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Kevin, Licensed Electrical Contractor
Category: Electrical
Satisfied Customers: 3663
Experience:  30 years Licensed Electrical Contractor in Illinois, Adjunct College Electrical Instructor, Former Electrical Inspector, Diploma: Digital Electronics, FCC Technician License
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A customer has moved a 150kw generator on a lot to feed

Customer Question

A customer has moved a 150kw generator on a lot to feed several reefer units coming off of ships. The reefers remain on generator power (3 ph. 480v) until they leave the yard. My question is Does the generator require a ground wire attached to the frame for a mechanical ground. and where in the code is this addressed.
Submitted: 1 year ago.
Category: Electrical
Expert:  Kevin replied 1 year ago.

Hello and welcome to Just Answer. My name is ***** ***** I will be happy to assist you with your electrical question.

1) Article 250.30 ----------> Grounding Separately Derived Alternating Current Systems within the National Electrical Code addresses this for generators.

Expert:  Kevin replied 1 year ago.

If you have any additional questions, just let me know and I’ll be glad to answer them for you.

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Customer: replied 1 year ago.
I agree but this doesn't make it clear about a portable generator . The definitions are different. Seperately derived and portable or listed seperately I don't have my book to give you the article . This is not a seperately derived circuit it is stand alone and the only power source available. It does not need a grounded conductor. I'm not completely satisfied with your answer. I particularly don't understand how you get the answer from the article you gave me. Maybe I'm not understanding it the way you do. That's why I contacted you. I read it a dozen times and don't see it.
Expert:  Kevin replied 1 year ago.

1) If the gen set is NOT connected to an existing direct connection between the gen set and the service (standalone only), then the gen set is considered as a "separately derived system".

2) Refer to page 225 (Article 250.30) in the 2011 NEC Handbook if you have a copy of this and they provide the following explanation.

"Section 250.30(A)(1) requires separately derived systems to have a system bonding jumper connected between the generator frame and the grounded circuit conductor (neutral). The grounding electrode conductor from the generator is required to be connected to a grounding electrode"

3) How can a standalone generator not be a separately derived system if it is NOT connected to a service thru a transfer switch? It's not connected to anything except downstream loads. Therefore, the gen set is a separately derived system since it is the only available power source.

4) Refer to Article 100 Definitions of a Separately Derived System

5) Separately derived = no other available power source and no other interconnection (100% standalone) with any other power sources in my opinion:).

Expert:  Kevin replied 1 year ago.

If you have any additional questions, just let me know and I’ll be glad to answer them for you.

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Customer: replied 1 year ago.
There is no grounding electrode conductor in this system. The generator is straight 3 ph 480 and there is no place that I can find for a grounding electrode conductor . If this article covers the situation fully what exactly is the answer . Do I case grnd or not?
Expert:  Kevin replied 1 year ago.

1) I assume the generator contains a ground lug? The grounding electrode conductor lands on the ground lug.

