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Correctly grounding a house with back-up generator and multiple sub-panels

Correctly grounding a house with back-up generator and multiple sub-panels

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  #1  
Old 04-04-13, 09:18 PM
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Correctly grounding a house with back-up generator and multiple sub-panels

Welcome to my nightmare....

I'm replacing my main panel and adding a back-up generator, and in the end it will be something like this:

The power drop to my home comes in by my shop, where the electrical meter sits in a panel with a 200 amp main breaker. There is a single sub-panel from this panel that powers the shop, and a back-up generator with an automatic transfer switch for sending power to the house.

Power leaves the switch and runs underground in conduit to a primary panel on the house 100 feet away, where two 100 Amp breakers split it off to two more separate sub-panels in different parts of the house.

Here is the question: how to I correctly GROUND all of this?

Since this is complicated, I've made diagrams of the four possible options I see:

#1 Have everything ground back to a single rod at the main panel, INCLUDING the generator, and have the main panel be the ONLY place where neutral and ground are connected together.

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#2 Same as above, but the generator gets its own ground rod.

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#3 Have the primary panel on the HOUSE have its own ground rod (with no ground wire connecting the shop to the house) and have neutral and ground in this panel also be connected.

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#4 Same as above, but the generator gets its own ground rod.

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Or......something else entirely? I know sub-panels in theory should never have neutral and ground connected, but in this case would the primary panel at the house still be considered a sub-panel if it had its own rod and there was no ground wire connecting it to the main panel back at the shop?

And is it bad/wrong/correct/better for the generator to have its own grounding rod as well?

All responses are appreciated, and thanks for even checking out my absurdly long post. I'm surprised you made it this far....
 
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  #2  
Old 04-04-13, 10:06 PM
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The correct answer can be different from city to city. You need to check your local building code.
 
  #3  
Old 04-04-13, 10:07 PM
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There is only ONE way to properly connect this and none of your pictures show it. First of all, ignore the presence of the generator since it has no bearing on how to run the grounding electrode and equipment grounding wiring.

The meter and main circuit breaker is the first point of disconnection and this is where the neutral/ground bond is made and NO OTHER LOCATION! You will most likely need two, eight-foot long grounding electrodes (ground rods) driven fully into the ground a minimum of six feet apart and connected together and then to the neutral/ground bond with #6 solid copper wire.

ALL panels after this point MUST be wired as "sub-panels" with the neutral bus isolated from the enclosure and the equipment ground bus (probably need to be purchased and installed separately) "bonded" to the enclosure. The panel at the house will also need to have a grounding electrode system consisting of ground rods and #6 minimum connecting to the grounding bus in the panel. This is in addition to having a minimum of a #6 copper equipment ground running from the transfer switch to the house panel(s).

Your transfer switch will need to be of three-pole construction AND with a 200 ampere rating. You will need to switch the neutral with this switch. The generator needs to have the neutral and ground bonded at the generator and the frame of the generator will need to be grounded to the grounding electrode system.
 
  #4  
Old 04-04-13, 10:27 PM
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Well covered. I just have one question ...... why the 3 pole transfer switch. Switching neutral ?
 
  #5  
Old 04-04-13, 10:39 PM
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Yes, switching the neutral.

Your transfer switch will need to be of three-pole construction AND with a 200 ampere rating. You will need to switch the neutral with this switch.
 
  #6  
Old 04-05-13, 07:48 AM
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Thanks for the reply I have one question:

The generator I'm using was originally installed near the house panel, and the transfer switch consisted of two 100 amp switches---one for each of the 100 amp breakers that lead to the each of the sub-panels in the house. These were switches that came with this system (Generac) and they were only two pole. I have a 200 amp switch that will replace them in the new switch box, and it is ALSO only two pole. I'm not questioning your knowledge/wisdom....but I'm curious why you feel is it necessary to switch the neutral when the OEM system before didn't?

Again, thanks for the reply
 

Last edited by deathvalleydog; 04-05-13 at 08:46 AM.
  #7  
Old 04-05-13, 11:52 AM
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Switching the neutral is the preferred method in the latest code and it also eliminates the probability of unwanted parallel paths for return currents.
 
  #8  
Old 04-05-13, 12:46 PM
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Gotcha. The old system was installed about ten years ago, so that makes sense. Thanks again for all your help
 
  #9  
Old 04-05-13, 05:17 PM
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Furd, great explanation!

