Forums | Home Repair | Home Improvement | Painting | Interior Decorating | Remodeling | Landscaping


Go Back   DIY Chatroom Home Improvement Forum > Home Improvement > Electrical

CLICK HERE AND JOIN OUR COMMUNITY TODAY...IT'S FREE!

Reply
 
Thread Tools Search this Thread Display Modes
Old 09-05-2011, 09:43 PM   #1
Newbie
 
Join Date: Jul 2011
Posts: 15
Share |
Default

Detached Garage Subpanel + Generator Back Feed


I'm a couple weeks away from closing on a house and in the process of taking care of some electrical things. First and foremost, my friend, a licensed electrician, is getting rid of a bunch of knob and tube. At the same time, I'm having him upgrade the service from 100 amp to 200 amp and put in a new load center.

Right now, the detached garage is fed in a highly unsafe way with a piece of Romex hanging about 7 feet in the air from the house to the garage. Fixing this is one of the first things I will be taking care of myself.

My current plan is the following:

(1) Bury 2" PVC conduit from house to garage.
(2) Install a 100 amp subpanel in the garage.
(3) Feed subpanel with 2/2/2/6 THWN and properly bond the panel with 2 ground rods.

Not having power for about 5 days after Hurricane Irene has added another item on my list. I plan to get a ~10 kW portable generator and back feed it into the main panel with a 50A breaker. Since I want to do this by the book, I will be installing an interlock on the main panel to prevent the back feed from being on when the main panel is on.

The convenient place for the generator to sit and hook up is near the garage. Thus, I plan to put the back feed receptacle on the exterior of the garage, and leverage the same 2" conduit to run the back feed cabling. I plan to run 6/6/6/10 THWN for the back feed. However, I can get rid of the need for running the neutral and ground if I simply tie them into the garage subpanel.

The question: is this code (NEC 2008) compliant? It's not as if the neutral and ground are switched/fused anywhere, so it doesn't change the "topology" of the circuitry too much.

Whether or not I have to run a separate neutral and ground, I'm also curious about derating. In steady state, there are normally only 3 CCCs, since the genset isn't active. When the genset is active, worst case is 100% of the 50A from the genset go to the main panel and the back to the subpanel. This gives us 6 CCCs (assuming dedicated neutral for the generator) and in this case, the 2/2/2/6 at an 80% derate is way over 50A. The 6/6/6/10 at 75C is also > 50A with the derate.

Thoughts?

insaneirish is offline   Reply With Quote
Old 09-05-2011, 10:30 PM   #2
Member
 
Join Date: Nov 2007
Location: Nashua, NH, USA
Posts: 6,781
Default

Detached Garage Subpanel + Generator Back Feed


It is not permissible to feed a generator into the garage subpanel unless the latter is restricted to generator power. You would need to put the transfer switch in the line between the main panel and subpanel or put the or cam/slider interlock on the breaker that feeds the subpanel.

To have both the main panel and subpanel fed by utility power you would need to run separate hot and neutral generator lines up to the main panel where a backfeed together with cam/slider interlock is okay.

For a standard hot-hot-neutral 120/240 volt single phase feed the neutral carries the difference between the loads on each of the hot lines; if both hot lines were supplying the same current for example 50 amps each then there would be zero amps on the neutral. Here the neutral is not counted as a "current carrying conductor" for derating purposes.

__________________
The average homeowner who lost his house in the Oklahoma tornadoes should move for good and not rebuild. Too much complexity watchdogging the contractor. Too much a chance to be defrauded.

Last edited by AllanJ; 09-05-2011 at 10:50 PM.
AllanJ is offline   Reply With Quote
Old 09-05-2011, 10:38 PM   #3
Newbie
 
Join Date: Jul 2011
Posts: 15
Default

Detached Garage Subpanel + Generator Back Feed


So as long as I do keep the cables completely separate is everything okay? In other words, none of the back feed cabling will touch the garage subpanel, it will just utilize the same conduit back to the main panel. Make sense?
insaneirish is offline   Reply With Quote
Old 09-06-2011, 02:09 AM   #4
Semi-Pro Electro-Geek
 
Join Date: Jul 2009
Location: Arizona, USA
Posts: 2,461
Default

Detached Garage Subpanel + Generator Back Feed


Quote:
Originally Posted by insaneirish View Post
So as long as I do keep the cables completely separate is everything okay? In other words, none of the back feed cabling will touch the garage subpanel, it will just utilize the same conduit back to the main panel. Make sense?
Yes, this is fine. The neutral for the generator must be separate from the subpanel neutral. I'm not sure a second grounding conductor is necessarily required. Can it be tied to the subpanel's equipment grounding bus instead of running a second EGC? An electrician or inspector can probably answer this from a code standpoint.
mpoulton is offline   Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Restoring power to detached garage alexmce Electrical 4 05-31-2011 05:27 PM
Painting Detached Garage Exterior bsmith95610 Painting 3 02-15-2011 06:52 PM
Standby generator connected via detached garage? vsheetz Electrical 22 04-04-2009 02:44 AM
detached garage with a subpanel amakarevic Electrical 10 03-24-2009 05:27 PM
detached garage subpanel help edmart2 Electrical 7 03-19-2009 12:01 PM




Top of Page | View New Posts

Copyright © 2003-2014 Escalate Media. All Rights Reserved.