# 3 wire circuit help, cant solve voltage drops



## Wire Tags (May 11, 2016)

Yeah so there is a break in the neutral and the fuse is blown open, I want to know what are the voltage drops across each resistor. Every resistor is 
60 ohms. I know the voltage across R1 is 120 volts.

So I guess R2//R3? the 120V in red is just a guess.









Answers:
VR1=120V VR2=48V VR3=72V VR4=24V VR5=24V


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## Otterinaround (Feb 13, 2018)

*hmmm.*

So I hope the smarter folks on here correct me when I am wrong ....

(I'm sure to have botched the notation!)

but the way my pea brain sees it....

anything after R1 runs // to R1 

but this is where it seems to get fun.. 

R2 and R3 are // to each other but in series with R4 and R5...

so I guess you figure out R(2&3) 

and 
the series that is R(2&3) R4 and R5 to all the while understanding that the 
Red lines are HALF of a // circuit... 

so as stated above you do kinda have to figure them all out to get the drop 
across the spots you indicated. 

Of course that is my own personal W.A.G and I could be COMPLETELY WRONG. But I posted to tag along and find out what others came up with.


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## Wire Tags (May 11, 2016)

I posted the answers in the OP, this way its easier for others to try and crack it.
if VR2 is indeed // with VR3 then their voltage drop would be the same at 48V but the answer says otherwise.


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## splatz (May 23, 2015)

This is a really good problem. The key is to unfold it like this, then it's manageable: 


```
L1                       N
|--R5----R4--|           |
|            |           |
|            |-----R3----|
|            |           |
|-----R2-----|           |
|                        |
|------------R1----------|
```


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## gpop (May 14, 2018)

judging by the red lines it looks like you presumed that there was a N connection under r2. 

The drawing doesnt show a dot so technically its the same phase so the voltage should be zero


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## splatz (May 23, 2015)

gpop said:


> judging by the red lines it looks like you presumed that there was a N connection under r2.
> 
> The drawing doesnt show a dot so technically its the same phase so the voltage should be zero


Right, R2 was originally a 240V load - look at my awesome ascii art ... R2 has a path through R3 to N


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## just the cowboy (Sep 4, 2013)

*Real life*

Splatz gave the correct way to break it down, now its just ohms law to get the rest.


Lesson learnt.


Look for everything as small as connection dots.
Start at voltage and make a path back.
Draw it out.


This would be a great example of how things get smoked when sharing a neutral in multi wire branch circuits if resistance values were different. .


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## Wire Tags (May 11, 2016)

thanks guys.


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## telsa (May 22, 2015)

The voltage across R1 is 0 because a short to the neutral has been permitted.

R4 + R5 should see 240 VAC but since the rail is shorted... I guess not... maybe 120 VAC instead... no wait... the other rail has no voltage at all... its fuse is blown.

So we have one rail shorted to neutral, the other blown open.

Gee, I guess that sets every voltage difference to zero.

I think the original wiring diagram is all fouled up.

Tell me again how the one rail fails to blow its fuse?


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## JasonCo (Mar 23, 2015)

Correct me if I'm wrong, but isn't the diagram incorrect? Or am I just reading it incorrectly. 

This is a 120/240 single-phase delta-wound transformer coming off the secondary side? Lets say the fuse never blew out. Wouldn't you have 120v (A-phase) on one side of the resistor R2 and 120v (B-phase) on the other side of the resistor R2? There is no dot in the crossing lines and I don't even think the vertical line on the bottom half of that cross should even be there. If it was eliminated and a dot was drawn in, the drawing would make more sense. I'm guessing that's why the fuse blew out lol.


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## gpop (May 14, 2018)

JasonCo said:


> Correct me if I'm wrong, but isn't the diagram incorrect? Or am I just reading it incorrectly.
> 
> This is a 120/240 single-phase delta-wound transformer coming off the secondary side? Lets say the fuse never blew out. Wouldn't you have 120v (A-phase) on one side of the resistor R2 and 120v (B-phase) on the other side of the resistor R2? There is no dot in the crossing lines and I don't even think the vertical line on the bottom half of that cross should even be there. If it was eliminated and a dot was drawn in, the drawing would make more sense. I'm guessing that's why the fuse blew out lol.