OP, I noticed in all of your scenarios you wanted the generator connected to a ground rod. As I recall, the NEC does not require a ground rod at the generator unless it's a separately derived system, but I also seem to remember that most inspectors want to see one. Furd, can you weigh in on this? What's your opinion?
 
  #10  
Old 04-05-13, 05:26 PM
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As I recall, the NEC does not require a ground rod at the generator unless it's a separately derived system, but I also seem to remember that most inspectors want to see one. Furd, can you weigh in on this?
Originally Posted by Furd
the frame of the generator will need to be grounded to the grounding electrode system.
And I agree. OSHA requires it on construction sites. Not sure why the NEC hasn't added it for homes.
 
  #11  
Old 04-05-13, 05:38 PM
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If your generator DOES NOT have a neutral/ground bond then you do not have to switch the neutral per Code.

Check with the generator manufacturer - actually, it is required to be labeled as such.


The generator I'm using was originally installed near the house panel, and the transfer switch consisted of two 100 amp switches---one for each of the 100 amp breakers that lead to the each of the sub-panels in the house. These were switches that came with this system (Generac) and they were only two pole. I have a 200 amp switch that will replace them in the new switch box, and it is ALSO only two pole.
 
  #12  
Old 04-05-13, 06:00 PM
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I am not sure I have ever seen a standby generator connected through an automatic transfer switch that had a neutral/ground bond.
 
  #13  
Old 04-05-13, 08:41 PM
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If your generator DOES NOT have a neutral/ground bond then you do not have to switch the neutral per Code.
What section is that in ?
 
  #14  
Old 04-06-13, 01:51 AM
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When the neutral is NOT switched the NEC does not consider the generator to be a "separately derived system". That means that the generator may NOT have an internal neutral/ground bond. Since most portable generators made today DO have a neutral/ground bond they MUST be connected as a separately derived system to get around having two neutral/ground bonds; that means that the neutral DOES need to be switched.

Think of it this way: When you install a transformer from a higher voltage to a lower voltage, let's use a 480Y/277 primary to a 208Y/120 secondary for example, you make a neutral to (transformer) case bond AT THE TRANSFORMER and connect a grounding electrode conductor to that bond point and the grounding electrode system. You will run separate equipment grounding and neutral conductors to the 208Y/120 volt panelboard and you will terminate the neutral on an insulated neutral bus while the equipment ground is terminated on a bonded (to the panelboard enclosure) equipment grounding bus. This is common and every commercial/industrial electrician has done it, or at least seen it in the field. Remember that there are NO disconnects or overcurrent protective devices between the point of the neutral/case bond and the first OCPD in the panelboard. There is also NO point beyond the neutral/case bond where the neutral and the equipment ground is ever again connected except under fault conditions.

Now, substitute a generator for the transformer. If the generator is to be a separately derived source the same rules apply, you MUST have a neutral/case/equipment ground bond AT THE GENERATOR and this bond must also be made to the grounding electrode system. You MUST isolate this neutral from the utility's neutral/ground bond or else you WILL have parallel current flow on the neutral and equipment grounding conductors. The option is to NOT have the neutral/ground bond at the generator, not switch the neutral and therefore not have a separately derived system.

Obviously if you have a large fixed generator, and especially if that generator has an ungrounded Delta-wound 3 phase output feeding only 3 phase loads and transformers then the issue of grounding and bonding is moot. The power going through step-down transformers, properly connected, then becomes separately derived system with the requisite bonding of the neutral and equipment grounding conductors.

However, with single phase, 240/120 generators it isn't so easy. With no transformer(s) there isn't the isolation inherent in a transformer installation. You need to properly ground and bond the generator output for safety while at the same time eliminate any chances of parallel current paths under normal conditions.

It gets more involved but I'm tired of typing now.
 
  #15  
Old 04-06-13, 08:28 AM
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Here is the interconnection diagram for the generator in question, scanned from the manual.

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I have to say I'm a bit confused: at first glance it seems to show neutral and ground bonded at the generator, because it looks like there are TWO wires coming out of the neutral connection at the generator control panel. Along with the two hot leads from the breaker, FOUR wires total enter the conduit.

But there are only THREE wires exiting the conduit at the switch box: two hots and one neutral....and the neutral is definitely not switched.

