Backfeeding a sub-panel?

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  #1  
Old 02-19-11, 07:21 PM
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Backfeeding a sub-panel?

Hi all, first time poster long time reader

So we recently bought a property that has a number of out-buildings in addition to the main home.

In one of these buildings is a horrific mess of a sub-panel that needs to go.

I wanted to add a kill switch upstream of the new panel I am putting in so I could shut off all power to the downstream sub-panel (and thus the entire building) with a simple throw of the lever. When i went to the local electrical supply store they told me that this was a needlessly expensive addition and instead advised me to backfeed the sub-panel through a 100A 2-pole breaker (and yes, 100A is far more than I need in this building) so if I need to shut power off to the panel I could just manually trip the breaker. They said as long as 100A is far and away more than I would ever need in the building, and to make sure to split my circuit loads evenly, that back-feeding like this is perfectly safe.

So they sold me the breaker and I installed it as suggested. (supply is 6 AWG threaded that come from 60 AMP breakers in the main panel)

A carpenter that is working on an unrelated project on the property happened to pass by the panel as I was working on adding a few light circuits and flipped the freak out. He said that no one should ever back-feed a sub-panel, that it was a huge fire risk should the 100A breaker ever fail, and that I should immediately disconnect and feed the box properly.

Am I missing something? Who is right? The electrical guys down at the supply store, or the carpenter?
 
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  #2  
Old 02-19-11, 07:29 PM
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Welcome to the forums! You should have a breaker in the house that feeds the subpanel. How does the subpanel get it's power, now, without going through a breaker?
"Backfeeding" isn't what you are doing, so I fail to understand either point of view. Only one right way to do it.
 
  #3  
Old 02-19-11, 07:32 PM
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Yes, a pair of 60amp breakers back at the main panel feeds the out-building. I wanted to add a kill switch in the building in question before the subpanel in said building. That way if there is ever a problem I wouldn't have to sprint across the property (50 yards or so) to trip the breaker in the main panel as an emergency shutdown.

I was told that instead of adding the kill switch, to backfeed that sub-panel with the power coming into the sub through a breaker instead of bringing in through the main lugs.
 
  #4  
Old 02-19-11, 07:47 PM
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Backfeeding a circuit breaker is an acceptable practice for what you want to do however the 100 ampere circuit breaker MAY be too large for the specific panel. Backfed circuit breakers also need to have a "hold down" device approved for the purpose by the manufacturer if the only thing holding the breaker in place is the dead-front cover (i.e. not a bolt-on breaker).
 
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Old 02-19-11, 08:07 PM
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I see. The sub-panel in question is 200-amp. About 3x bigger than my needs, but due to a sale it was half the price of a sub-panel of appropriate original capacity. So now I have a way-over-sized panel. Had a bunch of blown 15amp breakers I was gonna paint red and fill in the unused slots with.

Anyway, so the carpenter was essentially over-reacting then? He made a rather animated and some-what convincing argument at the time. Maybe he's just not familiar with the practice?

Image for clarity:



Note - alternate plan was to have a different type of kill switch that was essentially 2 fuses - one on each of the hot wires - that had a lever disconnect
 

Last edited by Oukachiru; 02-19-11 at 08:28 PM.
  #6  
Old 02-19-11, 08:50 PM
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You can't switch a neutral wire.
 
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Old 02-19-11, 09:02 PM
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I think you got over sold a bit. If it is fed by a 60 amp breaker you could have used an unfused 60a AC disconnect at the subpanel. Probably less then half the cost of that 100a breaker and no need to find a hold down kit assuming one exists.

Of course if you had bought a main breaker panel kit it would have been maybe cheaper then the main lug, I assume it's a main lug you bought, and would have come with the main breaker.
 
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Old 02-20-11, 12:53 AM
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Doh, massive brain fart. Have updated the image to reflect that one does not put switches on neutrals. Exceptionally stupid, my bad.

The reason I have the over-powered 100amp breaker in the sub is because in the main, the previous owner had the power coming to this building out of a 100 amp breaker. after i bought the 100 amp breaker for the sub I discovered the wiring went from 4 awg that was visible at both ends (main and old sub) to 6 awg in the conduit that connected them.

I removed the 4 awg lengths and replaced with 6 awg making the wire sizes constant, and therefore dropped the main's breakers to the sub from 100 amps to 60 amps. which left me with the new 100 amp in the sub that I can't return so I figured I'd use it instead of buying another 60amp.
 
