voltage on common

[kentriolinda]

Today I was replacing all exhisting kitchen & bath recp. with GFCI recp at an appartment complex 80% were changed out without incident. Two of the appt. had the same problem they had looped the wires into the boxs ( all wiring is hard piped) and without cutting wire stripped back insulation and just looped it around recp. screws and continued on. The problem was that when I changed out the recp. in these two apt.s ( I chose to do them hot because they were so far away from the breaker panel ) I cut the looped common and the light bulbs in the kitchen and hall instantly burned out. So upon further investagation I found that from hot to one common leg I had 144 volts and from hot to other half of common I had 121 volts and volts between the two commons was 121. My question is how could one common being cut in half give me 121 v between the two halves. A second why cutting that wire caused such a spike in voltage.

 

[mivey]

You opened the neutral on a MWBC.

 

[growler]

You cut an actual neutral and it was carrying the unbalanced load of a multiwire branch circuit.

Most of the time the white conductor is just a return path but not so with a multiwire branch circuit it really is a neutral and we all know what happens when you open the neutral.

If you draw the circuit out it's real easy to see why what you did caused a power surge.

You may want to think about turning off those breakers it's a lot safer.

 

[infinity]

You opened the neutral on a MWBC.

Mivey summed it up in seven words. Good thing you didn't blow up the $3000 TV.

 

[e57]

Feel lucky that you don't own the smoldering building and its charred occupants...

(Yes that's extreme...)

But if the building were occupied - you most likely would own more than a few lamps. A smokey computer, a flat screen tv melted into a puddle...

Once you cut the neutral - the connected loads were in series across the 240v feeding the two circuits. Since the neutral is keeping both circuits at 120 - cutting it makes the neutral point between the two legs float depending on load. A higher load on one side of the circuit would give you measured voltages like 10v/230v - A more equal load 110v/130v - But 240 is going across the two circuits, as the other leg is now the return path to the transformer.

 

[kentriolinda]

I understand that I should have turned off power for safty but I still dont really understand how a multiwire circut can do that and is that common in multi family dwellings more so than sgl fam. dwellings?

 

[Jim W in Tampa]

You were very lucky that it only cost a few bulbs. You never cut a neutral live on a MWBC.
Highly suggest you get some training on that area fast. While what you did would been limited to only those 2 circuits the same rules apply as if it was service.

 

[mivey]

There are some nice graphics members have posted here if you feel like searching. I don't right now.

 

[jumper]

480Sparky has a good one.

 

[roger]

There are some nice graphics members have posted here if you feel like searching. I don't right now.

Here's one.

Roger

 

[e57]

I understand that I should have turned off power for safty but I still dont really understand how a multiwire circut can do that and is that common in multi family dwellings more so than sgl fam. dwellings?

Beware - the same could happen in a commercial building as well - on a 3 phase 120/208 it operates the same way... Instead of splitting the voltage one way or the other - it splits 3 ways... More complicated to explain - but trust me...

There are many who see the MWBC as a hazard... And some who feel that unqualified persons should not be working on them. And limit the use of them to accomidate the unqualified person who might find themselves working on it. I'm neither of those people... I think everyone should be educated and qualified for what they work on. This may serve as your learning experiance - we ALL have had one. Either by mistake or ignorance or initiation... (when you have 2-6 of them - thats when it's time to worry... )


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Roger if thats your diagram? Maybe add "damaging over-current" as well on the 60v side...

 

[kentriolinda]

Thank you all for your input on my ignorance and I plan to get some extensive training on MWBC ASAP

 

[walkerj]

I had the pleasure of watching someone else learn the hard way.

Opened a neutral on a 480/277 lighting circuit feeding some high bays.

The 17 lights were not very big fans of the overvoltage.

To this day, the guy that did it says there is no way he could have done that because the circuit he was working on was off:roll:

 

[dbuckley]

Just a note on the diagram above - it precisely illustrates the principle but shows the loads, the resistors, as static; it has no time element.

What happens in real life is the loads are not single resistors but several loads in parallel. When the neutral vanishes the voltages "balance" as per the diagram. But... in short order a load fails such as a bulb burns out. The voltages then rebalance, usually putting the overvoltage ontothe other leg, and one by one the over-voltage items pop usually going open-circuit until there is just one survivor, which then gets zero volts.

Never pretty...

 

[Dom99]

Got booby trapped a few years ago and burned out a tv in another room changing a hall outlet in a house. Found out the owner had an addtion built and the so called electrician must have unknowingly created a MWBC.

My question is looking at an outlet that has a feed and sub-feed, how are you to know it's a MWBC?

 

[roger]

My question is looking at an outlet that has a feed and sub-feed, how are you to know it's a MWBC?

There will be more than one ungrounded conductor in the box if it's run correctly. Otherwise you can always put an ammeter on the neutral to see if it has current on it after you have turned off the circuit you are working on.

Roger