4.3 Equipment Grounding Conductors & Bonding Jumpers

4.3 Equipment Grounding Conductors & Bonding Jumpers

Quick Answer: The Equipment Grounding Conductor (EGC) provides a dedicated, low-impedance path to return ground-fault current directly to the electrical source to trip the circuit breaker. Sized using NEC Table 250.122 based on the overcurrent protective device (OCPD) rating, the EGC must be proportionally upsized under NEC 250.122(B) whenever ungrounded conductors are increased in size for voltage drop. In parallel raceways, a full-size EGC must be installed in each conduit per NEC 250.122(F). The Main Bonding Jumper (MBJ) joins neutral to ground exclusively at the main service equipment; downstream neutral-to-ground bonding in subpanels is strictly illegal.

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1. Definition and Purpose of the Equipment Grounding Conductor (EGC)

Under NEC Article 100, the Equipment Grounding Conductor (EGC) is defined as:

"The conductive path(s) that provides a ground-fault current path and connects normally non-current-carrying metal parts of equipment together and to the system grounded conductor or to the grounding electrode conductor, or both."

The Effective Ground-Fault Current Path (NEC 250.4(A)(5))

An effective ground-fault current path must satisfy three strict performance criteria:

1. It must be intentionally constructed, permanent, and electrically continuous.
2. It must possess low impedance to limit touch voltage to ground and facilitate the operation of overcurrent protective devices (circuit breakers or fuses).
3. It must have ample current-carrying capacity to conduct safely the maximum ground-fault current likely to be imposed on it without burning open.

                      GROUND FAULT CLEARED BY EGC (CORRECT):

       [Utility XFMR] === Hot Phase (120V) ===> [Appliance Load]
             ^                                         |
             |                                      (FAULT!)
             | (Neutral)                               v
       [Service Panel] <=== Low-Z Metal EGC <=== [Metal Frame]
             |
     (Overcurrent device sees 1,000+ Amps of fault current and trips instantaneously in 0.01 sec)

-------------------------------------------------------------------------------------------------

                      GROUND FAULT WITHOUT EGC (DEADLY HAZARD):

       [Utility XFMR] === Hot Phase (120V) ===> [Appliance Load]
             ^                                         |
             |                                      (FAULT!)
             | (Earth return)                          v
       [Earth Ground Rod (25 Ohms)] <=== Earth === [Metal Frame (Energized at 120V!)]
             |
     (Ohm's Law: 120V / 25 Ohms = 4.8 Amps. Breaker NEVER trips! Metal frame remains lethal!)

Permitted Types of EGCs (NEC 250.118)

An EGC does not necessarily have to be a green insulated copper wire. NEC 250.118 lists approved types, including:

• Bare, covered, or insulated copper, aluminum, or copper-clad aluminum conductors.
• Rigid Metal Conduit (RMC) and Intermediate Metal Conduit (IMC).
• Electrical Metallic Tubing (EMT).
• Listed Flexible Metal Conduit (FMC) and Liquidtight Flexible Metal Conduit (LFMC) (subject to 20A limit and 6-ft length restrictions under 250.118(5)–(7)).
• Armor of Type AC cable (BX) with internal bonding strip.
• Listed Type MC cable with combined armor and ground assembly.

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2. Sizing the EGC: NEC Table 250.122

Unlike the GEC (which is sized from Table 250.66 based on conductor size), the Equipment Grounding Conductor is sized strictly from NEC Table 250.122 based on the rating or setting of the upstream overcurrent protective device (OCPD) protecting the circuit.

Full NEC Table 250.122 Reference Table

Note on Intermediate Breakers: If a circuit is protected by a 30A, 40A, or 50A breaker, Table 250.122 dictates that the minimum copper EGC size is 10 AWG Copper (under the 60A bracket).

