paralleling / backfeeding load question

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pegggu

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hi, just a general question here.

what happens if you connect at two seperate points to feed some load?
considering the feed is from the same source?

see attached picture for more details (red is the original feed, say someone didnt know where the feed was coming from, so they ran the blue cable to pick up the load, what would happen in this case?)
 

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The result would be similar to a UK style ring wired circuit.
It is not allowed under the NEC because it would be paralleling conductors over very different paths.
But I do not see your rationale for adding the blue wire. Either the red wire is adequate to handle the load, whether you know the routing or not, or you feed the new load from the new blue wire without interconnecting it to the red.
 
If I understand you correctly you are asking if two different branch circuits where connected together by mistake?

It depends on which lines are connected. With a single phase 120/240 source one way would be a line to line short circuit the other way would not be a short but the available current on the circuit would be increased. Assuming two 20 amp circuits you would now have 40 amps of protection on 20 amp conductors.
 
Bob -
I'm not saying the ring circuit is okay - it's not. But I don't see how this could be true:

iwire said:
.... Assuming two 20 amp circuits you would now have 40 amps of protection on 20 amp conductors.
Click to expand...

Which set of conductors could possibly have 40A on them?

Label conductors
Left Feed
Left conductors
Center Conductors
Right Conductors
Right feed

Label loads
L1, L2, R2, R1


ice
 
iceworm said:
Bob -
I'm not saying the ring circuit is okay - it's not. But I don't see how this could be true:



Which set of conductors could possibly have 40A on them?

Label conductors
Left Feed
Left conductors
Center Conductors
Right Conductors
Right feed

Label loads
L1, L2, R2, R1


ice
Click to expand...


No thanks, sounds confusing. :D

You are assuming a perfect ring circuit when in reality the chances are very low it will just be a ring. The sections of branch circuit that are outside the ring will be protected above their ratings.

Also this combined circuit will be supplying something, most likely something that has a max overcurrent rating such as receptacle outlets, lighting outlets, appliances etc. We don't place overcurrent devices at outlets as other areas do with ring circuits.

How does this happen? I have seen it where people have made wrong connections in splice cans. I found it when I could not turn off a circuit. Two circuits on the same phase from two different panels where joined in a spice can.

Another time I have seen it like the OP describes, someone decides to back feed a 'dead circuit' (well it was when they where there) at the easiest point they can.




Besides, both line to line shorts and paralleled overcurrent devices are directly prohibited by the NEC. :cool:
 
pegggu said:
...(red is the original feed, say someone didnt know where the feed was coming from, so they ran the blue cable to pick up the load, what would happen in this case?)
Click to expand...


peg -
Commenting only on the circuit you drew.
As noted, it doesn't meet the NEC. But that is a legal issue, not a physics issue. There is no physics that will put 40A on any set of conductors.

However, it still is not a good idea - iwire listed a bunch of good reasons (besides the 40A one).

ice
 
iceworm said:
Bob -
I'm not saying the ring circuit is okay - it's not. But I don't see how this could be true:



Which set of conductors could possibly have 40A on them?

Label conductors
Left Feed
Left conductors
Center Conductors
Right Conductors
Right feed

Label loads
L1, L2, R2, R1


ice
Click to expand...
Well, anything plugged into any of the outlets would have up to 40A of current available before tripping a breaker.

Also, if the connection were accidental there would be a 50:50 chance that the second connection would be to the opposite leg of the 240 supply from the first one. That wouldn't be good.
 
i'm a bit confused here lol.
let me clarify, so lets ignore the 120/240v and just say it's a 240V bus from the utility feeding streetlights.
so you take a feed (2 hots, 240v) to the first streetlight and pick up the rest (shown in red).
now let's say for some odd reason another contractor gets in there not realizing this feed is already there and decides to feed the streetlights from the same bus (in phase, same source) just from a point further down the line (shown in blue).

what would happen in this case? would this be considered backfeeding or no (cause they are from the same supply)?
also, how would the current travel in this case?

hope that clarifies things, i was just curious, general question.
 

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I am having a problem understanding your drawing.


Are the circles in your drawing light poles?

Do the single lines represent both circuit conductors?

If I am correct in those assumptions nothing would happen as long as they did not swap phases or lines.


If it is covered by the NEC it would likely be a violation of the parallel conductor rules.
 
iwire said:
I am having a problem understanding your drawing.
Click to expand...

Same here. If in the last drawing the circles are "loads" then what I see is three loads connected in a series - still with some confusion of how you get voltage from the single line representing the source.
 
pegggu said:
... say it's a 240V bus from the utility feeding streetlights.
so you take a feed (2 hots, 240v) to the first streetlight and pick up the rest (shown in red).
now let's say for some odd reason another contractor gets in there not realizing this feed is already there and decides to feed the streetlights from the same bus (in phase, same source) just from a point further down the line (shown in blue). ....
Click to expand...
That was my understanding from your first post. Loads are fixed, as in lighting, as opposed to receptacles. Second power feed connected such as to not to cause a short. No invisible t-taps. One-line is two conductors, loads are connected in parallel.

pegggu said:
... what would happen in this case? ....
Click to expand...
As I said, it will work fine. It is a legal issue - not a physics problem. Although an inspector walking by could burst into flames:eek:hmy:

pegggu said:
... also, how would the current travel in this case? ....
Click to expand...
Given there is a minor variation in lead length, connection resistances, the two feeds will share (within the minor variations) feeding all three loads. Or maybe share feeding the center load.

pegggu said:
... would this be considered backfeeding or no (cause they are from the same supply)? ....
Click to expand...
As noted, by others as well as me, this is not a valid/normal NEC wiring method. I don't know what term would be used. Back fed? Double fed? Illegally fed? Poor design? Not a good idea? What was done is clear from the context - what you wish to call it likely doesn't matter.

pegggu said:
I'm a bit confused here ....
Click to expand...
I think you got it.

ice
 
kwired said:
Same here. If in the last drawing the circles are "loads" then what I see is three loads connected in a series - still with some confusion of how you get voltage from the single line representing the source.
Click to expand...

Never mind, they call that a "one line" drawing.

I will not get into the debate over whether or not this is an illegal parallel - I have been in that debate before in a long thread or two with no general consensus - it is a common practice in some places that do not use NEC and there are advantages and disadvantages to it.

I will say if the second feed is from a separate overcurrent device - you probably do have an ability to overload a conductor without opening any overcurrent protection, plus a risk of someone turning off a switch or breaker thinking they have killed power when in fact there is an additional feed to the circuit, but of course for safety reasons we all disconnect then check for voltage right?
 
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