240 circuit only reading 15v on one leg

[Uncle Ben]

I'm having a recurring power issue on one leg of my 240 circuit that goes directly into my attic furnace. About a week ago my HVAC suddenly quit working, and I realized I was getting 120v on one leg of the furnace's incoming power and only 15v on the other. I loosened both legs of the 240v at the terminal block in the furnace and then re-connected, as well as tightened some other connections inside the furnace and then was getting 120v on both legs and 240 combined, so my AC was able to run again (but not actually cooling...different story).

Yesterday while I was continuing to troubleshoot my AC, the outside condenser unit and compressor was running for a few minutes and suddenly shut off, and ever since then I am NOT getting 120v on one leg (happens to be the black wire) of my 240v...and I'm getting about 15v on that leg instead.

When I took off the electrical panel cover and checked the lugs coming out of the breaker, I am getting 120v on both legs individually and 240v combined, so I don't know how I could be getting only 15v on the black leg of that circuit up in the furnace. The only other thing I can figure is that there is a break in the wire, which would be disastrous since the electrical panel is on the first floor and the furnace is on the 3rd floor, so I can only imagine what it would look like (and cost) to replace that wire if there is a break in it. I also know that a wire break is the most unlikely cause, so I'm really hoping that is not the issue.

Any other possibilities that I am not thinking of? What else can I test to figure out the cause of the lack of voltage on one leg of my 240v?
Thanks in advance for your input!

 

[FredWanaker]

I would turn that breaker off, and look for other circuits that die when it is off. When you find them, look inside for a bad junction. If no other circuit is affected, and the wire loss is still between the breaker and the HVAC, then pulling a new wire is the simplest solution. A mouse, rat or squirrel in the attic may have damaged a wire. FWIW a couple years ago we went to every single electrical outlet and light etc., in the house and figured what circuit was on. I built a very large spread sheet that listed what was on each circuit. Now if there is an issue, I can go to the spread sheet and instantly figure out if there are any boxes involved. There were some things on circuits I would never have imagined. A junction for something in room "A" might come thru a box in the garage or bathroom etc.. many feet away. It all depends on how the house was wired originally, and then there is always the possibility someone made some illegal additions to the wiring.

 

[Hohn]

Sounds to me like you have a short on the leg such that the resistance of the short is high enough to allow 15V still on your leg of interest. A dead short obviously would read zero, but a partial short can provide enough resistance that some voltage/current will still be present on the leg of interest.
You have 120V on that leg at the panel and 105V of it disappeared somewhere. It didn't disappear-- you need to find out where it went.
I'd check for some kind of rodent damage to the insulation. Could also be damage to the wiring from improper wire routing. You'd be surprised how often it ends up being something like a nail or plumbing hanger or such.

This is a hazard you need to address seriously because it could mean something like plumbing strapping or such is live to the tune of 105V. Definitely a fire hazard. Call a pro ASAP IMO.

 

[Uncle Ben]

Thanks Hohn for your input!
The only thing I would add is that the system was working properly up until a week ago, and there has not been any construction, repairs or renovation that would have damaged the wiring. But I do know that sometimes things work just fine...until suddenly they don't.

 

[Hohn]

Thanks Hohn for your input!
The only thing I would add is that the system was working properly up until a week ago, and there has not been any construction, repairs or renovation that would have damaged the wiring. But I do know that sometimes things work just fine...until suddenly they don't.

Yes, which strongly suggest a change due to an unauthorized actor (i.e. rodent).

 

[Uncle Ben]

Ok, so what is the best method to confirm a wire break and locate the break itself? Any tricks to this? There is only about a foot or two of wire coming up out of the floor underneath the furnace in the 3rd floor attic space, and it feeds directly into the furnace and connects to the terminal block in the furnace (old system), so there is very little exposed wire as the rest is inside the walls across 3 stories.

 

[Innovate1]

Sounds to me like you have a short on the leg such that the resistance of the short is high enough to allow 15V still on your leg of interest. A dead short obviously would read zero, but a partial short can provide enough resistance that some voltage/current will still be present on the leg of interest.
You have 120V on that leg at the panel and 105V of it disappeared somewhere. It didn't disappear-- you need to find out where it went.
I'd check for some kind of rodent damage to the insulation. Could also be damage to the wiring from improper wire routing. You'd be surprised how often it ends up being something like a nail or plumbing hanger or such.

This is a hazard you need to address seriously because it could mean something like plumbing strapping or such is live to the tune of 105V. Definitely a fire hazard. Call a pro ASAP IMO.

Why do people call any issue with electricity a SHORT? What you describe is an OPEN or a high resistance partial open not a short. A short is when conductors that aren't supposed to be connected become connected. A short resulting in zero volts would trip the breaker. -- rant off --

Your other comments are good. Unlikely it is causing some exposed metal to be come energized but the fire hazard is real. I wouldn't expect any splices in the run but could be one near the HVAC unit - there could be others but unlikely.

