8.4 Equipment Grounding Conductors (EGC) Sizing & Identification

8.4 Equipment Grounding Conductors (EGC) Sizing & Identification

1. Function and Purpose of EGCs (NEC Article 100 & 250.4(A)(3))

NEC Article 100 defines the Equipment Grounding Conductor (EGC) 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.

Under normal conditions, an EGC carries zero current. Its sole function is to return fault current back to the system neutral, permitting high currents to trip breakers in fractions of a second.

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2. Approved Types of Equipment Grounding Conductors (NEC 250.118)

NEC 250.118 recognizes 14 distinct materials and wiring methods as approved Equipment Grounding Conductors:

1. Wire Conductors: Copper, aluminum, or copper-clad aluminum; solid or stranded; insulated, covered, or bare.
2. Rigid Metal Conduit (RMC) & Intermediate Metal Conduit (IMC): Heavy-wall threaded steel conduits provide approved ground paths without an internal wire.
3. Electrical Metallic Tubing (EMT): Fully approved as an equipment grounding conductor by itself, provided couplings and connectors are made up wrench-tight.
4. Flexible Metal Conduit (FMC): Permitted as an EGC only where listed, installed with listed grounding fittings, protected by an OCPD rated 20 amperes or less, length not exceeding 6 feet (1.8 m), and where flexibility is not required.
5. Liquidtight Flexible Metal Conduit (LFMC): Permitted as an EGC only where listed with listed grounding fittings, length not exceeding 6 feet, and OCPD ratings not exceeding 20 amperes for 3/8-in and 1/2-in sizes, or 60 amperes for 3/4-in through 1-1/4-in sizes (where flexibility is not required).
6. Cable Armor: Armor of Type AC cable; listed Type MC cable with interlocked armor and internal bare aluminum grounding tape in continuous contact with armor, or smooth/corrugated metallic sheaths.

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3. Conductor Identification and Re-Identification Rules (NEC 250.119)

NEC 250.119 establishes strict color-coding rules for wire-type Equipment Grounding Conductors:

• Permitted Finishes: Bare, continuous green outer insulation, or green insulation with one or more continuous yellow stripes.
• Conductors 6 AWG and Smaller: Must have factory-extruded continuous green outer finish or green with yellow stripes along their entire length (or be bare). Re-identifying conductors 6 AWG or smaller with green tape is strictly prohibited.
• Conductors 4 AWG and Larger (NEC 250.119(A)): Conductors 4 AWG and larger are permitted to be permanently re-identified as an EGC at each termination and accessible point using green tape, green paint, or green adhesive labels.

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4. Sizing EGCs Using NEC Table 250.122

Unlike the GEC (sized from Table 250.66 by conductor size), the Equipment Grounding Conductor is sized strictly by the rating or setting of the automatic overcurrent protective device ahead of the equipment.

NEC Table 250.122: Minimum Size Equipment Grounding Conductors

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5. Proportional Up-Sizing for Voltage Drop (NEC 250.122(B))

Under NEC 250.122(B), where phase conductors are enlarged for voltage drop, wire-type EGCs must be proportionally increased in circular mils.

The Proportional Formula

$\text{Required EGC Circular Mils} = \frac{\text{Actual (New) Phase Circular Mils}}{\text{Original Minimum Phase Circular Mils}} \times \text{Table 250.122 Minimum EGC Circular Mils}$

Relevant Conductor Circular Mils (NEC Chapter 9, Table 8)

• 14 AWG = $4{,}110\text{ cmils}$; 12 AWG = $6{,}530\text{ cmils}$; 10 AWG = $10{,}380\text{ cmils}$; 8 AWG = $16{,}510\text{ cmils}$; 6 AWG = $26{,}240\text{ cmils}$; 4 AWG = $41{,}740\text{ cmils}$

Worked Example 1: Voltage Drop Calculation

A 30A branch circuit protected by a 30A breaker supplies equipment 350 feet away. The minimum phase conductor is 10 AWG copper ($10{,}380\text{ cmils}$), and Table 250.122 requires a 10 AWG copper EGC ($10{,}380\text{ cmils}$). To limit voltage drop to under 3%, phase conductors are up-sized to 4 AWG copper ($41{,}740\text{ cmils}$).

1. Calculate expansion ratio: $\text{Ratio} = \frac{41{,}740\text{ cmils}}{10{,}380\text{ cmils}} \approx 4.0212$
2. Calculate required EGC area: $\text{New EGC Area} = 4.0212 \times 10{,}380\text{ cmils} = 41{,}740\text{ cmils}$
3. Select conductor from Table 8: A 6 AWG conductor is only $26{,}240\text{ cmils}$. A 4 AWG copper conductor ($41{,}740\text{ cmils}$) must be installed as the equipment grounding conductor.

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6. Sizing EGCs in Multiple Parallel Raceways (NEC 250.122(F))

Under NEC 250.122(F)(1), where circuit conductors are installed in parallel in multiple raceways or cables, the wire-type equipment grounding conductor installed in each raceway must be sized based on the full rating of the overcurrent protective device protecting the entire circuit.

Worked Example 2: Parallel Feeder Raceways

An 800-ampere feeder protected by an 800A breaker is installed in two parallel IMC raceways with 500 kcmil copper conductors.

• Dividing EGC size among raceways is prohibited.
• Per Table 250.122, an 800A overcurrent device requires a 1/0 AWG copper (or 3/0 AWG aluminum) equipment grounding conductor.
• A full-size 1/0 AWG copper EGC must be installed in each parallel raceway to carry fault current safely.