8.3 Equipment Grounding Conductors (EGC) Sizing & Installation (NEC 250.118, 250.120 & 250.122)
Key Takeaways
- NEC 250.118 recognizes specific permitted types of equipment grounding conductors (EGCs), including copper/aluminum wire conductors, Rigid Metal Conduit (RMC), Intermediate Metal Conduit (IMC), Electrical Metallic Tubing (EMT), listed Type AC armor, and limited lengths of Flexible Metal Conduit (FMC/LFMC).
- Flexible Metal Conduit (FMC) is permitted as the sole EGC under NEC 250.118(5) only if terminated in listed fittings, protected by an OCPD rated 20A or less, total ground path length does not exceed 6 feet (1.8 m), and flexibility is not required after installation.
- Under NEC Table 250.122, wire-type EGCs are sized strictly based on the rating or setting of the upstream automatic overcurrent protective device (fuse or circuit breaker) protecting the circuit.
- NEC 250.122(B) mandates that if ungrounded circuit conductors are increased in size from the minimum required ampacity (such as for voltage drop compensation), wire-type EGCs MUST be increased in size proportionally according to the circular mil area of the ungrounded conductors.
- In parallel raceway or multiconductor cable installations under NEC 250.122(F), a full-sized EGC based on the full rating of the circuit OCPD must be installed in EACH parallel conduit or raceway; dividing the EGC into smaller sizes across parallel conduits is strictly prohibited.
Equipment Grounding Conductors (EGC) Sizing & Installation (NEC 250.118, 250.120 & 250.122)
The Equipment Grounding Conductor (EGC) is the unsung hero of electrical safety. Unlike the Grounding Electrode Conductor (which connects to the earth), the EGC runs alongside circuit conductors directly to utilization equipment, ensuring that any short circuit or ground fault to a metallic chassis finds an immediate, ultra-low-impedance metallic path back to the supply source to instantly trip the circuit breaker. On the Oklahoma Journeyman examination, sizing EGCs from NEC Table 250.122, calculating proportional increases for voltage drop under 250.122(B), and installing EGCs in parallel raceways under 250.122(F) are among the most heavily tested mathematical problems.
1. Permitted Types of Equipment Grounding Conductors (NEC 250.118)
NEC 250.118 defines the specific materials and wiring methods permitted to serve as equipment grounding conductors:
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| PERMITTED EQUIPMENT GROUNDING CONDUCTORS (NEC 250.118) |
| |
| METALLIC RACEWAYS & CABLES (Inherent EGC) WIRE-TYPE CONDUCTORS |
| ----------------------------------------- -------------------------------------------- |
| 1. Rigid Metal Conduit (RMC) 1. Copper, Aluminum, or Copper-Clad Aluminum |
| 2. Intermediate Metal Conduit (IMC) 2. Bare, Covered, or Insulated Wire |
| 3. Electrical Metallic Tubing (EMT) |
| 4. Armor of Type AC (BX) Cable with internal RESTRICTED RACEWAYS (Conditional EGC) |
| bonding strip -------------------------------------------- |
| 5. Listed Type MC Cable marked MCI-A 1. Flexible Metal Conduit (FMC) |
| 6. Copper sheath of Mineral-Insulated (MI) Cable 2. Liquidtight Flexible Metal Conduit (LFMC) |
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Limitations on Flexible Raceways as EGCs (NEC 250.118(5) & (6)):
| Raceway Type | Maximum OCPD Rating | Maximum Length in Ground Path | Listing & Fitting Rules | Flexibility After Install |
|---|---|---|---|---|
| Flexible Metal Conduit (FMC) | 20 Amperes | 6 feet (1.8 m) | Must use listed fittings | Prohibited if flexibility required |
| Liquidtight FMC (LFMC) (3/8" to 1/2") | 20 Amperes | 6 feet (1.8 m) | Must use listed fittings | Prohibited if flexibility required |
| Liquidtight FMC (LFMC) (3/4" to 1-1/4") | 60 Amperes | 6 feet (1.8 m) | Must use listed fittings | Prohibited if flexibility required |
Critical Journeyman Exam Rule: If Flexible Metal Conduit (FMC) or LFMC exceeds 6 feet in length, or is protected by a breaker larger than 20A (or >60A for 3/4"+ LFMC), or supplies vibrating equipment where flexibility is required after installation (e.g., motors, transformers, rooftop HVAC units), a separate wire-type EGC must be pulled inside or outside the conduit (NEC 250.118(5)(d)).
2. Sizing Equipment Grounding Conductors (NEC Table 250.122)
Wire-type EGCs are sized strictly based on the rating or setting of the upstream automatic overcurrent protective device (OCPD) protecting the circuit conductors:
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| NEC TABLE 250.122: MINIMUM SIZE EQUIPMENT GROUNDING CONDUCTORS |
| |
| RATING OR SETTING OF UPSTREAM MINIMUM SIZE OF EGC |
| OVERCURRENT DEVICE (AMPERES) ----------------------------------------------- |
| COPPER (AWG/kcmil) ALUMINUM (AWG/kcmil) |
| -------------------------------------------- ----------------------- ---------------------- |
| 15 14 AWG 12 AWG |
| 20 12 AWG 10 AWG |
| 30 10 AWG 8 AWG |
| 40 or 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 |
| 500 2 AWG 1/0 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 Adjustment for Conductor Upsizing / Voltage Drop (NEC 250.122(B))
When ungrounded phase conductors are increased in size from the minimum size required for the load (most commonly to prevent excessive voltage drop on long circuit runs), the wire-type EGC must be increased in size proportionally according to the circular mil area of the ungrounded conductors.
