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Home > Home Wiring USA Archive: NEC 1999 > Accessory Structures to Dwellings > Installing a Generator (NEC 1999)

Installing a Generator (NEC 1999)

By Warren Goodrich
Installing a Generator (NEC 1999)

If you are attempting to wire a generator, there are several different methods and designs. The key factor, you must consider, is to ensure that your generated power does not re-energize the utility company’s primary lines !

If you produce a 120/240-volt power source from your generator, it will back feed through your meter base, through your Utility company’s transformer and jump up to primary voltages. The primary voltages run approximately 7,200 volts. The power company’s transformer will jump your 120-volt power source from your generator to approximately 7,200 volts.

The following loose description is an attempt to make a picture of the high voltage danger you may be producing without knowing it. A 20-amp 120-volt circuit has approximately 4,000 times what it would take to kill a full-grown man. It is my understanding that if your body took a shock of approximately one-one thousandth of one amp your body would feel a tingling sensation. Two-one thousandth of one amp and your body would feel your hair stand on end. Three-one thousandth of one amp and your body would feel severe pain. Four-one thousandth of one amp and your body would most likely have blacked out and be without thought capability. Over five-one thousandth of one amp and your body would most likely BE DEAD! Electrical shock will vary in intensity depending on the grounding source your body is in contact with. The better you are grounded the more amps would be created.

200 amps at 240 volts would equal approximately 6.7 amps at 7,200 volts. As you can see, the higher the voltage the less grounding contact you would have to experience before major bodily harm would occur. The following is a good description of what would happen if you came in contact with a 7200-volt line. You would probably consider yourself lucky if you only lost an arm or leg during the accident and you were still alive. This damage to a person can happen in a split second contact with a primary line. If you were to install a generator and energize the utility company’s primary lines by a back feed from your generator, the authorities would most likely trace the power source back to your generator. It is my understanding that if you are found to be at fault for the death of a service man, you could be criminally charged in this described incident.

Please make double sure that you have installed some type of safety transfer switch between your generator powered dwelling and the serving utility company’s feeder from their transformer to your dwelling!

In the following , I will attempt to describe a few wiring designs of different type projects.

Please remember this is designed as a self-help guide only.

Here are some pictures showing accepted designs providing power to a limited number of emergency lighting circuits, or a complete dwelling, or a complete farm load:

Remember that a generator is a separately derived source and must have its own grounding source provided for the generator.

The National Electrical Code does not require an overcurrent device until the nearest point of entry of the structure (just as your feeder enters the structure), Please check with your serving Utility Company to make sure that they do not require a disconnect at the line side of metering device. I believe that you will find your serving utility company will accept the pole top disconnect as their required disconnect form. The Authority having Jurisdiction (Electrical Inspector) should accept the pole top disconnect as your generators transfer box, if the pole top is designed as a transfer box. You should confirm with the supplier of the pole top disconnect that it is UL, approved as a transfer box.

Feeders from the pole top disconnect to your buildings may be ran either overhead or underground. Please keep in mind that the National Electrical Code does not allow you to install both the line conductors and the load conductors in the same conduit.

Remember that the grounding service conductor must be unbroken from the grounding service in the main service panel or meterbase which ever is required.

Definition of a grounding service conductor is that sole conductor and its entire length between the grounding source and the meterbase or main service panel.

Remember that the pole supporting the pole top disconnect is considered a structure and requires its own grounding source (ground rod).

You may run a three wire cable to each structure and install a new grounding source at each separate structure.

You may run a four wire cable instead of a three wire cable using the fourth wire as a grounding conductor. If you run a four wire cable to each structure then each structures main service will become a non service rated panel. (Sub Panel) using the service structure as the main panel. If you do remember to separate the neutrals and ground in the non service rated panels so that you can avoid a paralleling affect causing the bare grounding conductor to become a current carrying conductor, if the insulated neutral conductor fails.

This document is based on the 1999 national electrical code and is designed to give you an option, as a self-help, that should pass minimum code requirements. While extreme care has been implemented in the preparation of this self-help document, the author and/or providers of this document assumes no responsibility for errors or omissions, nor is any liability assumed from the use of the information, contained in this document, by the author and / or provider.

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