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Old 09-28-2011, 06:33 PM   #1
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To bond or not to bond.....


Hi, Got a whole house generator question maybe someone can help with. here is the setup. I have a 200 amp SE entering the house with a 200 amp load center and a 100 amp sub panel branching off of that. The main load center has the neutral and the ground bonded together as it should be and the 1oo amp sub panel is not bonded as it should be ( I believe that is correct?) Anyway utilizing a whole house automatic standby generator.....a 200 amp ATS (automatic transfer switch) installs between the SE and the main load center. Here is the question:
The generator comes with a jumper enabling you to bond the neutral and the ground at the generator connection. They state it will work either way...but there is a right way and a wrong way and I wonder what is the right way in this case. The generators will power the main panel and everything it feeds including the non-bonded 100 sub panel.
Does the generator need the bonding jumper or should the neutral and the ground remain isolated?
If I had to guess I would say it remains non-bonded at the generators output.....but I would like to know for certain. I hope I made my question clear for those with the knowledge I am seeking. Thanks in advance for any info.

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Old 09-28-2011, 06:59 PM   #2
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To bond or not to bond.....


I think bonding them would override the ats.

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Old 09-28-2011, 08:57 PM   #3
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To bond or not to bond.....


Great question. And it does not need the bonding jumper to operate properly. If you are installing this generator per local code, the AHJ will be certain to point out what you need to do....either by red tagging it if you do it wrong, or actually offering advice. (good luck with the advice part) Bottom line is it will work either way. Mine is not bonded. You can also contact the mfg. of the generator and seek advice there. Does your generator switch the neutral as well? And how large is your generator? A 200 amp service with a 100 amp sub will have you running around switching off breakers if you generator isn't large enough.
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Old 09-28-2011, 09:24 PM   #4
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To bond or not to bond.....


If your transfer switch does not interrupt the neutral, then the whole discussion is a moot point. The neutral is already bonded at the main service panel, and that does not change when you are in standby power mode.

Any ground faults will be ultimately connected thru the neutral at the bonding point, and that carries the fault current back to the source, whether it is the utility transformer, or your gen set.
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Old 09-28-2011, 10:32 PM   #5
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To bond or not to bond.....


kbsparky>>> Thanks. That is exactly what I thought because the transfer switch does not interrupt the neutral. ....therefore the generator need not be bonded. P.S.to Missouri Bound..it is a 20KW generator...should not need to be switching off breakers...hopefully Thank you both for great advice!
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Old 09-29-2011, 03:50 PM   #6
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>>>>TO KBSPARKY>>>>>I have another question I would like your advice on. This is a whole house generator rated at 120/240 18kW at 75 amps utilizing natural gas / 20 kW @ 83 amps utilizing LP gas. The pre-wired connection at the gen set uses #6 copper (I believe #6 copper is rated @75 amps?) At any rate...with a distance of approx 25 feet from gen set to transfer switch...what would be the required (copper) AWG size per leg? If #4 copper is rated @ 95 amps at 90 degrees....would that be sufficient..or is there some other calculation used to determine cable feed size from gen to ATS? Thanks for any info you can provide.
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Old 09-29-2011, 07:51 PM   #7
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I'd go with #3 THHN copper conductors, as you have to upsize those wires by 15%.

You also have to use the 75 degree column, since most electrical equipment termination lugs are not rated for 90 degree connections.

That would put you above the capacity of the #4 wires you were proposing to use.

As for the size of the internal wiring in the gen set itself, those are part of a listed assembly, and not subject to the limitations of the NEC.
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Old 09-30-2011, 09:37 AM   #8
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To KBSPARKY: To expand on the bonding issue...here is another question if you will: Prior to the installation of a Automatic Transfer Switch..as I stated..the200 amp Main load center is bonded...the 100 amp sub panel is not. However; Once you put a transfer switch between the service entrance and the Main 200 amp load center.....does the bonding screw in the 200 amp main load center need to be removed.....(because the bonding now takes place in the Automatic Transfer Switch?)In the transfer switch..the neutral is not broken..the neutral bar AND the ground bar are both mounted to the metal box (no insulators) so the neutral is bonded to the ground in the transfer switch. So ,again...would that require the 200 amp main AND the 100 sub panel to be separated IE: UN bonded??
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Old 09-30-2011, 09:52 AM   #9
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Are you using a service rated transfer switch?
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Old 09-30-2011, 11:41 AM   #10
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Correction: Kohler Model RRT-JFNC-0200A

Checked deeper and it is NOT the service entrance model.(sorry if you saw prior post)
thanks.

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Last edited by jodejr; 09-30-2011 at 12:22 PM. Reason: information error
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