Portable generator backfeed wiring

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  #1  
Old 01-05-07, 11:55 AM
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Portable generator backfeed wiring

I am considering the following setup for a portable, gasoline-powered backup generator. I would appreciate advice as to whether this is safe and professional. I would like to connect the generator at a garage subpanel and backfeed to the main panel.

My current setup: Utility feeds meter box and 200 amp main load center in house. Main load center feeds detached garage subpanel through 60 amp 2 pole breaker.

My proposed generator setup:
1. Add an interlock switch and 2 pole backfeed breaker to the main load center.
2. Add a generator connection panel to the garage subpanel.
3. Add wiring connected between the generator connection panel and backfeed breaker. This wiring would be fished through same underground conduit feeding garage subpanel.

The point of this configuration is the ability to locate the generator connection near its storage area in the garage. The interlock seems beneficial because the main load center supports it and it avoids relocating the circuits to a transfer switch panel. The generator connection panel would be located inside the garage near the garage door so that the generator can be operated outside the structure.

Thanks for any advice.
 
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  #2  
Old 01-05-07, 12:06 PM
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It seems like you have the right idea in that the generator would only connect to the main panel via an interlocked breaker.

> This wiring would be fished through same underground conduit feeding
> garage subpanel.

What size and type is the conduit, and what conductors are currently installed? Also, what is the distance between the house and garage panels "as the wire flies" and what size is the generator?
 
  #3  
Old 01-05-07, 02:21 PM
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>What size and type is the conduit, and what conductors are currently >installed? Also, what is the distance between the house and garage panels "as >the wire flies" and what size is the generator?

Thanks for your reply. The conduit is 1" PVC. Currently there are 3 #14 awg wires and 3 #6 awg wires. The distance is ~52 feet. The generator would likely be 4500 watts peak / 4000 rated (20 amp twist lock interface).
 
  #4  
Old 01-05-07, 02:23 PM
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Square D is the only panel I have heard of that make the interlock. What ever setup you use it MUST BE IMPOSSIBLE to have the generator and the line power connected at the same time. It can't be you have to remember to turn off the main.
 
  #5  
Old 01-05-07, 03:01 PM
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[QUOTE=crosstalk;1102200]>What size and type is the conduit, and what conductors are currently >installed? Also, what is the distance between the house and garage panels "as >the wire flies" and what size is the generator?

> The conduit is 1" PVC....52 feet

Conduit size and distance shouldn't be a problem.

> Currently there are 3 #14 awg wires and 3 #6 awg wires.

This has me a little concerned; it looks like your subpanel is wired incorrectly. The (3) #6 are obviously the hot-hot-neutral to the subpanel. What are the (3) #14 for? Switched lighting perhaps? The fact that you have those #14 conductors installed requires that you add a #10 green or bare ground from the house to the subpanel ground bar and isolate the grounds and neutrals at the garage.

I'm fairly confident that you could pull black, red, white #12s for the hot-hot-neutral of the generator receptacle (in addition to the #10 ground). I think you're aware that the hot and neutral wires to the generator cannot connect to anything in the garage subpanel. The ground wire from the generator receptacle would land on the garage panel ground bar with the #10 from the house.

Does anyone see a problem with using this conduit for both the subpanel feeder and the generator receptacle conductors?

FYI, http://www.interlockkit.com sells UL listed interlock kits for a variety of panel types.
 
  #6  
Old 01-05-07, 04:22 PM
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Thanks again for your detailed reply. Yes, the #14s are for a three-way switched light above the entrance to the garage (one switch in the garage, one switch in the house). The subpanel in the detached garage has a separate earth ground. Do you still think it is wired incorrectly? In this configuration, do I land the generator ground in the subpanel (i.e., locally grounded)?

Thanks for the link to the interlock site. If I go forward with this project, I think I would rather purchase the one that is made by Siemens for my main panel. The ones sold at this site seem to mount on top of the trim plate instead of inside the cabinet. I supposed it's safe either way, but I would prefer it inside. Plus I think the 20 amp backfeed breaker needs a retention clip in this setup.
 
  #7  
Old 01-05-07, 06:40 PM
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Yes, your sub-panel is wired incorrectly.

You need a ground wire from the main panel in the house to the ground buss in the sub panel, and your ground buss and neutral buss in the sub panel must be separate. The ground rod you have connects to the ground buss as well.

Assuming you add the green ground wire as ibpooks directed, and connect the grounds to the ground buss I think you will be fine.

So pull a total of four new wires.
A #10 green insulated wire for the ground for the sub panel.
A #12 white insulated wire for the generator neutral.
2 #12 insulated wires for generator hots (these can be black, blue, red, etc., but not white or green).
 
