NEC Questions and Answers based on 2020 NEC^® - March 2021

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For EC&M Magazine
By Mike Holt, NEC^® Consultant
Here's the follow-up to yesterday's newsletter. This includes the answers to the questions sent, so you can see how you did.

Note: The answers to these questions are based on the 2020 NEC.

Q1. What is the Code rule for connecting a grounding electrode system to a building supplied by a feeder?
A1. 250.32 Buildings Supplied by a Feeder

(A) Grounding Electrode. A building supplied by a feeder must have a grounding electrode system and a grounding electrode conductor installed in accordance with Part III of Article 250. [250.32].

Ex: A grounding electrode is not required for a building if it is supplied by a single branch circuit or multiwire branch circuit.

Q2. What is the Code rule for the connection of a portable generator to a grounding electrode?
A2. 250.34 Generators—Portable and Vehicle- or Trailer-Mounted

(A) Portable Generators. A portable generator is not required to be connected to a grounding electrode (grounded) if both [250.34(A)]:

(1) The generator only supplies equipment and/or receptacles mounted on the generator.

(2) The normally noncurrent-carrying metal parts of equipment and the equipment grounding conductor terminals of the receptacles are connected to the generator frame.

Q3. What is the Code rule for the connection of a vehicle- or trailer-mounted generator to a grounding electrode?
A3. (B) Vehicle- and Trailer-Mounted Generators. Vehicle- and trailer-mounted generators are not required to be connected to a grounding electrode (grounded) [250.34(B)] if:

(1) The generator frame is bonded to the vehicle or trailer frame

(2) The generator only supplies equipment or receptacles mounted on the vehicle, trailer, or generator 

(3) The normally noncurrent-carrying metal parts of equipment and the equipment grounding conductor terminals of the receptacles are connected to the generator frame.

Q4. What is the Code rule for neutral connection bonding of a portable or vehicle-mounted generator?
A4. (C) Neutral Conductor Bonding. The generator manufacturer is required to bond the neutral conductor to the generator frame where the generator is a component of a separately derived system.

Informational Note: For grounding portable generators supplying fixed wiring systems, see 250.30.

Q5. What are the NEC conditions that allow the installation of high-impedance grounded systems?
A5. To limit ground-fault current to a low value, a high-impedance grounded system is permitted to be installed on three-phase systems of 480V up to 1,000V where all the following conditions are met [250.36]:

(1) Conditions of maintenance and supervision ensure that only qualified persons service the installation.

(2) Ground detectors are installed on the system.

(3) Only line-to-line loads are served.

Informational Note: According to Annex O of NFPA 70E, Standard for Electrical Safety in the Workplace, high-impedance grounding is an effective tool to reduce arc flash hazards.

Author’s Comment:
►High-resistance grounding will insert an impedance in the ground return path and will typically limit the fault current to 10A or less, leaving insufficient fault energy and thereby reducing the arc flash hazard level. High-resistance grounding will not affect arc flash energy for line-to-line faults [Annex O NFPA 70E].
►High-impedance grounded systems are generally referred to as “High-Resistance Grounded Systems” in the industry. These systems are generally used where sudden interruption of power will create increased hazards and where a reduction of incident energy is needed for worker safety.

(A) Grounding Impedance Location. A grounding impedance, usually a resistor is installed between the neutral point of the separately derived system and the grounding electrode conductor.