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Portable Generator House Backup Grounding

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DennisCTB
Join Date: Nov 1998
Posts: 2644 NorthWest NJ
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2012-12-05          185611


I have a Portable Generator that is connected to a transfer switch via a 240 twist connector. Is it true that since it is connected to the transfer switch which is grounded via the main panel that I don't have to ground the Portable Genset with a ground stake directly to the Portable Genset?

My generator a manual just says to obey local codes in regards to using the grounding terminal. My dealer says you would only ground the generator if it was not connected to a whole house grounded panel.



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Portable Generator House Backup Grounding

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greg_g
Join Date: Jan 2004
Posts: 1816 Western Kentucky
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2012-12-05          185613


All current from the generator must return to the generator. If you do not have a grounded (neutral) conductor from the generator to the house you will have an open neutral and will damage the household electrical equipment.

My backup generators are usually stored in the shed until I need emergency power, then one or both are usually wheeled to their operating location as required. As such, I have run a permanent 10ga solid copper wire from my service entrance ground to that location. Once I move a generator into place, the ground wire's already there waiting for hookup. When I run them both simultaneously, I simply bond them together with another piece of 10ga BC.

//greg// ....

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Portable Generator House Backup Grounding

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DennisCTB
Join Date: Nov 1998
Posts: 2644 NorthWest NJ
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2012-12-05          185614


I searched for the electric code on grounding generators which I posted below. It seems that if my set up is not a "Separately derived system" I do not need to ground it at the generator. The code suggests that most transfer switches do no require a separate Gen-set ground because you would have to go out of your way to make it a separately derived system, not normal practice.

Anyway I am in an area I don't have any knowledge on and appreciate everyone's input.

I will have to check with the Electrical inspector to make sure that is the standard that they approved when it was installed 3 years ago. I'd much rather sink the grounding rod then be wrong, can grounding a system that does not not require grounding at the Genset be a problem or is it just redundant?


"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.

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.

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.

Most generators are connected through transfer switches that do not transfer the neutral. Switching the neutral is done by design, due to other electrical requirements.
Where required for a generator, the purpose of the ground rod(s) at a generator is to limit the voltage from surges, lightning or unintentional contact with high voltage lines, and to limit the voltage to ground on conductive metal parts."
....

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Portable Generator House Backup Grounding

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greg_g
Join Date: Jan 2004
Posts: 1816 Western Kentucky
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2012-12-05          185619


Asking the contractor is probably best, because the answer is in both my response and in the article you cited - which tells me you don't quite grasp either. So let me summarize by combining the two; start by determining which of the two types of transfer switches you have. One type transfers neutral. The other type doesn't. If you've got the former, it satisfies the "all current must return to the generator" requirement. If it doesn't, you need to bond the generator housing to your structure common ground.

Unless you're going to drive it in more than 150' linear feet from the house ground, don't waste your money on a new ground rod (supplemental electrode) just for the generator. Code requires that supplemental electrodes be bonded to the common ground anyway. You can do that with the 10ga solid copper I originally mentioned. If you drive in another rod within 150' of the common, you risk introducing ground loop interference. Your electronics won't like that.

//greg// ....

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Portable Generator House Backup Grounding

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taogden
Join Date: Sep 2012
Posts: 81 United States
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2012-12-06          185625


Dennis, I agree with Greg on this, you don't sound comfortable with this, I would get some local help. You need to identify the types of switches you have and how it will be tied to your house. I would not tie in a generator to my house without using the housing ground connection, to do so could cause problems or actual damage to your generator, house equipment and you or your family. This would not be something to practice on. I have an uncle who did his own dryer installation, I am surprised it did not kill my aunt, the dryer skin had electricity to it and it still ran, he thought he did fine until I showed him why she was complaining of being shocked. This is not a place to play if you don't know and understand what you are doing. ....

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Portable Generator House Backup Grounding

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DennisCTB
Join Date: Nov 1998
Posts: 2644 NorthWest NJ
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2012-12-06          185628


Quote:
Originally Posted by greg_g | view 185613
If you do not have a grounded (neutral) conductor from the generator to the house you will have an open neutral and will damage the household electrical equipment. ....


Hi Greg thanks for your help! I am asking my towns electrical inspector for help on this.