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3. Mandatory Upsizing of EGC (NEC 250.122(B))

One of the most common calculation questions on the Massachusetts Journeyman exam involves NEC 250.122(B):

"Increased in Size. Where ungrounded conductors are increased in size from the minimum size that has sufficient ampacity for the intended installation, wire-type equipment grounding conductors, where installed, shall be increased in size proportionally according to the circular mil area of the ungrounded conductors."

Why Upsizing Is Necessary

When ungrounded phase conductors are upsized to counteract voltage drop on long circuit runs, the overall resistance of the phase wire drops. However, if the EGC were left at its standard Table 250.122 size, its higher resistance over a long distance could choke fault current, preventing the breaker from opening within the required instantaneous trip curve.

Proportional Upsizing Step-by-Step Method

1. Find Minimum Required Conductor Size: Determine the minimum ungrounded conductor size required for the circuit ampacity under NEC Table 310.16 (e.g., 75°C rating).
2. Look Up Circular Mil Areas (NEC Chapter 9, Table 8):
   • Find the circular mil area of the minimum ungrounded conductor ($\text{cmil}_{\text{min}}$).
   • Find the circular mil area of the actual, upsized ungrounded conductor ($\text{cmil}_{\text{actual}}$).
3. Calculate Proportional Ratio ($R$): $R = \frac{\text{cmil}_{\text{actual}}}{\text{cmil}_{\text{min}}}$
4. Find Standard EGC Size: From Table 250.122, find the standard EGC size for the circuit OCPD and look up its circular mil area ($\text{cmil}_{\text{egc_orig}}$).
5. Calculate New EGC Circular Mil Area: $\text{cmil}_{\text{egc\_new}} = \text{cmil}_{\text{egc\_orig}} \times R$
6. Select Conductor: Consult Chapter 9, Table 8 and select the next larger standard conductor size with equal or greater circular mils.

Comprehensive Worked Example: Voltage Drop Upsizing

• Circuit Specifications: A 30A, 240V single-phase circuit supplying an exterior compressor located 300 feet away, protected by a 30A circuit breaker.
• Conductor Upsizing: To keep voltage drop under 3%, the installer increases the ungrounded conductors from 10 AWG Copper to 4 AWG Copper.
• Question: What is the minimum size copper equipment grounding conductor required?

Calculation Steps:
1. Minimum ungrounded conductor for 30A: 10 AWG Copper.
2. Area from Chapter 9, Table 8:
   - 10 AWG = 10,380 circular mils
   - 4 AWG (actual) = 41,740 circular mils
3. Proportional Ratio (R):
   R = 41,740 / 10,380 = 4.021
4. Standard EGC from Table 250.122 for 30A OCPD: 10 AWG Copper (10,380 circular mils).
5. Required New EGC Area:
   Area = 10,380 cmil * 4.021 = 41,740 circular mils
6. Conductor Selection from Chapter 9, Table 8:
   - 6 AWG = 26,240 cmil (Too small!)
   - 4 AWG = 41,740 cmil (Exact match!)
   Result: The EGC MUST be upsized to 4 AWG Copper!

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4. EGC Sizing in Parallel Raceways (NEC 250.122(F))

When large feeders are divided into multiple parallel conduits (e.g., an 800A feeder routed in two parallel raceways):

The Parallel EGC Mandate (NEC 250.122(F)(1)): Each raceway or cable must contain a full-size equipment grounding conductor sized based on the ampere rating of the circuit overcurrent device from Table 250.122!

                  PARALLEL CONDUIT EGC SIZING (NEC 250.122(F))

               800A Feeder Protected by 800A Overcurrent Protective Device
       =========================================================================
       CONDUIT 1:                               CONDUIT 2:
       - Phase A: 500 kcmil Cu                  - Phase A: 500 kcmil Cu
       - Phase B: 500 kcmil Cu                  - Phase B: 500 kcmil Cu
       - Phase C: 500 kcmil Cu                  - Phase C: 500 kcmil Cu
       - Neutral: 500 kcmil Cu                  - Neutral: 500 kcmil Cu
       - EGC: 1/0 AWG Copper (FULL SIZE!)       - EGC: 1/0 AWG Copper (FULL SIZE!)

