Okay, let's break down this equipment grounding conductor (EGC) fill calculation, addressing the potential error in your NEC handbook and the treatment of pigtails. I'll assume you're working with box fill calculations as outlined in NEC 314.16.

Understanding the Basics of Box Fill Calculations (NEC 314.16)

The general principle is that you need to account for the volume of conductors, devices (like switches or receptacles), and internal cable clamps within an electrical box. The goal is to ensure you don't overfill the box, which can lead to overheating, loose connections, and safety hazards.

Key Factors in EGC Fill



General Rule

All equipment grounding conductors entering a box count as a single conductor, based on the largest EGC entering the box (NEC 314.16(B)(5)).

Pigtails

The NEC does NOT provide a general exception rule to reduce the EGC count for those conductors that only serve as pigtails to connect devices (receptacles, etc.) All EGCs entering the box are to be counted.

Volume Allowance

The volume allowance is determined by the conductor size and listed in NEC 314.16(B)(1). For 12 AWG, it's typically 2.25 cubic inches.

Your Scenario: 6 EGCs (2 Pigtailed)

Let's analyze your calculation, step by step. Assuming your handbook follows the NEC 314.16:1. Identify the Largest EGC

You state the largest EGC is 12 AWG.2.

Count all EGCs as ONE

Regardless of whether they're pigtailed or not, all EGCs entering the box are treated as one conductor, based on the size of the largest EGC (in your case, 12 AWG). Therefore, the entire 6 conductors are treated as ONE.3.

Determine the Volume Allowance

Since the largest EGC is 12 AWG, the volume allowance is 2.25 cubic inches (based on the NEC table).

Therefore, the correct EGC volume allowance for your situation is 2.25 cubic inches.Why Your Book Might Be Incorrect (or Misinterpreted)



Outdated Edition

Double-check that your NEC handbook is based on the current edition of the NEC (2023). Older editions might have had slightly different rules, although this particular rule has remained largely unchanged.

Typographical Error

Handbooks can contain errors. Compare the relevant section with the official NEC text.

Misinterpretation

It's possible you're misinterpreting the wording of the handbook. Carefully reread the section on box fill, paying attention to the specific rules for EGCs.

"Helpful Hint" Gone Wrong

Sometimes, handbooks include simplified explanations or "helpful hints" that can be misleading if taken out of context. If there's a note about pigtails, make sure you understand its limitations.

Example illustrating NEC 314.16(B)(5)

Let’s assume your box has the following: (3) 12 AWG EGCs (3) 14 AWG EGCsAll 6 EGCs are treated as one, using the volume allowance of the largest EGC in the box - which in this case is 12 AWG (2.25 cubic inches).

Addressing Your Original Calculation

Your original breakdown: "1) 4X EGC = 2.25 (1 single volume allowance)" -- This is correct in the sense that all EGCs are considered ONE allowance.* "2) 2 x EGC (pig tail jumpets) 1/4 x 2 x 2.25" -- This is where the calculation is incorrect. The NEC does not grant an allowance to reduce the volume of EGCs that are used as pigtails.

Conclusion

The correct EGC fill calculation for your scenario (6 EGCs, largest being 12 AWG) is

2.25 cubic inches. It seems likely that your handbook contains an error, or that the section you are reading is misinterpreted. Double-check against the official NEC document.

Important Note: Always consult the latest edition of the National Electrical Code (NEC) for the most accurate and up-to-date information. This explanation is for informational purposes and should not be considered a substitute for professional electrical advice. When in doubt, consult a licensed electrician. Flag for review