In reference to your statement above, if I buy a cheap outlet tester that tests for seven conditions: ground fault interruption, open ground, open neutral, open hot, hot/ground reverse, hot/neutral reverse, and correct wiring, and test outlets while under generator power will that help determine if there is an issue with my set up ?

By the way I have been using a portable generator with current switch for a couple of years during several multi week outages, running the well, two furnaces, hot water heater, wide screen led TV, computers etc. with no problems.

I just sold my old gen and bought an new Honda EM6500 and reading the manual prompted my question and research on this. ....

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Portable Generator House Backup Grounding

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greg_g
Join Date: Jan 2004
Posts: 1816 Western Kentucky
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2012-12-07          185630


It never hurts to have an electrical tester around. But you really need to get the owner manual/installation manual/spec sheet on your transfer box. Given no history of electrical casualties, you may have the type that returns neutral to the generator. But it would be reassuring to know for sure.

//greg// ....

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____________________________________________________________________________________
Portable Generator House Backup Grounding

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DennisCTB
Join Date: Nov 1998
Posts: 2644 NorthWest NJ
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2012-12-11          185677


I bought an outlet tester and tested the circuits in the house under generator power and they show the outlets being correctly wired. Same on the Generator 120 outlets.

The tester tests for open ground,open neutral, open hot,hot grd reverse,hot neutral reverse.

On the the transfer switch, I guess I don't have a transfer type switch really. Transfer switches take splices off the main panel and fill your main power box with twist connectors, not a pretty site in my opinion.

What I have is a Generator Sub Panel. My house wiring that will be run by the generator has home run wires that connects directly to the Generator subpanel. The generator subpanel is fed from one of my main panels by a 60 amp breaker.

The generator sub panel has two breakers on an interlock that toggles between Genset 30amp and House Panel 60 amp, only allowing one or the other breaker to be on.

So my set up has no user manual that an after market "transfer switch" would have.



....

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Portable Generator House Backup Grounding

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greg_g
Join Date: Jan 2004
Posts: 1816 Western Kentucky
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2012-12-11          185679


In that case, I don't see a neutral/fault return path to the generator. In addition to the mismatched ground potential I described earlier, you also risk isolating the generator circuit breakers. I still recommend you follow my original advice; bond the generator housing to the residential common ground (typically under the electric meter).

//greg// ....

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Portable Generator House Backup Grounding

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greg_g
Join Date: Jan 2004
Posts: 1816 Western Kentucky
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2012-12-17          185738


Here's a power company brochure on the subject Hal. The separate housing ground I recommended is illustrated in the diagram

//greg// ....


Link:   How to Safely Install Your Standby Generator

 
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Portable Generator House Backup Grounding

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dododo
Join Date: Aug 2005
Posts: 19
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2014-12-13          191625


I have pondered this myself as I find the documentation that came with genset confusing. According to my dealer if I am connecting the generator with a 30 amp 4 prong plug to a generator subpanel I do not have to ground the generator with a ground stake at the genset.

My research finds the following:

Generator 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]
....

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Portable Generator House Backup Grounding

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dododo
Join Date: Aug 2005
Posts: 19
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2014-12-13          191626


This is a Separately derived System ....

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Portable Generator House Backup Grounding

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dododo
Join Date: Aug 2005
Posts: 19
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2014-12-13          191627


Not Separately Derived System ....

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Portable Generator House Backup Grounding

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Duddy03
Join Date: Sep 2011
Posts: 3 Mascoutah, IL
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2015-04-20          192600


Interesting discussion. A little off topic: I have a small Generac generator. The 3-prong outlets have an open ground. That is, the neutral and ground terminals are open when you would assume and expect them to have no resistance between them. My first generator had this condition and a replacement generator from Generac has the same situation. Can any of the forum members use a common electrical tester on their own generator and see if it checks out OK or if it also shows an open ground? I don't have a transfer switch for use at home. In a blackout situation I would connect the "floating ground" terminal on the generator with the house ground rod. The generator could be connected into the house wiring, I assume, if (and only if) I open the main circuit breaker to prevent voltage from entering the commercial electrical grid. When camping, I would like to tie the generator to a GFCI device, a temporary ground rod and the chassis of my RV. I've asked Generac for help with the wiring diagram. The open ground on their generator has me concerned since this doesn't meet electrical codes and a GFCI will not "set" if there is a wiring error. Your ideas? ....

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