Effective Ground-Fault Current Path

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hhmel413

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Chapter 2, Article 250, Part I, Section 250.1; gives us the definition of "Effective Ground-Fault Current Path".
Question: Where in the National Electrical Code, do you get guidance in constructing this "Effective Ground-Fault Current Path"?
I think the answer is in, proper Electrical Engineering Design outside the Code, but here is the problem..... Electrical Inspectors, Electrical Contractors, Electricians, ect. can not seem to understand that compliance with the NEC includes proper Electrical Engineering.
Example: #14-2 W/G Non-metallic sheathed cable (Romex). Ampacity 15 amps with a 15 amp circuit breaker. NO ATTENTION is given to the length of this circuit. NO ONE CARES IN TEXAS! How can the circuit "Equipment Grounding Conductor" provide an "Effective Ground-Fault Current Path", if the total circuit is 150 feet or MORE in length???
The rule for "Voltage Drop" was moved from a section (210-6), to a Fine Print Note in the 1975 NEC. Compliance with this requirement made the circuit comply, but NOW no AHJ seems to care.
Comments Please.
 
I'd think that complying with or exceeding the minimum guidelines in Article 250 would establish a properly constructed effective ground fault current path.
 
This a great question. I believe this is often overlooked both in construction and inspection because it is not understood. The NEC gives little guidance on this subject in Article 250. Soares Book on Grounding provide an excellent explanation of this subject. Soares defines Effective Grounding as 1. Provide a permanent and continuous path 2. Has ample ampacity to carry the fault current 3. Has impedance sufficiently low to limit the voltage to ground and facilitate the operation of the protective device. The NEC provides no figures for “Low impedance” but does say is should be low enough to operate the overcurrent device . This current path should be low enough to allow the circuit breaker to reach its instantaneous pickup operating range. Using a Square D operating trip curve for a 15 amp breaker, the required current is 150 amps or 10 time the breaker rating. This will vary with different breakers. Georgia Institute of Technology did a study on conduit systems with fault currents. It is know that most faults are arcing faults as opposed to bolted faults. GT developed the software program GEMI to make calculations under different fault conditions. Using this program, the maximum distance for a circuit of 2 # 14 cu with the 15 amp breaker is 110 ft. This calculation includes an arc impedance with 40 volt drop.
Additional information on GEMI http://www.steelconduit.org/gemi.htm
 
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This issue is also addressed in a Note to Table 250.122 - "here necessary to comply with 250.4(A)(5) or (B)(4), the equipment grounding conductor shall be sized larger thangiven in this table."

I agree many are not familair with this note nor what to do about it. In addition to the information provided by bob, it is important for the electrical professional to learn the concept of conductor withstand ratings during a fault condition. This along with the understanding of fuse and circuit breakr trip characteristics, one can properly evaluate circuits and make adjustments as necessary.
 
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