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#91731 - 02/01/05 09:34 PM Article 702 Optional Standby Systems  
shortcircuit  Offline
Member
Joined: Jun 2004
Posts: 608
massachusetts
OK...here we go again with generator questions...

I want to go over some of the requirements of the installation of a permanently installed backup generator for supplying optional standby power for a single family dwelling.

Customer has an old diesel powered 25kva generator he wants to use to back feed his home.The house has a 200amp meter main outside with 4/0 SER run into a 40 circuit 200 amp sub panel.

I plan to cut a 200 amp, 3pole, double throw safety switch(GE cat#TC35324)by the sub panel. I will be switching the grounded conductor as well as the ungrounded conductors through this switch.

Should the generator neutral be bonded to the generator frame? Should a grounding electrode be established at the generator?

I will have to locate a disconnect with overcurrent protection at the generator.

How can I determine the available fault current of the generator for proper selection of equipment as required by 702.5

Any advice on proper installation would be appreciated.

shortcircuit


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#91732 - 02/01/05 11:30 PM Re: Article 702 Optional Standby Systems  
Ryan_J  Offline
Moderator
Joined: Aug 2003
Posts: 1,374
West Jordan, Utah, USA
Quote
Should the generator neutral be bonded to the generator frame?


Yes, because the nuetral is switched and is therefore a seperatley derived system.

Quote
Should a grounding electrode be established at the generator?


Yes, because the nuetral is switched and is therefore a seperatley derived system.

Quote
How can I determine the available fault current of the generator for proper selection of equipment as required by 702.5


I have been told to use the KVA rating of the generator as though it were a transformer and use the infinite bus primary method. Assuming a 3% impedance you would have about 3,475 Isc available.
I=KVA/V/Z%
I=25,000/240/.03


Ryan Jackson,
Salt Lake City

#91733 - 02/01/05 11:34 PM Re: Article 702 Optional Standby Systems  
tdhorne  Offline
Member
Joined: Mar 2001
Posts: 345
Maryland, USA
Since you are supplying an unbonded panel through a switch that isolates the neutral you should bond the neutral to the generator frame and provide the appropriate grounding electrode system for the derived system that is the generator. You can run an appropriately sized Grounding Electrode Conductor (EGC) to the existing grounding electrodes for the utility service equipment.

If the service equipment is by any chance SquareD then they make a kit for interlocking the main breaker with the position 2-4 breaker. That would not switch the neutral but if you can unbond the generator neutral from the frame and use the service equipment main bonding jumper as the bonding point you will save a lot of money over the cost of three pole transfer switch.
--
Tom H


Tom Horne

"This alternating current stuff is just a fad. It is much too dangerous for general use" Thomas Alva Edison

#91734 - 02/02/05 12:24 AM Re: Article 702 Optional Standby Systems  
Bob  Offline
Member
Joined: Feb 2002
Posts: 182
Mobile, AL, USA
Why do you want to switch the neutral? There is no advantage and the hook up is simpler by not switching the neutral.
There should be on the name plate a symbol
Xd' or Xd". Use it as you would the %Z on a transformer. If this is a 3 phase unit don't forget the sqroot of 3 in the caculatons.


#91735 - 02/02/05 07:20 AM Re: Article 702 Optional Standby Systems  
shortcircuit  Offline
Member
Joined: Jun 2004
Posts: 608
massachusetts
Thanx for the replies guys...I seached and found an old discussion on generators on this forum: http://electrical-contractor.net/ubb/Forum2/HTML/001482.html

So...if the XO is bonded at the generator, I must switch the neutral and establish a grounding electrode system.

If the neutral is floating at the generator,I would not switch the neutral and do not need to install a electrode at the generator.

Where I float the neutral and use the dwellings remote grounding electrode system,signage as required by 702.8(A)&(B) must be located...(A) at the service entrance equipment...and (B) at the "grounding location"...grounding location being my point of connection of the grounding electrode conductor to the systems grounding electrode?

shortcircuit


#91736 - 02/03/05 05:05 PM Re: Article 702 Optional Standby Systems  
Bob  Offline
Member
Joined: Feb 2002
Posts: 182
Mobile, AL, USA
Shortcircuit
You make the decision whether to switch the neutral. It is your choice. There is no advantage to switching the neutral. If you decide to switch it you must
1. Bond the frame and neutral terminal together.
2 Install a suitable grounding electrode
(ground rod or other) connect to frame.


If you choose not to switch the neutral you must
1. remove the neutral to frame bond.
2 install an equipment grounding conductor from the existing electrical system.



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