Why whats wrong with a 60ohm load connect across 240v. Not every load is 120v. 

If you dont see a dot then the lines cross they do not connect. Dots are signs that the wires are connected. (there's 30 different ways this could be drawn to show the wires cross. Understanding how its drawn is half of the question the rest is learning how to simplify a drawing like splatz has shown)


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## splatz (May 23, 2015)

JasonCo said:


> Correct me if I'm wrong, but isn't the diagram incorrect? Or am I just reading it incorrectly.
> 
> This is a 120/240 single-phase delta-wound transformer coming off the secondary side? Lets say the fuse never blew out. Wouldn't you have 120v (A-phase) on one side of the resistor R2 and 120v (B-phase) on the other side of the resistor R2? There is no dot in the crossing lines and I don't even think the vertical line on the bottom half of that cross should even be there. If it was eliminated and a dot was drawn in, the drawing would make more sense. I'm guessing that's why the fuse blew out lol.


It could be part of a high leg delta or it could just be a standard residential three wire edison service, doesn't really matter for purposes of the problem. The fuse on one leg is blown. 

Since there's no dot where it crosses N, R2 is just a L1-L2 (240V) load.


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## JasonCo (Mar 23, 2015)

:icon_eek:Yeah it could just be a 240v load. lol. oof.. Thanks for the help.


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## Wire Tags (May 11, 2016)

so in real life scenario, if I was an electrician staring at a residential house panel a problem like this would be like the neutral connection of branch circuit R5 and branch circuit R4 are disconnected from their breaker and tied together, and one of the hots of branch circuit R2 have been removed from its breaker, and the hots from R3 and R4 have been removed from their respective breaker, and all 3 removed hots from R2, R3, R4 have been tied together?


Wire Tags said:


>


Edit: Sorry I meant to say one of the hots of branch circuit R2 have been removed from its breaker, and the hots from R3 and R4 have been removed from their respective breaker, and all 3 removed hots from R2, R3, R4 have been tied together.


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## gpop (May 14, 2018)

The example does not really transfer to a real life scenario but the lessons that can be learnt do.

Sometime in the future you will come across a breaker that has dropped a leg. The example shows you that electricity will try to find balance using any path available. 

If you have a main breaker panel that drops a leg everything running on 240 and everything using the bad side of the breaker should stop working. 
In real life the water heater will create a back feed to the lost phase. If a single phase load on the bad side happens to be plugged in then the water heater will now be in series with the 110v load. Depending on amp draw and resistance you can drop a leg with out noticing the problem. 

A coffee pot plugged into the broken phase will seem like it takes for ever to heat the water but the light will probably come on and it will seem like its working. Testing with a meter live to N or G may read close to 120v (water heaters are low resistance so the voltage drop isnt huge).

The main lesson here is testing a phase to N or G does not count out a backfeed. Only by testing A to B for 240 can you count out a bad main breaker.

Breaking a N is another problem you will come across and this one can be a royal pain. In your example R5 and R4 are the same resistance which never happens in real life. 
In a home it will be something like a coffee pot and lights on one side and a microwave plus lights on the other side. each phase will use items that are plugged in to make a artificial N (it will treat 2 120v loads as one 240v load) but as this artificial N has no reference to ground or transformer center tap it will float based on resistance.

In real life someone will turn on the microwave and the kitchen light will go dim and the bathroom light will go super bright. Measuring phase to N you will read anything between 60 and 170 volts (once you get outside these numbers you normally see smoke). From phase to G you will read 120v (unless the problem is on the poco side of the G & N bond in which case it will read the same as measuring to N).

Ok im rambling so back to your example. One day you may come come across a problem like your example. Its not uncommon to have more than one problem at the same time especially if the home owner or a (not going to say it) has already had a go at fixing it.


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