backfeeding voltage question.

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pegggu

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hi, just a general question for my own knowledge.
if i had a 240v system fed from transformer #1 picking up a bunch of lights lets say, now say i were to connect another 240v feed from transformer #2 and tie it to the end of the circuit, now it is back feeding into that circuit as well.

what would happen to the voltage in this case? would it be doubled and get 480v cause we have two feeds? or would they cancel eachother out? would the lights be damaged due to excessive current and voltage?

thanks
 
You need to acquire a lot of general knowledge!
1. If the two sources are not synchronized you will get fireworks, and not from the lights. (You did not mention circuit breakers.)
2. If the two sources are synchronized and matched in voltage, you will see no change in the voltage and the lights will pull the same current, half from each source. If you put in breakers, the wires will be able to draw twice the current before the OCPD opens.
3. The NEC generally does not allow this kind of connection, for a variety of very good reasons. :)
 
Electrically it makes no difference where where the second transformer is. You could install it at the end, beginning, middle....Provided the transformers are connected in phase all you will do is increase your available current. If they are not connected in phase you will get some really big sparks.

I have not seen it real life with transformers but have seen it when a knucklehead was testing a miswired manual transfer switch with the generator running.
 
GoldDigger said:
2. If the two sources are synchronized and matched in voltage, you will see no change in the voltage and the lights will pull the same current, half from each source. If you put in breakers, the wires will be able to draw twice the current before the OCPD opens.
Click to expand...
How so? I would think the breaker would trip on overcurrent no matter how much current is available.
 
Everybody:

Everybody:

Go read 450.6 and 450.7 which are the sections of the NEC that cover secondary ties which is what the OP asked.
It is about a page long.
 
ActionDave said:
How so? I would think the breaker would trip on overcurrent no matter how much current is available.
Click to expand...
If you have two sources, each with a 30A breaker you can pull 60A from anywhere along the wire. If it is not just a straight line, that entire 60A could be flowing through a 30A wire. Note that this would require changing the load.
Note also that the OP left out any mention of OCPD.
 
does it matter whether the source is in phase?
if transformer 1 & 2 are both from the Red phase then it would just share the load?
if transformer 1 & 2 are from different phases, the voltage would double and cause damage?
 
pegggu said:
hi, just a general question for my own knowledge.
if i had a 240v system fed from transformer #1 picking up a bunch of lights lets say, now say i were to connect another 240v feed from transformer #2 and tie it to the end of the circuit, now it is back feeding into that circuit as well.

what would happen to the voltage in this case? would it be doubled and get 480v cause we have two feeds? or would they cancel eachother out? would the lights be damaged due to excessive current and voltage?

thanks
Click to expand...

pegggu said:
does it matter whether the source is in phase?
if transformer 1 & 2 are both from the Red phase then it would just share the load?
if transformer 1 & 2 are from different phases, the voltage would double and cause damage?
Click to expand...
The second post there hits an important area. If the sources are in phase with each other - there is no voltage potential between them - they are in parallel to one another and you have to look at conductor differences in impedance to determine any differences in current supplied by each source to the load(s). If you ran exactly the same size and length from each source to the load, they each should carry equal current. When no load is present there should be no current - unless the sources are out of phase - then there will be voltage potential between them and there will be some circulating currents between the two sources even if they are just slightly out of phase from one another.

But you will not get slightly out of phase from two identical transformers supplied by the same source. You get out of phase much easier by independent sources like generators, all utility derived sources have a pretty stout supply and if connected to same input phase will be so much closer to being in phase with one another then you may ever see with a smaller local generator or other source.
 
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pegggu said:
hi, just a general question for my own knowledge.
if i had a 240v system fed from transformer #1 picking up a bunch of lights lets say, now say i were to connect another 240v feed from transformer #2 and tie it to the end of the circuit, now it is back feeding into that circuit as well.

what would happen to the voltage in this case? would it be doubled and get 480v cause we have two feeds? or would they cancel eachother out? would the lights be damaged due to excessive current and voltage?

What type lighting of problem are you trying to solve?
Click to expand...
 
pegggu said:
does it matter whether the source is in phase?
Click to expand...
Yes, it matters a lot.
if transformer 1 & 2 are both from the Red phase then it would just share the load?
Click to expand...
Pretty much.
if transformer 1 & 2 are from different phases, the voltage would double and cause damage?
Click to expand...
Nothing would double, there would a short circuit. Think about touching a wire from the red phase to a wire from the black phase....crack, boom.
 
mwm1752 said:
pegggu said:
hi, just a general question for my own knowledge.
if i had a 240v system fed from transformer #1 picking up a bunch of lights lets say, now say i were to connect another 240v feed from transformer #2 and tie it to the end of the circuit, now it is back feeding into that circuit as well.

what would happen to the voltage in this case? would it be doubled and get 480v cause we have two feeds? or would they cancel eachother out? would the lights be damaged due to excessive current and voltage?

What type lighting of problem are you trying to solve?
Click to expand...

not trying to solve any problems, just for my own general knowledge.
Click to expand...
 
ActionDave said:
Yes, it matters a lot.
Pretty much.
Nothing would double, there would a short circuit. Think about touching a wire from the red phase to a wire from the black phase....crack, boom.
Click to expand...

thanks for that, definitely helped put things into perspective.
thank you to everyone else for the help as well. just trying to learn is all.
 
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