10.2 Proportional Upsizing of EGC for Voltage Drop (NEC 250.122(B))

10.2 Proportional Upsizing of EGC for Voltage Drop (NEC 250.122(B))

Quick Answer: Under NEC Section 250.122(B), whenever ungrounded (phase) conductors are increased in size from the minimum size required for the load (most commonly to overcome voltage drop over long circuit runs), any wire-type equipment grounding conductor (EGC) installed in the raceway or cable must be increased proportionally in circular mil area. The calculation requires finding the ratio of the upsized phase conductor circular mils to the original minimum phase conductor circular mils (using NEC Chapter 9, Table 8), multiplying that ratio by the minimum Table 250.122 EGC circular mils, and selecting the next larger standard conductor size.

Few provisions in the National Electrical Code trigger more unexpected failures on master and journeyman licensing exams than NEC Section 250.122(B). Electricians often understand the necessity of upsizing ungrounded phase conductors to keep voltage drop below the 3% branch-circuit value suggested in the Informational Notes to 210.19. However, many fail to realize that increasing the size of the phase conductor without proportionally increasing the equipment grounding conductor compromises the entire ground-fault safety system.

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The Electrical Physics Behind NEC 250.122(B)

Why does the code enforce this strict proportional upsizing requirement? The answer lies in the fundamental physics of total ground-fault loop impedance ($Z_{\text{loop}}$) and the operating characteristics of inverse-time circuit breakers.

Inverse-Time Breaker Trip Curves

A standard molded-case circuit breaker possesses two distinct trip mechanisms:

1. Thermal Element (Bimetallic Strip): Responds to sustained overloads. It takes minutes to hours to trip at 125% to 200% of rated current.
2. Magnetic Element (Electromagnetic Coil): Responds to high-magnitude short circuits and ground faults. To achieve instantaneous tripping (within 1/2 to 1 cycle, or roughly 8 to 16 milliseconds), the fault current must reach 5 to 10 times the breaker rating.

The Long-Distance Impedance Hazard

Consider a 30-ampere, 120-volt branch circuit supplying an exterior parking lot sign located 300 feet from the electrical panel:

• A standard 30A circuit breaker requires a magnetic inrush current of at least 150 to 300 amperes ($5\times$ to $10\times$) to open instantaneously.
• Conductor resistance is inversely proportional to cross-sectional area: $R = \frac{\rho L}{A}$.
• To combat voltage drop over the 300-foot run, the installer upsizes the phase conductors from 10 AWG to 4 AWG copper.
• If the installer leaves the equipment grounding conductor at the standard Table 250.122 size of 10 AWG copper (which has an alternating-current resistance of roughly $1.2\ \Omega$ per 1000 feet per Chapter 9 Table 9):
  • Total round-trip fault loop resistance ($300\text{ ft}$ of 4 AWG + $300\text{ ft}$ of 10 AWG) exceeds 0.45 ohms.
  • An arcing ground fault at the sign delivers a maximum fault current of: $I_{\text{fault}} = \frac{120\text{ V}}{0.45\ \Omega} \approx 266\text{ A}$
  • If the arcing fault exhibits impedance or contact resistance, actual current drops to 100 to 140 amperes.
  • At 120 amperes, the 30-ampere breaker fails to trip magnetically. It falls into its slow thermal trip curve, requiring between 5 and 20 seconds to trip.
  • During those 20 seconds, thousands of joules of explosive arcing energy melt the sign framing, ignite combustible building materials, and hold the exterior metal sign at a lethal touch-potential of 80 to 100 volts to ground.

By upsizing the EGC proportionally to 4 AWG copper, the return path impedance is slashed, ensuring that available fault current comfortably exceeds 600 amperes, forcing the breaker into its instantaneous magnetic trip region within milliseconds.

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The NEC 250.122(B) Code Mandate

NEC Section 250.122(B) establishes the governing rule:

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

The Proportional Sizing Formula

To determine the required minimum circular mil area of the upsized equipment grounding conductor, apply the circular mil ratio formula:

$\frac{\text{cmil}_{\text{EGC-new}}}{\text{cmil}_{\text{EGC-min}}} = \frac{\text{cmil}_{\text{Phase-new}}}{\text{cmil}_{\text{Phase-min}}}$

Rearranging to solve for the new equipment grounding conductor:

$\text{cmil}_{\text{EGC-new}} = \text{cmil}_{\text{EGC-min}} \times \left( \frac{\text{cmil}_{\text{Phase-new}}}{\text{cmil}_{\text{Phase-min}}} \right)$

Where:

• $\text{cmil}_{\text{Phase-min}}$ = Circular mil area of the minimum ungrounded conductor required for the load or overcurrent protective device rating (from NEC Chapter 9, Table 8).
• $\text{cmil}_{\text{Phase-new}}$ = Circular mil area of the actual enlarged ungrounded conductor installed in the raceway (from NEC Chapter 9, Table 8).
• $\text{cmil}_{\text{EGC-min}}$ = Circular mil area of the standard equipment grounding conductor required by NEC Table 250.122 for the overcurrent device.
• $\text{cmil}_{\text{EGC-new}}$ = Minimum calculated circular mil area required for the upsized equipment grounding conductor.