  #8  
Old 01-08-07, 06:42 PM
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Thanks very much for the help. Do you think that I have the capacity in my 1" PVC conduit for a 30 amp generator feed? Can I build my own generator outlet (junction box or enclosure, receptacle and cover) or should I purchase one (like sold at the interlock site)? Also, why is the ground for the subpanel sized as a #10?
 
  #9  
Old 01-09-07, 09:25 AM
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> Do you think that I have the capacity in my 1" PVC conduit for a 30
> amp generator feed

You are at 29.1% fill in the 1" PVC with (3) #6, (3) #12, (3) #14, and (1) #10. Maximum allowed fill is 40%.

> Can I build my own generator outlet

Yes, but by the time you buy the components it may not be any cheaper. If the generator hookup is outdoors, I recommend buying one as those have been approved for wet locations.

> why is the ground for the subpanel sized as a #10?

The ground is sized to match the largest ampacity circuit in the conduit which is the 60A subpanel feed. NEC Table 250.122 specifies a #10 copper ground should be used for 30-60A circuits.
 
  #10  
Old 01-24-07, 10:26 AM
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Originally Posted by crosstalk View Post
Thanks again for your detailed reply. Yes, the #14s are for a three-way switched light above the entrance to the garage (one switch in the garage, one switch in the house). The subpanel in the detached garage has a separate earth ground. Do you still think it is wired incorrectly? In this configuration, do I land the generator ground in the subpanel (i.e., locally grounded)?

Thanks for the link to the interlock site. If I go forward with this project, I think I would rather purchase the one that is made by Siemens for my main panel. The ones sold at this site seem to mount on top of the trim plate instead of inside the cabinet. I supposed it's safe either way, but I would prefer it inside. Plus I think the 20 amp backfeed breaker needs a retention clip in this setup.
Question: Why do you have three circuits coming out from the house bypassing the protection of the subpanel? I would just re-route them to spaces on the panel. Perhaps I am missing something here.

As for the Safety Interlock, the Nat'l. Elec. Code (NEC) stipulates four (4) requirements for a transfer switch to meet code:

1) Both breakers must be able to be OFF simultaneously
2) Both breakers must NEVER be on simultaneously
3) Both breakers must be permanently affixed to the panel ( a retaining clip for the Gen breaker will suffice)
4) Safety Interlock protection must remain in place when the deadman's cover is removed. It appears that NONE of the kits listed at the website will meet this fourth condition. beside, for $150.00, you could

I would recommend contacting Siemens directly and inquire about safety interlock kits that install into existing panels.

For a distance of 52', you will need to de-rate the wire size one factor because of distance and voltage drop. So for a 20A feed, upgrade the #12 wire to #10 stranded, as stranded is easier to work with and can carry more current than solid.
 
  #11  
Old 01-24-07, 10:46 AM
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Why can stranded carry more current than solid? #10 is #10 wehter or not is it solid or stranded.
 
  #12  
Old 01-25-07, 08:07 AM
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Stranded -v- Solid

AC current suffers from what is called "skin effect". At DC (frequency = 0 Hz), current flows throughout the entire conductor because there are no eddy currents inside the conductor. For AC, as frequency rises, more and more of the current moving back-and-forth starts to contrate near the surface.

The way to mitigate this is to increase the surface area. As stranded conductors have greater surface area, their ampacity is greater than the same gauge solid wire, for a given frequency.
 
  #13  
Old 01-25-07, 09:55 AM
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whoa there..

it's been a long time since college, but I recall skin depth in copper at 60Hz to be about 0.3 inches. You need to be talking about wire much larger than house wire (00) for this effect to be significant at power line freqs.
Also, to the point about stranding, that helps a lot if EACH STRAND is INSULATED from one another. A Very difficult wire to find, indeed...
 
  #14  
Old 01-26-07, 10:31 AM
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Talking Stranded -v- Solid

ORIGINAL QUOTE FROM telecom guy: so, to the point about stranding, that helps a lot if EACH STRAND is INSULATED from one another. A Very difficult wire to find, indeed. [/QUOTE]

Yes, this is called Litz wire. Mosty used for Switched-Mode Power Supplies (SMPSs), where frequencies are getting into the MHz. Not cheap, and not easy to find.

Oops, I forgot, we're dealing with 60Hz & not 60kHz. OK, my argument still stands for the ease factor: stranded is easier to bend and work with than solid.
 
  #15  
Old 01-26-07, 06:17 PM
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This depends on termination and application respects.
 
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