Customer: replied 1 year ago.
KEVIN your last response did not make it through the system. But no there is no ground lug on the generator. if there is a tap anywhere other than the phase conductors I can't find it. Here is what I have been able to findGenerator Grounding
When are Ground Rods Required at Portable Generators?Portable generators are often used for backup power at traffic signals, buildings, structures and special events. Ground rods (grounding electrodes) are only required if the generator is a separately derived system. (For the complete text of sections cited please see the 2002 NEC)What is a Separately Derived System?
The NEC in Article 100 defines a Separately Derived System as:
Separately Derived System. A premises wiring system whose power is derived from a battery, from a solar photovoltaic system, or from a generator, transformer, or converter windings, and that has no direct electrical connection, including a solidly connected grounded circuit conductor, to supply conductors originating in another system. [See Fig 1]The key to knowing if a generator is a Separately Derived System is not the generator, but rather the transfer switch. If the transfer switch does not transfer the neutral (grounded conductor), then the generator has a “solidly connected” grounded circuit conductor and the generator is not a separately derived system. [See Fig 2]If the generator is connected to a transfer switch that transfers the neutral, and the generator does not have a “solidly connected” grounded circuit conductor, then it is a separately derived system. A separately derived system requires a connection to a grounding electrodes or ground rod(s).A transfer switch is required between the generator and connected load. It provides isolation and prevents backfeed to the utility source from the generator. New for the 2002 NEC is Section 702.6 Transfer Equipment, which requires suitable equipment for the use, designed and installed so as to prevent the inadvertent interconnection of normal and alternate sources of supply in any operation of the transfer equipment. Interlocked circuit breakers or manual double pole, double throw, center off switch can be used.Why Do Separately Derived Systems Require a Ground Rod?
Article 250 has the requirements for grounding of systems. In Section 250.20 Alternating-Current Circuits and Systems to Be Grounded, Part (D) states that systems covered in 250.20(A) or (B) are to be grounded per 250.30. A fine print note clarifies the connection of a generator:
FPN No. 1: An alternate ac power source such as an on-site generator is not a separately derived system if the neutral is solidly interconnected to a service-supplied system neutral.Part (B) is for AC systems over 50 V:
· 120V or 120/240V single phase
· 277/480Y 4 wire, three phase, 120/208V 4 wire, three phase
· 120/240V 4 wire, 3 phase deltaThese are the typical AC systems used for traffic signal or roadway lighting applications.Section 250.30 Grounding Separately Derived Alternating-Current Systems, gives the requirements for grounding. A common type of Separately Derived Service is a 480 x 120/240V step down transformer. When there is no direct electrical connection to the primary grounded conductor, and the secondary grounded conductor must be connected to a grounding electrode system.What is the Purpose of the Ground Rod?
The ground rods serve no purpose in clearing line to line or line to case faults. An electrical system will operate “ungrounded” and some industrial systems are designed to operate ungrounded. The purpose of the Ground Rod, or more accurately Grounding Electrode System is given in Section 250.4(A) (1) and (2):
(1) Electrical System Grounding. Electrical systems that are grounded shall be connected to earth in a manner that will limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operation.
(2) Grounding of Electrical Equipment. Non–current-carrying conductive materials enclosing electrical conductors or equipment, or forming part of such equipment, shall be connected to earth so as to limit the voltage to ground on these materials.Authors Note: A ground fault at a metal street lighting pole, with a ground rod but no low impedance (green wire) path back to the source, does not allow sufficient current to flow to open the over current protective device when using the earth as the sole return path. Even at 25 ohms ground resistance, a ground rod will not assist in clearing the fault for a 120 V supply source. At 25 ohms resistance the maximum current will be 4.8 amps at 120V. The metal lighting pole will remain energized and a serious shock hazard. A supplemental grounding electrode can be added at street lighting and traffic signal poles, but Section 250.54 states the earth shall not be used as the sole equipment grounding conductor.Most generators are connected through transfer s
Expert:  Kevin replied 1 year ago.

1) Yes those statements are all correct.

However, the key item to remember in your application is that there is no transfer switch installed and the generator is NOT connected to any other existing electrical service. The generator is 100% standalone and is completely isolated from any other available power source.

Your application falls under the NEC definition of a Separately Derived System in my opinion.

2) I recommend to contact your local electrical inspector and get their approval as to whether or not this application requires a Grounding Electrode Conductor. The AHJ (Authority Having Jurisdiction) can always over ride the NEC.

I would not consider a large 150kW generator as a portable gen set, but rather a permanently installed generator in this application. It's not like the gen set will only be utilized for a few days or a couple of weeks.

3) I've never come across a generator that does not have a grounding lug. Even the cheap $100 Chinese portable generators have grounding lugs. I suggest to contact the manufacturer.

Expert:  Kevin replied 1 year ago.

Hello again,

I just asked one of our electrical expert mentors here to review my answers and he also agrees with my answers and comments on your application. His name is***** and he has 46 years of experience as a licensed electrical contractor. He also agrees that a GEC is required for this application. I'm just a rookie as I only have 29 years as a licensed electrical contractor experience. Between Mike and myself we have a total of 75 years of combined electrical experience on numerous residential, commercial and industrial electrical construction projects with a vast knowledge of NEC code requirements.

So with this in mind, I suggest that you run this past your local AHJ and get their approval or disapproval as to whether or not a GEC is required. 2 licensed electricians here say you require the grounding electrode conductor for your application. The local AHJ has the final say so on the matter.

Take care and have a great day.

If you have any additional questions, just let me know and I’ll be glad to answer them for you.

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