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NEC Chapter 9, Table 8: Conductor Properties Reference

All circular mil values used in official exam calculations must be taken directly from NEC Chapter 9, Table 8 (Conductor Properties):

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Step-by-Step Worked Calculations

Worked Example 1: 30-Ampere Branch Circuit Upsized for Voltage Drop

Problem: A 240-volt single-phase branch circuit is protected by a 30-ampere circuit breaker. The circuit supplies an exterior gate motor 275 feet away. The minimum conductor required for ampacity is 10 AWG copper. To prevent excessive voltage drop during motor starting, the ungrounded phase conductors are increased in size to 4 AWG copper. What is the minimum size copper equipment grounding conductor required by NEC 250.122(B)?

Step 1: Identify minimum phase conductor and its circular mil area.

• Minimum phase conductor for 30A circuit = 10 AWG copper.
• From Chapter 9, Table 8: 10 AWG = 10,380 cmil.

Step 2: Identify upsized phase conductor circular mil area.

• Installed phase conductor = 4 AWG copper.
• From Chapter 9, Table 8: 4 AWG = 41,740 cmil.

Step 3: Calculate the area enlargement multiplier. $\text{Multiplier} = \frac{41,740\text{ cmil}}{10,380\text{ cmil}} = 4.0212$

Step 4: Identify the minimum Table 250.122 EGC size and circular mil area.

• From NEC Table 250.122, the minimum copper EGC for a 30-ampere overcurrent device is 10 AWG copper.
• From Chapter 9, Table 8: 10 AWG = 10,380 cmil.

Step 5: Calculate the required new EGC circular mil area. $\text{cmil}_{\text{EGC-new}} = 10,380\text{ cmil} \times 4.0212 = 41,740\text{ cmil}$

Step 6: Select the new conductor size from Chapter 9, Table 8.

• Referring to Table 8, a 4 AWG conductor has exactly 41,740 cmil.
• Final Answer: The equipment grounding conductor must be upsized to 4 AWG copper.

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Worked Example 2: 100-Ampere Feeder Upsized for Voltage Drop

Problem: A 100-ampere subpanel is fed from a main distribution board located 350 feet away in an adjacent warehouse wing. The circuit is protected by a 100-ampere circuit breaker with 75°C terminals. The minimum conductor required for ampacity is 3 AWG copper. To limit voltage drop to 2%, the ungrounded phase conductors are upsized to 1/0 AWG copper. What is the minimum size copper equipment grounding conductor required?

Step 1: Identify minimum phase conductor and its circular mil area.

• Minimum phase conductor for 100A at 75°C = 3 AWG copper (NEC Table 310.16 rating: 100A).
• From Chapter 9, Table 8: 3 AWG = 52,620 cmil.

Step 2: Identify upsized phase conductor circular mil area.

• Installed phase conductor = 1/0 AWG copper.
• From Chapter 9, Table 8: 1/0 AWG = 105,600 cmil.

Step 3: Calculate the area enlargement multiplier. $\text{Multiplier} = \frac{105,600\text{ cmil}}{52,620\text{ cmil}} = 2.00684$

Step 4: Identify the minimum Table 250.122 EGC size and circular mil area.

• From NEC Table 250.122, the minimum copper EGC for a 100-ampere overcurrent device is 8 AWG copper.
• From Chapter 9, Table 8: 8 AWG = 16,510 cmil.

Step 5: Calculate the required new EGC circular mil area. $\text{cmil}_{\text{EGC-new}} = 16,510\text{ cmil} \times 2.00684 = 33,133\text{ cmil}$

Step 6: Select the new conductor size from Chapter 9, Table 8.

• Looking at Chapter 9, Table 8:
  • 6 AWG = 26,240 cmil (less than 33,133 cmil, INSUFFICIENT)
  • 4 AWG = 41,740 cmil (exceeds 33,133 cmil, COMPLIANT)
• Final Answer: The equipment grounding conductor must be upsized to 4 AWG copper.

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Worked Example 3: 200-Ampere Feeder Upsized for Voltage Drop

Problem: A 200-ampere, 480Y/277V, 3-phase feeder runs 500 feet to an outbuilding. The minimum ungrounded conductor required for ampacity is 3/0 AWG copper (rated 200A at 75°C). The designer upsizes the phase conductors to 350 kcmil copper to combat voltage drop. What is the minimum required size of the copper equipment grounding conductor?

Step 1: Identify minimum phase conductor cmil.

• 3/0 AWG copper = 167,800 cmil.

Step 2: Identify upsized phase conductor cmil.

• 350 kcmil copper = 350,000 cmil.

Step 3: Calculate the multiplier. $\text{Multiplier} = \frac{350,000\text{ cmil}}{167,800\text{ cmil}} = 2.0858$

Step 4: Identify minimum Table 250.122 EGC.

• For a 200A overcurrent device, Table 250.122 specifies 6 AWG copper.
• 6 AWG copper = 26,240 cmil.

Step 5: Calculate required EGC cmil. $\text{cmil}_{\text{EGC-new}} = 26,240\text{ cmil} \times 2.0858 = 54,731\text{ cmil}$

Step 6: Select conductor size from Chapter 9, Table 8.

• Evaluating standard sizes:
  • 3 AWG = 52,620 cmil (less than 54,731 cmil, INSUFFICIENT)
  • 2 AWG = 66,360 cmil (exceeds 54,731 cmil, COMPLIANT)
• Final Answer: The equipment grounding conductor must be upsized to 2 AWG copper.

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Critical Examination Pitfalls & Exceptions