       *VIOLATION:* Installing a 2 AWG conductor in each conduit (splitting 1/0 cmil)
       is strictly prohibited! A ground fault in Conduit 1 will carry the full 800A!

The Electrical Fault Physics

Electricians often ask: "Why can't I split the circular mil area of the EGC between the two pipes?"

If a ground fault occurs inside Conduit 1, magnetic coupling ensures that virtually all of the fault current returns along the EGC inside Conduit 1. If that EGC was half-sized, it would instantly fuse and disintegrate before the 800A breaker could clear the fault, leaving the conduit and switchboard energized at line voltage!

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5. Main Bonding Jumper (MBJ) & System Bonding Jumper (SBJ)

The Main Bonding Jumper (NEC 250.24(B) & 250.28)

The Main Bonding Jumper (MBJ) is the critical link in electrical safety: it connects the grounded circuit conductor (system neutral) to the equipment grounding conductor and the service disconnect enclosure.

• Location: Installed at the service disconnecting means only.
• Construction: May be a wire, busbar, or screw. Where a green bonding screw is supplied with listed service equipment, it must be driven tightly to bond the neutral bus to the metal enclosure.
• Sizing (NEC 250.28(D)): Sized from NEC Table 250.102(C)(1) based on the largest ungrounded service-entrance conductor.
  • Where service conductors exceed 1,100 kcmil copper or 1,750 kcmil aluminum, the MBJ must have an area not less than 12.5% of the area of the largest ungrounded conductor.

System Bonding Jumper (SBJ) (NEC 250.30(A)(1))

For separately derived systems (such as the secondary of a step-down dry-type transformer), the System Bonding Jumper (SBJ) performs the exact same function as the MBJ, connecting the transformer secondary neutral to the equipment ground bus and enclosure.

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6. Strict Prohibition of Downstream Neutral-to-Ground Bonding

Under NEC 250.24(A)(5) and NEC 250.142(B), a grounded neutral conductor shall never be connected to the equipment grounding conductor or metal enclosures on the load side of the service disconnecting means.

+-------------------------------------------------------------------------+
|          THE LETHAL HAZARD OF DOWNSTREAM NEUTRAL-TO-GROUND BONDING      |
+-------------------------------------------------------------------------+
|                                                                         |
|   MAIN SERVICE PANEL                           SUBPANEL (LOAD SIDE)     |
|   +-----------------------+                    +---------------------+  |
|   | [Neutral Bus]         | === Normal Neutral ==> | [Neutral Bus]   |  |
|   |       |               |     Return Current |         |           |  |
|   |     (MBJ)             |                    |   (ILLEGAL BOND!)   |  |
|   |       v               |                    |         v           |  |
|   | [Ground Bus / Encl]   | <== Parallel Path === | [Ground / Encl]  |  |
|   +-----------------------+     Through Metal  +---------------------+  |
|                                 Conduit & Piping                        |
|                                                                         |
|   DANGERS CREATED:                                                      |
|   1. Continuous "objectionable neutral current" on metal enclosures.     |
|   2. Severe shock hazard: worker opening conduit gets full neutral shock.|
|   3. Nuisance tripping: GFCI and AFCI circuit breakers cannot hold.     |
|   4. Arcing and fire hazard at loose conduit fittings and junction boxes|
+-------------------------------------------------------------------------+

In Field Practice: The Subpanel Rule

In every subpanel (feeder panelboard):

1. The neutral bus must be fully floating (isolated) from the metal enclosure by removing the green bonding screw and ensuring the neutral bar sits on its insulating plastic base.
2. All equipment grounding conductors must terminate exclusively on a separate, dedicated ground bar bolted directly to the metal enclosure.