3.3 Equipment Grounding & Bonding

3.3 Equipment Grounding & Bonding

The Equipment Grounding Conductor (EGC) is the low-impedance electrical backbone that connects all non-current-carrying metal enclosures, raceways, junction boxes, and equipment frames back to the grounded circuit conductor at the service or separately derived system. When an insulation breakdown occurs, the EGC carries the massive fault current necessary to instantly trip the circuit breaker or blow the fuse.

For the electrical plans examiner, verifying EGC conductor types, OCPD coordination, parallel conduit sizing, proportional upsizing for voltage drop, and piping bonding schedules is essential to ensure public safety.

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1. Permitted Equipment Grounding Conductors: NEC 250.118

NEC Section 250.118 recognizes fourteen distinct wiring methods and materials as suitable for use as Equipment Grounding Conductors:

+-----------------------------------------------------------------------------------------+
|                       PERMITTED EGC TYPES (NEC 250.118 SUMMARY)                         |
+------------------------------------+----------------------------------------------------+
| Wiring Method / Material           | Conditions & Code Limitations                      |
+------------------------------------+----------------------------------------------------+
| Copper, Aluminum, or Copper-Clad   | • Bare, covered, or insulated (250.118(1)).        |
| Aluminum Conductor (Wire Type)     | • If insulated, green or green with yellow stripes |
+------------------------------------+----------------------------------------------------+
| Rigid Metal Conduit (RMC)          | • Permitted as sole EGC with threaded or listed    |
| & Intermediate Metal Conduit (IMC) |   threadless fittings (250.118(2), (3)).           |
+------------------------------------+----------------------------------------------------+
| Electrical Metallic Tubing (EMT)   | • Permitted as sole EGC with listed fittings       |
|                                    |   (set-screw or compression) (250.118(4)).         |
+------------------------------------+----------------------------------------------------+
| Flexible Metal Conduit (FMC)       | • Grounding path ONLY if total length <= 6 ft,     |
| (250.118(5))                       |   circuit OCPD <= 20A, and listed fittings used.   |
|                                    | • If > 6 ft or > 20A: Wire EGC is MANDATORY!       |
+------------------------------------+----------------------------------------------------+
| Liquidtight Flexible Metal         | • Grounding path ONLY if total length <= 6 ft,     |
| Conduit (LFMC) (250.118(6))        |   and OCPD <= 20A (3/8"-1/2") or <= 60A (3/4"-1"). |
|                                    | • If > 6 ft or larger OCPD: Wire EGC is MANDATORY! |
+------------------------------------+----------------------------------------------------+
| Type AC Cable (250.118(8))         | • Armor in combination with internal 16 AWG        |
|                                    |   aluminum bonding strip qualifies as EGC.         |
+------------------------------------+----------------------------------------------------+
| Type MC Cable (250.118(10))        | • Interlocked armor listed as EGC (MC-AP), OR      |
|                                    |   MC cable containing dedicated internal wire EGC. |
+------------------------------------+----------------------------------------------------+

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2. Standard Sizing of EGCs: NEC 250.122 & Table 250.122

Unlike Grounding Electrode Conductors (which are sized by conductor size under Table 250.66), Equipment Grounding Conductors are sized strictly according to the rating or setting of the upstream Overcurrent Protective Device (OCPD) protecting the circuit.

+-----------------------------------------------------------------------------------------+
|                       NEC TABLE 250.122 EGC SIZING BENCHMARK TABLE                      |
+-----------------------------------+-----------------------------------------------------+
| Rating or Setting of Upstream     | Minimum Size Equipment Grounding Conductor          |
| Overcurrent Device (Amperes)      | Copper (AWG / kcmil)      | Aluminum (AWG / kcmil)  |
+-----------------------------------+---------------------------+-------------------------+
| 15                                | 14 AWG                    | 12 AWG                  |
| 20                                | 12 AWG                    | 10 AWG                  |
| 30                                | 10 AWG                    | 8 AWG                   |
| 60                                | 10 AWG                    | 8 AWG                   |
| 100                               | 8 AWG                     | 6 AWG                   |
| 200                               | 6 AWG                     | 4 AWG                   |
| 300                               | 4 AWG                     | 2 AWG                   |
| 400                               | 3 AWG                     | 1 AWG                   |
| 600                               | 1 AWG                     | 2/0 AWG                 |
| 800                               | 1/0 AWG                   | 3/0 AWG                 |
| 1000                              | 2/0 AWG                   | 4/0 AWG                 |
| 1200                              | 3/0 AWG                   | 250 kcmil               |
| 1600                              | 4/0 AWG                   | 350 kcmil               |
| 2000                              | 250 kcmil                 | 400 kcmil               |
+-----------------------------------+---------------------------+-------------------------+

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3. Proportional Upsizing for Voltage Drop: NEC 250.122(B)

One of the most frequently tested and inspected provisions is NEC 250.122(B). When ungrounded conductors are increased in size from the minimum size required for the load or ampacity (such as to compensate for voltage drop or high ambient temperature), the equipment grounding conductor(s) must be increased proportionally in circular mil area.