 

[Innovate1]

Ok, so what is the best method to confirm a wire break and locate the break itself? Any tricks to this? There is only about a foot or two of wire coming up out of the floor underneath the furnace in the 3rd floor attic space, and it feeds directly into the furnace and connects to the terminal block in the furnace (old system), so there is very little exposed wire as the rest is inside the walls across 3 stories.

Time Domain Reflectometer. But that's not something most electricians are going to have in the tool box.

 

[mm08822]

Turn the cb off and remove it. Visually inspect the stabs that the cb plugs onto and also mating area on cb.

 

[Innovate1]

Turn the cb off and remove it. Visually inspect the stabs that the cb plugs onto and also mating area on cb.

First post states they checked at output lugs of circuit breaker and it was good. Although perhaps the bus connection is bad and removing the cover and pressing the lugs with the probe "fixed" it. Wouldn't hurt to check this and wiggle the breaker around to check any loose connections there.

 

[FredWanaker]

It is also possible that we are dealing with a bad connection to the breaker. The screw can show proper voltage, but the wire be loose and corrosion, especially if aluminum. Likewise, the wire at the receptacle can have voltage on it but that connection be bad. I found a bad wire twist in a junction box once where the wire inbound would have shown proper voltage if I put the prob on it but the wire outbound would have shown low. Only found it because when I shut the garage door the microwave on the other side of the house beeped as it lost power.

 

[larry_g]

Sounds to me like you have a short on the leg such that the resistance of the short is high enough to allow 15V still on your leg of interest. A dead short obviously would read zero, but a partial short can provide enough resistance that some voltage/current will still be present on the leg of interest.
You have 120V on that leg at the panel and 105V of it disappeared somewhere. It didn't disappear-- you need to find out where it went.
I'd check for some kind of rodent damage to the insulation. Could also be damage to the wiring from improper wire routing. You'd be surprised how often it ends up being something like a nail or plumbing hanger or such.

This is a hazard you need to address seriously because it could mean something like plumbing strapping or such is live to the tune of 105V. Definitely a fire hazard. Call a pro ASAP IMO.

Not that I believe this to be true, but for the sake of argument throw a amp meter on the black wire and measure the current draw. If a short is drawing down the voltage to cause the low voltage then you will be drawing some big amps. If while measuring the 15vac low to no amps are being drawn then you have an open.

lg
no neat sig line

 

[Innovate1]

Not that I believe this to be true, but for the sake of argument throw a amp meter on the black wire and measure the current draw. If a short is drawing down the voltage to cause the low voltage then you will be drawing some big amps. If while measuring the 15vac low to no amps are being drawn then you have an open.

lg
no neat sig line

No way the load is pulling the voltage down to 15V if the connections are good. It's a connection issue.

 

[Uncle Ben]

Thank you all for the input...much appreciated!
I will check the breaker and connection to the breaker lug, make sure there is no corrosion and reconnect to ensure a solid connection and then go from there.

 

[FredWanaker]

I concur that many people incorrectly use the word "Short" for both an open circuit, and a shorted error condition. A "short" is properly used to refer to a wiring connection that is causing no load current to flow between the wires in the circuit. In this situation, the amount of current needed to pull down the circuit from 120 volts to 15 volts with a no load direct "short" would be enough to make the wire glow and the breaker pop. If the resistance in the circuit were 3 ohms for example, a dead short would pull 35 amps to drop the voltage on that leg from 120 V to 105 V. It would produce 3700 watts. A hair dryer puts out 1100 +/-. A portable plug-in heater can be say 1500 watts. A "short" in this condition would be like almost 3 portable heaters. By now the house would have burned down or the breaker popped. Most likely there is corrosion at one of the connections, or in an unknown junction box in between which is creating an "open" condition. In the "open/corroded" situation, a 1-amp load on a circuit that has 100 ohms of resistance at a joint would drop the voltage 105V. That would produce the heat of a 100-watt light bulb. But, if the corrosion, either aluminum oxide, or copper oxide is say 10,000 ohms then the current needs only be .01 amps which is about a 1-watt loss at the junction on a 105V drop. Copper oxide and aluminum oxide conduct poorly. I used an online calculator to get the numbers, I used to be able to do it long hand or in my head. The old Ohm's law thing.

 

[wyliesdiesels]

Why do people call any issue with electricity a SHORT? What you describe is an OPEN or a high resistance partial open not a short. A short is when conductors that aren't supposed to be connected become connected. A short resulting in zero volts would trip the breaker. -- rant off --

Your other comments are good. Unlikely it is causing some exposed metal to be come energized but the fire hazard is real. I wouldn't expect any splices in the run but could be one near the HVAC unit - there could be others but unlikely.

You beat me to it.