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| NEC 250.122(B) PROPORTIONAL EGC SIZING FORMULA & LOGIC |
| |
| Area_new_EGC = Area_orig_EGC × ( Area_upsized_phase / Area_orig_phase ) |
| |
| Why is this mandatory? |
| On a long conductor run, if phase conductors are upsized to reduce resistance and voltage drop, |
| leaving the EGC at standard size leaves the ground-fault loop impedance high. In a fault, |
| the voltage drop across the undersized EGC would elevate chassis touch potential to lethal levels|
| and delay the instantaneous trip of the circuit breaker! |
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Step-by-Step Worked Voltage Drop / Proportional Upsizing Problem:
Scenario: A 50-Ampere, 240V branch circuit supplies an outdoor water pumping station located 350 feet from the panelboard. The minimum conductor size required for a 50A circuit is #8 AWG THHN Copper (rated 50A at 75°C). To limit voltage drop to under 3%, the ungrounded phase conductors are upsized to #4 AWG Copper.
Calculation Steps:
- Find Circular Mil Area of Original Minimum Phase Conductor:
- From NEC Chapter 9, Table 8: #8 AWG = $16,510\text{ cmil}$.
- Find Circular Mil Area of Upsized Phase Conductor:
- From NEC Chapter 9, Table 8: #4 AWG = $41,740\text{ cmil}$.
- Find Standard EGC Size for a 50A OCPD:
- From NEC Table 250.122: 50A OCPD requires #10 AWG Copper.
- Area of #10 AWG Copper (Ch. 9, Table 8) = $10,380\text{ cmil}$.
- Apply the Proportional Adjustment Formula:
- Select Next Standard AWG Size from Chapter 9, Table 8:
- #8 AWG = $16,510\text{ cmil}$ (Too small)
- #6 AWG = $26,240\text{ cmil}$ (Standard industry round: $26,240\text{ cmil} \approx 26,242\text{ cmil}$, #6 AWG is selected; if strict rounding applies, next size is #6 AWG).
- Answer: The required equipment grounding conductor must be upsized to #6 AWG Copper.
4. EGCs in Parallel Raceways or Cables (NEC 250.122(F))
Where circuit conductors are run in parallel in two or more raceways or cables (e.g., an 800A feeder split across two parallel conduits), the rules for EGC installation are strictly enforced:
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| PARALLEL RACEWAY EGC INSTALLATION (NEC 250.122(F)(1)) |
| |
| 800A FEEDER OCPD |
| +-----------------------------------------------------------------------------------+ |
| | | |
| | Conduit 1: (3) 500 kcmil Phase + (1) 1/0 AWG Cu EGC <-- FULL SIZE EGC | |
| | | |
| | Conduit 2: (3) 500 kcmil Phase + (1) 1/0 AWG Cu EGC <-- FULL SIZE EGC | |
| +-----------------------------------------------------------------------------------+ |
| |
| CRITICAL CODE MANDATE: |
| Each parallel raceway must contain a FULL-SIZED equipment grounding conductor sized from |
| Table 250.122 based on the full 800A OCPD rating (1/0 AWG Cu in each pipe). |
| You CANNOT divide the 1/0 AWG EGC into two smaller conductors (such as two #4 AWG wires)! |
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Why Must Each Parallel Conduit Contain a Full-Sized EGC?
If a ground fault occurs inside Conduit 1, the fault current flows almost entirely through the EGC inside Conduit 1 due to magnetic coupling and mutual inductance. If that EGC were undersized (half-sized), it would instantly vaporize under the 800A fault current before the upstream circuit breaker could clear the fault, resulting in conduit blowout and structural fire.
5. Cord-and-Plug Connected Equipment Grounding (NEC 250.114)
NEC 250.114 specifies which appliances and portable tools must have their exposed metal frames grounded via an equipment grounding conductor in the cord and attachment plug:
- Residential Occupancies: Refrigerators, freezers, room air conditioners, clothes washing machines, clothes dryers, dishwashers, electrical aquarium equipment, sump pumps, and motor-operated tools (saws, drills, lawn mowers, snowblowers, hedge clippers).
- Commercial & Industrial Occupancies: Hand-held motor-operated tools, cord-connected appliances, and all equipment operating at over 150 volts to ground.
- Double-Insulated Exception (NEC 250.114 Exception): Listed tools and appliances protected by a system of double insulation (distinctively marked with the square-in-a-square symbol) are NOT required to be grounded.
An 800-ampere feeder is installed in two parallel rigid PVC conduits between a main switchboard and a distribution panelboard. Each conduit contains three 500 kcmil copper ungrounded phase conductors and one neutral conductor. What size copper equipment grounding conductor must be installed in EACH of the two parallel conduits under NEC 250.122(F)(1)?
A journeyman electrician is connecting a 20-ampere, 120V stationary commercial appliance using a 5-foot length of 1/2-inch Flexible Metal Conduit (FMC) with listed end fittings. Under NEC 250.118(5), under what condition is this FMC permitted to serve as the sole equipment grounding conductor without pulling an internal wire EGC?
A 30-ampere, 240V single-phase branch circuit supplies an outdoor air compressor located 250 feet from the panel. The minimum conductor size required for a 30A circuit is #10 AWG copper (10,380 cmil). To prevent excessive voltage drop, the electrician increases the phase conductors to #6 AWG copper (26,240 cmil). What size copper equipment grounding conductor must be installed under NEC 250.122(B)?
A commercial feeder protected by a 400-ampere molded-case circuit breaker supplies a panelboard located in a manufacturing facility. What is the minimum size copper equipment grounding conductor required for this feeder under NEC Table 250.122?