Mathematical Formulation

$\text{New EGC Area } (\text{cmil}) = \text{Standard EGC Area } (\text{cmil}) \times \left( \frac{\text{Upsized Phase Area } (\text{cmil})}{\text{Minimum Phase Area } (\text{cmil})} \right)$

Step-by-Step Worked Voltage Drop Upsizing Example

• Circuit Specifications: A 60A, 240V single-phase circuit feeding an outdoor chiller located 350 feet away.
• Standard Sizing without Voltage Drop:
  • Minimum ungrounded conductor for 60A = #6 AWG THHN Cu ($26,240\text{ cmil}$ per Chapter 9, Table 8).
  • Standard EGC for 60A breaker per Table 250.122 = #10 AWG Cu ($10,380\text{ cmil}$).
• Engineering Voltage Drop Upsizing:
  • To limit voltage drop to $< 3%$, the electrical engineer upsizes the ungrounded phase conductors to #1 AWG THHN Cu ($83,690\text{ cmil}$).
• Proportional EGC Calculation:
  1. Calculate conductor enlargement ratio: $\text{Upsize Ratio} = \frac{83,690\text{ cmil}}{26,240\text{ cmil}} = 3.1894$
  2. Multiply standard EGC circular mil area by upsize ratio: $\text{Required New EGC Area} = 10,380\text{ cmil} \times 3.1894 = 33,106\text{ cmil}$
  3. Look up resulting area in Chapter 9, Table 8 (Conductor Properties):
     • #6 AWG Cu = $26,240\text{ cmil}$ (Too small: $< 33,106\text{ cmil}$)
     • #4 AWG Cu = $41,740\text{ cmil}$ (Compliant: $\ge 33,106\text{ cmil}$)
  4. Required EGC: The Equipment Grounding Conductor must be upsized from #10 AWG to #4 AWG Copper!

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4. EGCs in Parallel Raceways: NEC 250.122(F)

When a feeder or circuit is installed in parallel raceways or cables (NEC 310.10(G)), the equipment grounding conductor rules depend on whether single conductors or multi-conductor cables are used:

+-----------------------------------------------------------------------------------------+
|                     PARALLEL RACEWAY EGC SIZING (NEC 250.122(F)(1))                     |
|                                                                                         |
|   FEEDER RATING: 800A (Two Parallel Conduits)                                           |
|                                                                                         |
|   +---------------------------------------+   +---------------------------------------+ |
|   | Conduit 1: 3-500 kcmil + 1-500 kcmil  |   | Conduit 2: 3-500 kcmil + 1-500 kcmil  | |
|   | + FULL 1/0 AWG Cu EGC                 |   | + FULL 1/0 AWG Cu EGC                 | |
|   +---------------------------------------+   +---------------------------------------+ |
|                                                                                         |
|   CRITICAL CODE RULE:                                                                   |
|   • A FULL-SIZE EGC based on the 800A OCPD (1/0 AWG Cu from Table 250.122) must be       |
|     installed in EACH parallel conduit!                                                 |
|   • Splitting the EGC (e.g. installing #4 AWG in each) is STRICTLY PROHIBITED!         |
+-----------------------------------------------------------------------------------------+

[!WARNING] The Parallel EGC Division Trap: Designers sometimes mistakenly divide the EGC area across parallel conduits (e.g., dividing an 800A EGC requirement between two pipes). This violates NEC 250.122(F)(1). In a ground fault inside one conduit, that single conduit's EGC must carry the entire ground-fault current until the 800A breaker opens. If undersized, the EGC will vaporize before the breaker trips.

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5. Bonding of Piping Systems & Structural Metal: NEC 250.104

+-----------------------------------------------------------------------------------------+
|                        NEC 250.104 BONDING REQUIREMENTS MATRIX                         |
+----------------------------+------------------------------------------------------------+
| System to be Bonded        | Sizing Basis & Installation Mandates                       |
+----------------------------+------------------------------------------------------------+
| Metal Water Piping         | • Sized per Table 250.102(C)(1) based on service-entrance  |
| (NEC 250.104(A))           |   conductor size.                                          |
|                            | • Must bond around water meters, filtering equipment, and  |
|                            |   dielectric fittings with listed bonding jumpers.         |
+----------------------------+------------------------------------------------------------+
| Other Metal Piping / Gas   | • Sized per Table 250.122 based on the rating of the       |
| Piping (NEC 250.104(B))    |   circuit likely to energize the piping.                   |
|                            | • Standard gas piping is bonded via the appliance EGC.    |
|                            | • CSST (Corrugated Stainless Steel Tubing) requires direct |
|                            |   bonding per manufacturer specs (typically min #6 Cu).    |
+----------------------------+------------------------------------------------------------+
| Structural Metal Framing   | • Sized per Table 250.102(C)(1) based on service size.     |
| (NEC 250.104(C))           | • Required where exposed structural steel is               |
|                            |   interconnected to form a building frame.                 |
+----------------------------+------------------------------------------------------------+