So...... What is that other wire? Is it the ground? And where did it go? Or am I just misreading the diagram?...( I'll be the first to admit I might be doing just that )

Furd: If I understand your last post correctly, you're saying that if the neutral is NOT switched, as this diagram shows, then ground and neutral should NOT be bonded at the generator?
 
  #16  
Old 04-06-13, 10:16 AM
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NEC 702.

I'm referring to a NON separately derived system. It's very common for residential services. Anyone that use an "interlock" on their panel is operating under such a system as the neutral is NOT switched.
 
  #17  
Old 04-06-13, 11:21 AM
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What does the schematic for the generator show?
 
  #18  
Old 04-06-13, 11:31 AM
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Originally Posted by Nashkat1
What section is that in?
NEC 702.

I'm referring to a NON separately derived system.
That sounds like Section 702.11(B). OK, at the risk of getting a bit more technical that most DIYers like to go, here's what Section 702.11 says:
702.11 Portable Generator Grounding.

(A) Separately Derived System
. Where a portable optional standby source is used as a separately derived system, it shall be grounded to a grounding electrode in accordance with 250.30.

(B) Nonseparately Derived System. Where a portable optional standby source is used as a nonseparately derived system, the equipment grounding conductor shall be bonded to the system grounding electrode.
DVD, are you using a portable generator to supply your standby power? If so, you can choose to ground it either way, depending on whether you are using it as a separately derived system or not.
 
  #19  
Old 04-06-13, 02:23 PM
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Furd: If I understand your last post correctly, you're saying that if the neutral is NOT switched, as this diagram shows, then ground and neutral should NOT be bonded at the generator?
That is correct. With no neutral/ground bond at the generator the system is a "non separately derived system" according to the NEC. The neutral/ground bond at the utility service entrance will be the bond point for such a system. It is not the best but in my opinion the hazard is minimal, especially for a standby generator that is permanently wired to the transfer switch. Be that as it may, some local authorities are requesting/requiring that any generator installation be neutral switching and the generator neutral be bonded to equipment ground and the earth at the generator. Also, most portable generators manufactured today DO have such a bond, making the use of a neutral switching transfer switch or panel a necessity when using the portable to supply permanent premises wiring.
 
  #20  
Old 04-07-13, 09:23 PM
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Yeah, that's pretty technical for my pay-grade

It's definitely NOT a portable generator: GENERAC Guardian model 04079-1. It's bolted to a concrete pad and is powered by the same propane tank that supplies my house.

But I'm unclear on the difference between a "separately derived" and "nonseparately derived" system. Which one would mine be?
 
  #21  
Old 04-07-13, 09:28 PM
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So the neutral/ground bond (or lack thereof) is what determines "separately derived" vs. "non-separately derived"? I guess I'm still hazy on the difference...
 
  #22  
Old 04-07-13, 10:28 PM
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Section 702.11 only applies to a portable generator. For yours, see Section 702.7(B):
(B) Grounding. Where removal of a grounding or bonding connection in normal power source equipment interrupts the grounding electrode conductor connection to the alternate power source(s) grounded conductor, a warning sign shall be installed at the normal power source equipment stating:
WARNING
SHOCK HAZARD EXISTS IF GROUNDING
ELECTRODE CONDUCTOR OR BONDING JUMPER
CONNECTION IN THIS EQUIPMENT IS REMOVED
WHILE ALTERNATE SOURCE(S) IS ENERGIZED.
 
  #23  
Old 04-08-13, 01:07 AM
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So the neutral/ground bond (or lack thereof) is what determines "separately derived" vs. "non-separately derived"?
No. The determining factor is if the utility is completely disconnected from the premises wiring when the generator is used. In a non-separately derived source the neutral conductor from the utility is still connected to the grounding electrode system and the premises wiring. Using the three-pole (neutral switching) transfer switch will disconnect ALL the utility conductors completely from the premises wiring system. The generator would then be connected solely to the premises wiring system, including the grounding electrode system. While this then requires a neutral/ground bond at the generator for complete compliance the existence of lack of this bond has no bearing on whether or not it is a separately derived source.


Your generator ideally would have a neutral/ground bond at the generator and before the generator's circuit breaker but the bond can be made after the circuit breaker or at the transfer switch. Wherever the bond is made is also where the neutral and equipment grounding conductors part company and must continue as separate conductors.
 
  #24  
Old 04-08-13, 11:06 AM
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OK, that makes sense. Thanks!
 
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