You're right to question the need for external bonding jumpers on flexible metal conduit (FMC) and liquidtight flexible metal conduit (LFMC) when there's already an equipment grounding conductor (EGC) inside the conduit. Here's a breakdown of why the external bonding jumper is sometimes required, and the relevant code sections

Why External Bonding Jumpers are Required (Sometimes):

The primary reason for external bonding jumpers is to ensure a low-impedance fault path for ground fault currents and provide adequate grounding of the conduit itself. This is critical for quickly tripping overcurrent protection devices (like circuit breakers) during a fault. While the EGC within the conduit certainly plays a role, the metal conduit itself is intended to be part of the effective ground-fault current path, and under certain conditions, its ability to carry that current safely is questionable without the bonding jumper.Here's a more detailed breakdown: Continuity Concerns

FMC and LFMC are not inherently continuously conductive like rigid metal conduit (RMC) or intermediate metal conduit (IMC). The flexible nature of these conduits, especially the spiral construction of FMC, creates a high-impedance pathway for fault currents, especially across fittings.

Impedance and Length

Even with a good connection at each end, the impedance of FMC and LFMC increases significantly with length. A long run of FMC or LFMC, even with an internal EGC, may not provide a low-enough impedance path for the rapid clearing of ground faults, especially on higher voltage systems or those with very sensitive ground fault protection.

Mechanical Integrity

Flexibility means the conduit can be moved, bent, or accidentally pulled. Over time, connections can loosen, or the conduit can be damaged, further increasing impedance. The external bonding jumper provides a redundant, robust connection that is less susceptible to these issues.

Purpose of internal EGC The internal equipment ground is primarly there to provide a ground to the devices that are being powered. Yes, it also helps to clear ground faults but as stated before the metal conduit itself is intended to be part of the ground fault current path.

When is a Bonding Jumper Required (Code References)?

The requirements for bonding jumpers on FMC and LFMC are found in the National Electrical Code (NEC) Article 250, particularly section 250.102 and related sections concerning equipment grounding conductors, and 348.60 (FMC) and 350.60 (LFMC).Here's a summary of the key rules and when a bonding jumper (also called an "equipment bonding jumper" or EBJ) is generally required: General Rule (250.4(A)(5))

Metal parts that are likely to become energized shall be connected together to create a low impedance ground-fault current path.

FMC (348.60) and LFMC (350.60)

FMC and LFMC shall be connected to boxes, fittings, and other equipment, with fittings listed for grounding.

The Key Exemption (348.60(A) & 350.60(A)) The big exception to requiring an external bonding jumper is when the FMC or LFMC is installed in a manner that meets all of the following conditions simultaneously:1. Listed Fittings

You must use fittings that are specifically listed for grounding or bonding. Using regular set-screw connectors isn't enough. These fittings are designed to maintain a good electrical connection, even with vibration or movement.2.

Overcurrent Protection Limitations

The circuit must be protected by an overcurrent device (circuit breaker or fuse) rated at

20 amps or less for FMC, 20 amps or less for 3/8" trade size LFMC and 60 amps or less for LFMC sizes 1/2" through 4". This is based on the fault current carrying capacity of the conduit.3. Length Limitations

The total combined length of FMC or LFMC in any ground return path cannot exceed

6 feet.In plain English, this means that if you use listed grounding fittings, and the circuit is protected by a relatively small breaker (20A or 60A max, depending on conduit size), and the total run of FMC/LFMC is short (6 feet or less), then you usually don't need an external bonding jumper.When you MUST use a Bonding Jumper



Circuits Exceeding Amperage Limits

If the circuit breaker is rated above 20A for FMC, 20A for 3/8" LFMC, or 60A for 1/2" to 4" LFMC, you must use an external bonding jumper, regardless of fitting type or length.

Length Exceeding 6 Feet

If the total combined length of FMC/LFMC in the ground fault path exceeds 6 feet, you must use an external bonding jumper.

Non-Listed Fittings

If you're using fittings that aren't specifically listed for grounding, you must use an external bonding jumper. This is a common mistake.

Questionable Connections

Even if technically meeting the code requirements, if there's any doubt about the integrity of the connections (corrosion, damage, etc.), it's best practice to install a bonding jumper for added safety.

Examples Where You Would Need a Bonding Jumper


Connecting a Large Motor

Imagine a 480V, 3-phase motor with a 50A circuit breaker connected to a junction box with a run of 1" LFMC that's 8 feet long. Even if you use listed fittings, because the breaker is over 60A and the length is over 6 feet, you must have an external bonding jumper.2.

Long Run to a Panel

You're running several circuits to a subpanel. One circuit uses 3/4" FMC, protected by a 30A breaker, and the run is 10 feet. Even though the breaker is under 60A, the length exceeds the limit, so you need a bonding jumper.3.

Uncertain Fittings

You find an existing installation of 1/2" LFMC connecting a piece of equipment to a junction box. The breaker is 20A, the run is 5 feet, but you can't verify if the fittings are listed for grounding. You should install a bonding jumper for safety.

Why You See It Everywhere (Even When Possibly Not Required)



Over-Engineering/Conservative Practices

Many plants err on the side of caution. The cost of a bonding jumper is relatively low compared to the potential liability and downtime from a ground fault incident. It's a cheap insurance policy.

Standardization

It's often easier to implement a plant-wide standard requiring bonding jumpers on all FMC/LFMC installations, regardless of the specific circuit, rather than meticulously calculating and documenting compliance with the exceptions. This simplifies training and inspection.

Past Practices

Older installations may have been installed under previous code cycles that had stricter requirements.

Inspection Consistency

Some inspectors are more stringent than others and may require bonding jumpers even in situations where they might technically be optional.

In summary: While it's possible to avoid using bonding jumpers on FMC/LFMC under very specific conditions, it's often the safest and most practical approach to install them, especially in industrial settings. Knowing why* they're required helps you understand the potential risks and make informed decisions about electrical safety. Flag for review