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Old 02-19-2011, 10:33 PM   #1
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Backfeeding a sub-panel?


Hi all,

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. Otherwise it is about a 50-yard dash back to the main panel if I have to execute an emergency shut-off on the out-building.

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? If there is a danger, what is it?

Image for clarity:



Note: sub-panel is 200-amp.


Last edited by oukachiru; 02-20-2011 at 02:52 AM.
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Old 02-19-2011, 10:52 PM   #2
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Backfeeding a sub-panel?


You never ever switch the neutral in your situation. By feeding the main lugs your not backfeeding anything. I don't have time tonight to go over all the things you need to be concerned with but I will attach a drawing of mine to get you going in the correct direction.
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Backfeeding a sub-panel?-subpanel-2.jpg  

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Old 02-19-2011, 11:04 PM   #3
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Backfeeding a sub-panel?


In your second diagram your showing a configuration for 'back feeding' a double pole breaker. This is perfectly acceptable as long as it is labeled a 'Main' to satisfy the building disconnect requirement of the electrical code. You are also required to install a hold down kit on the breaker.

DO NOT SWITCH THE NEUTRAL ..... this will do things to the loads operated off that sub-panel that your not going to like.

Also it is not necessary to backfeed a 100 amp breaker from a 60 amp breaker in the main panel (service equipment). Just install a 60 amp breaker in the sub and feed it from the 60 amp in the main panel. There is considerable cost difference and the 100 amp breaker is not going to allow the sub panel to receive a 100 amp capability. The 60 amp breaker in the service equipment protects the feeder to the sub and makes the rating of the sub 60 amps even though it may be a factory rated maximum 100 amp panel.
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Old 02-20-2011, 02:52 AM   #4
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Backfeeding a sub-panel?


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 02-20-2011, 07:26 AM   #5
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Backfeeding a sub-panel?


How does the ground circuit look in this sub panel?
You should have a ground wire coming from the main to the sub along with the two hots and neutral.
The outbuilding should have its own ground rod.
The neutral bar should only have white wires and the ground bar only ground wires.
The ground bar should be mounted to the panel case or have a bonding strap.
The neutral bar should not have a connection to the case or to the ground bar so make sure the bonding screw or bonding strap is not installed.
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Old 02-20-2011, 07:37 AM   #6
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Backfeeding a sub-panel?


Depending upon which code cycle the sub was installed under the three wire feed may be acceptable. The sub does need a grounding electrode.
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Old 02-20-2011, 08:00 AM   #7
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Backfeeding a sub-panel?


The upstream panel has (should have) a breaker suitable for the feed to the subpanel, here, a 60 amp breaker.

Then it doesn't matter what size breaker you have to control the entire subpanel or whether it is backfed, provided that the subpanel breaker is suited for the (rating of the) subpanel itself.

If the previous owner had a backfed 100 amp breaker, you can leave it there and keep using it.
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Last edited by AllanJ; 02-20-2011 at 08:07 AM.
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Old 02-20-2011, 12:35 PM   #8
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Backfeeding a sub-panel?


Yes, I installed a grounding rod for the sub. All of the buildings now have their own grounding rod, as well as the two buried junction boxes mid-property. Just didn't want to overly complicate the diagram.

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