What 'Connected to Establish Electrical Continuity and Conductivity' Means for Bonding

When the National Electrical Code (NEC) defines a component as being "connected to establish electrical continuity and conductivity," it is explicitly defining bonding. In one sentence: bonding is the permanent joining of metallic parts to form an electrically conductive path that ensures continuity and the capacity to safely conduct any fault current likely to be imposed. While grounding connects a system to the earth, bonding connects metal parts to each other. What this changes in a real installation is the fault-clearing path: without a low-impedance bonded path back to the source, a line-to-case fault will simply energize the metal enclosure, waiting to shock the next person who touches it while grounded.

The Data: NEC Bonding and Continuity Specifications

To achieve the conductivity required by code, you cannot simply rely on mechanical connections like standard locknuts or painted enclosure seams. The NEC mandates specific wire sizes for bonding jumpers based on the size of the overcurrent device and ungrounded conductors. Below is a data-dense extraction from NFPA 70 (NEC) Table 250.102(C)(1), detailing the minimum size equipment bonding jumpers on the line side of the service or separately derived system.

Notice the jump in required copper AWG when moving from 6 AWG to 4 AWG feeders. Using a 10 AWG jumper on a 100A service (which typically uses 3 AWG copper or 1 AWG aluminum) violates the conductivity requirement. This creates a thermal bottleneck that could melt the jumper before the main breaker trips, defeating the entire purpose of the fault-clearing path.

Worked Numeric Example: Clearing a 120V Line-to-Case Fault

Let’s look at what happens when continuity and conductivity are correctly established versus when they are compromised by poor workmanship.

Assume a standard 120V, 20A branch circuit wired with 12 AWG THHN copper in a 100-foot run to a metal junction box. The source is a standard thermal-magnetic breaker in the main panel.

Scenario A: Properly Bonded
The metal box is bonded to the panel via a dedicated 12 AWG copper equipment grounding conductor (EGC).

• Resistance of 12 AWG copper: ~1.588 ohms per 1,000 ft at 75°C.
• Total loop length (Hot wire out + EGC back): 200 ft.
• Loop resistance: (200 / 1000) * 1.588 = 0.3176 ohms.
• A hot wire faults to the box. Fault current (I) = V / R = 120V / 0.3176Ω = 377.8 Amps.

A standard 20A thermal-magnetic breaker requires roughly 5x to 10x its rated current (100A–200A) to trigger the instantaneous magnetic trip. At 377.8A, the breaker trips in milliseconds (typically <0.025 seconds), clearing the fault before anyone can touch the box.

Scenario B: Poor Continuity (The "Hinge" Mistake)
Suppose an installer relies on a metal cabinet door hinge to bond a 120V receptacle to the grounded cabinet, skipping the dedicated bonding jumper. Paint, grease, and pivot wear introduce 5.0 ohms of resistance into the return path.

• Fault current = 120V / 5.0Ω = 24 Amps.

Because 24A is only slightly above the 20A rating, the magnetic instantaneous trip will not activate. The breaker relies on the thermal bimetallic strip, which may take 40 to 60 seconds to trip—or it may never trip if the ambient panel temperature is low. For that entire minute, the cabinet door sits at ~115V lethal potential. This is exactly why code requires parts to be explicitly "connected to establish electrical continuity and conductivity" rather than relying on mechanical happenstance.

Where You Meet This in Practice

You will encounter the requirement to establish electrical continuity and conductivity in several specific, often-missed jobsite scenarios:

1. Concentric and Eccentric Knockouts: Standard locknuts do not provide reliable conductivity across concentric KOs because the metal-to-metal contact is minimal and easily compromised by paint. You must use a bonding bushing (like the Arlington Industries GKB series) with a bonding jumper routed back to the ground bar.
2. Flexible Metal Conduit (FMC): While FMC can act as an EGC in very specific, short runs, NEC 250.118(5) restricts this. If the FMC run exceeds 6 feet, or if it is subject to flexing in use, you must pull a separate green or bare copper bonding jumper inside the flex to establish conductivity.
3. Metal Water Pipe Bonding: Per NEC 250.50 and 250.53(D), a continuous underground metal water pipe in direct contact with the earth for 10 feet or more must be bonded to the electrical grounding electrode system. This prevents the plumbing from becoming energized if an electrical fault finds a path through the water.
4. Hazardous Locations: In Class I, Division 1 areas (where flammable gases are present), standard wrench-tight threaded connections are not enough. You must use specific bonding locknuts or jumpers across every threaded joint to ensure continuity and prevent static sparking.

Bonding vs. Grounding: The Most Common Confusion

What people most commonly confuse with bonding is grounding. The terms are used interchangeably by DIYers, but they describe two entirely different physical connections with different purposes.

Grounding is the connection of an electrical system to the earth (dirt) via a grounding electrode like a copper-clad ground rod or a Ufer ground. Its primary purpose is to stabilize system voltage to earth and provide a path to dissipate high-voltage lightning strikes. According to OSHA 1910.399 definitions, grounding is explicitly about the earth connection.

Bonding is the connection of metal parts to each other. Its primary purpose is to provide a low-impedance fault-clearing path back to the source (the transformer or main panel), forcing the breaker to trip.