commit to memory:

1. MBJ at the service. SBJ at the separately derived system source. Never at a sub-panel. Installing either at the wrong location creates objectionable current on the EGC — a code violation and a safety hazard.
2. Size the MBJ, SBJ, and SSBJ from NEC Table 250.102(C)(1). For parallel conductors over 1,100 kcmil total, use the 12.5% rule in Note 1 of the table. The MBJ must be a single, unspliced conductor — it cannot be paralleled.
3. Size the EBJ from NEC Table 250.122. It is a load-side conductor sized like an EGC — based on OCPD rating, not on the phase conductor size.

Frequently Asked Questions

What is the difference between a main bonding jumper and a system bonding jumper?

The main bonding jumper (MBJ) is installed at the service equipment and connects the neutral (grounded conductor) to the equipment grounding conductor and service enclosure. The system bonding jumper (SBJ) performs the same function but at a separately derived system — such as a transformer secondary or generator output — rather than at a utility service. Both are sized from NEC Table 250.102(C)(1).

Can the main bonding jumper be run as multiple conductors in parallel?

No. NEC 250.28 specifies that the main bonding jumper and system bonding jumper shall be a wire, bus, screw, or similar suitable conductor — a single conductor. The conductor may need to be quite large (e.g., 250 kcmil or larger on a 2,000A service), but it must still be installed as a single unspliced run.

How long can an equipment bonding jumper be on the outside of a conduit?

Per NEC 250.102(E), an external equipment bonding jumper must not exceed 6 feet in length and must be routed with the raceway it bonds. This keeps the impedance increase insignificant while allowing flexible installation in tight spaces or where internal routing is not practical.

Do I need a bonding jumper across a water meter?

Yes, if the water meter interrupts the electrical continuity of the metal water pipe being used as a grounding electrode or bonding path. NEC 250.68(B) requires the connection to be made on the street side of the meter where possible, or a bonding jumper must bridge the meter to restore continuity.

What NEC table do I use to size the main bonding jumper?

NEC Table 250.102(C)(1). Locate the size of the largest ungrounded (phase) conductor supplying the service or separately derived system. For parallel conductors, compute the total equivalent circular mil area per phase. When that total exceeds 1,100 kcmil, Note 1 of the table directs you to apply the 12.5% rule to determine the minimum bonding jumper size.