# Back-feeding breakers



## bharbwyr (May 8, 2009)

I have encountered a partially upgraded service. The old 60 A fusebox still serves as the main. The new sq. D QO box has a 70A breaker, back-fed and interlocked with a 30A generator breaker. My question is- do QO breakers merely act as disconnects when back-fed, or do they offer over-current protection?


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## drsparky (Nov 13, 2008)

Current flow in either direction will make the breaker trip.


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## 480sparky (Sep 20, 2007)

It makes no difference which end a breaker is fed from. Current flowing one direction will be flowing the opposite in 1/120th of a second.


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## zen (Jun 15, 2009)

so why do some people feed a mlo panel from the main panel under a breaker and some feed from the main under a breaker to a breaker in the mlo panel


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## 480sparky (Sep 20, 2007)

zen said:


> so why do some people feed a mlo panel from the main panel under a breaker and some feed from the main under a breaker to a breaker in the mlo panel


Same reason some people wear boxers, others wear briefs.

Same reason some people drive vans, others drive pickups.

Same reason some people wear baseball caps, others don't.


If the other end of the feed has a properly-sized overcurrent device, you don't need one in the MLO panel.


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## chenley (Feb 20, 2007)

480sparky said:


> Same reason some people wear boxers, others wear briefs.
> 
> Same reason some people drive vans, others drive pickups.
> 
> ...


That just reminded me of this from Joe Dirt:

Hey, how exactly is a rainbow made?

How exactly does the sun set?

How exactly does the posi-trac
rear end on a Plymouth work?

-It just does.


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## bharbwyr (May 8, 2009)

I see that according to 408.36(D), I'm good to go as long as the backfed breaker is fastened in place, which, thanks to the generator interlock bracket, it is.


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## user5941 (Mar 16, 2009)

bharbwyr said:


> I see that according to 408.36(D), I'm good to go as long as the backfed breaker is fastened in place, which, thanks to the generator interlock bracket, it is.


 I don't think the interlock prevents removal of the breaker


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