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Old 02-20-11, 04:23 AM
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I see what you are trying to do, but I think a disconnect, like Ray said, would be the best alternative. If you have a problem (not sure what it would be), you can just pull the disconnect and work on the outbuilding. Keep the disconnect in sight of your subpanel.
 
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Old 02-20-11, 07:01 AM
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I would just replace the 100 amp breaker in the sub-panel with a two pole 60 amp breaker and use the required hold down device. The hold down device protects the breaker from coming loose should you or someone else in the future remove the cover. You could leave the 100 amp breaker and install the hold down device on that if you wanted to. The #6 wire is being protected by the 60 amp breaker back at the main panel. Nothing wrong with the over sized panel that you have. You actually have more room for wiring and for future circuits. The breaker at the main lug panel (sub panel) is only a requirement if there are more than 6 circuits installed but is a good idea like you said in case you want to shut it down quickly.
 
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Old 02-20-11, 07:22 AM
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I agree with Teamo. Since you now have the 100 amp breaker just go ahead and add a hold down kit. Only if no hold down is available would you need to look for an alternate solution such as a disconnect. It is important though for safety the hold down is installed as soon as possible.
 
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Old 02-20-11, 08:30 AM
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Wouldn't the main panel also need a 2 pole breaker, not 2 single pole as you have drawn in your image ?
 
  #13  
Old 02-20-11, 01:56 PM
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A carpenter that is working on an unrelated project on the property happened to pass by the panel as I was working on adding a few light circuits and flipped the freak out. He said that no one should ever back-feed a sub-panel, that it was a huge fire risk should the 100A breaker ever fail, and that I should immediately disconnect and feed the box properly

Oh and tell the carpenter to stick to sawing wood.
 

Last edited by ray2047; 02-20-11 at 02:17 PM. Reason: Fixed Quote for Clarity
  #14  
Old 02-20-11, 03:10 PM
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Originally Posted by teamo View Post
I would just replace the 100 amp breaker in the sub-panel with a two pole 60 amp breaker and use the required hold down device. The hold down device protects the breaker from coming loose should you or someone else in the future remove the cover. You could leave the 100 amp breaker and install the hold down device on that if you wanted to. The #6 wire is being protected by the 60 amp breaker back at the main panel. Nothing wrong with the over sized panel that you have. You actually have more room for wiring and for future circuits. The breaker at the main lug panel (sub panel) is only a requirement if there are more than 6 circuits installed but is a good idea like you said in case you want to shut it down quickly.
I would agree and I would also want a main breaker in the subpanel since this is a separate building; the breaker being backfed is not a concern nor a code issue as long as you have the hold down bracket.

Oh and tell the carpenter to stick to sawing wood.
Hahahaha In some areas, carpenters market themselves as electricians; I find this humorous.
 
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Old 02-20-11, 05:03 PM
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Originally Posted by CasualJoe View Post
Hahahaha In some areas, carpenters market themselves as electricians; I find this humorous.
Well, slick, it just depends on how much experience you have in the trades and what type of learning process you went through before your ears dried. It may be humorous to you, now, but you'll get over it.
 
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Old 02-20-11, 07:27 PM
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Originally Posted by CasualJoe View Post
Hahahaha In some areas, carpenters market themselves as electricians; I find this humorous.
Just like handymen. Funny yet scary.
 
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Old 02-21-11, 05:16 PM
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Originally Posted by chandler View Post
Well, slick, it just depends on how much experience you have in the trades and what type of learning process you went through before your ears dried. It may be humorous to you, now, but you'll get over it.
Sorry if I offended you Chandler, it was certainly not intended. There is more to the story, but I wasn't wanting to bring it up, but maybe now I should. I was thinking about the fact that the carpenter's union in some areas now markets themselves as a bonafide union of electrical workers (even has locals for them) and not about a person who is truly a skilled craftsman and mechanic. The carpenter mentioned in the OP reminded me of this and made me chuckle. Didn't want to and still don't want to talk unions.
 
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Old 02-21-11, 06:07 PM
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After I posted and went to work, I felt like you didn't mean anything derogatory. I just happen to be seasoned in both trades, and didn't want to be lumped into the jack-of-all-trades category. No harm. You'll not get me talking union, either, so we're safe, pal!!
I really think it more important, nowadays, for young people to get as much concentrated education in the trades as possible. I, like many others, have a degree, and have worked in Corporate America for years. But, you'd have to hog tie me and drag me through a slop yard to get me to do it again. I love what I do, and learn new stuff every day, even as old as I am
 
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