Its not a short… drives me nuts when people call every problem with electricity a short… smh

 

[Innovate1]

It's a short? Just lengthen it!

 

[Uncle Ben]

Opened up my panel again and this time checked the connection to the breaker lug for the suspect leg of the 240v double breaker. As soon as I pressed on the wire a little I could instantly see that it was not snug. I shut off the whole panel and pulled the breaker and discovered that while the breaker lug was as tight as could be, the wire was behind the clamp, so when the installer snugged down the lug, they were not actually clamping the wire inside it...the wire was only making connection to the breaker lug by touching the back side of the clamp! On top of that, the screw head was already a bit stripped out, and it was cranked down so tight that it was impossible for me to loosen the screw without ruining it, which made the breaker unusable since I could not loosen that screw at all, not even to replace it with a new screw.

Needless to say, I am quite surprised that this breaker/circuit has functioned for the last 2 1/2 years that I have lived in this house. When I discovered this last night it was already after 10pm and past store hours, so I couldn't go get a new breaker on the spot, but will be pick one up today. Hopefully this will solve my overall AC issue now that there will be a proper connection and proper voltage.

In a bad connection situation like this, does the voltage fluctuate as the circuit is heated up during use, and that is why the AC would suddenly shut off, and then later would "magically" have voltage and function again?

 

[Innovate1]

Sounds like whoever installed it had some trouble and instead of seeing they were clamping on the wire insulation they just over torqued the screw to try to make it work.

As far as why it worked sometimes and didn't other times very small shifts in things can change the connection when it is barely touching. Not sure you will figure out why beyond that.

 

[mm08822]

In a bad connection situation like this, does the voltage fluctuate as the circuit is heated up during use, and that is why the AC would suddenly shut off, and then later would "magically" have voltage and function again?

Yes, it can. As a poor connection heat up, contact surfaces can expand to the point of the connection completely opening. Think of a bimetal strip in thermostats. When fully opened (circuit incomplete), it creates the obvious need to troubleshoot. It is the transition from ambient to very hot where intermittent connections occur, causing arcing and heating. This is the state when erratic voltages start showing up. Many times the insulation on the conductor starts to fry, with the worst being closest to the bad connection. Also arcing between contact parts can occur melting them.

Not too different from the pics I showed of the stab end of the cb (but caused by other reasons).

Throw out the cb and install a new one. Trim off a few inches of the conductor when reconnecting.

 

[Framton Nuttel]

We just had a nearly identical situation, with voltage in one leg that was low and fluctuating, and it ended up being due to the service connection at the meter socket not being fully seated (there was a temporary jumper plate in place of the meter). It was especially annoying because all circuits in the house on that leg were affected (in addition to every 240V device).

In our case it was clear the bad connection was inside the meter box because you could see some arcing when tapping on the glass faceplate of the meter socket. It had been working ok for almost a year, but we've had some extreme temperature swings recently and the problems started when the homeowner (a family member) came back from a long trip.

So in both this thread and our meter socket saga, it seems like a bad connection was introducing resistance with a resultant voltage drop in one leg. In addition to causing circuits to stop working, this kind of situation is a fire hazard due to the potential for arcing.

It would be good to make sure none of the equipment (breakers, panel) was damaged from heat/arcing. As mm08822 suggested, I would replace the circuit breaker involved just to be safe.

While troubleshooting this kind of thing, it's also probably a good idea to turn off breakers on the affected leg and/or make sure there are no motors (pumps, compressors, etc) running on it as these could be damaged. In many cases these will go into a thermal protection mode if the voltage goes too low, which might be why your equipment was acting weird.

 

[Uncle Ben]

I picked up a new CB and will install it tonight, but I wanted to ask about another thing in the meantime.

At the device (in my case a furnace @ the incoming terminal block), could the poor circuit breaker connection cause it to read 120v on each leg individually, but read no voltage when checking both legs together at the same time...or would only a polarity issue cause this condition?

There have been times that I would get 120v on each leg when tested individually, but when I would put my leads on both legs at the same time I would get 0volts, instead of 240v.

 

[Innovate1]

Sounds like the connection through the furnace was feeding 120V from the hot wire to the wire that was not making connection at the breaker. So both lines were at the same potential. 120V from either line to ground or neutral but zero between the hot legs.

 

[FredWanaker]

Sounds like the connection through the furnace was feeding 120V from the hot wire to the wire that was not making connection at the breaker. So both lines were at the same potential. 120V from either line to ground or neutral but zero between the hot legs.

concur

 

[Hohn]

Why do people call any issue with electricity a SHORT? What you describe is an OPEN or a high resistance partial open not a short. A short is when conductors that aren't supposed to be connected become connected. A short resulting in zero volts would trip the breaker. -- rant off --

Your other comments are good. Unlikely it is causing some exposed metal to be come energized but the fire hazard is real. I wouldn't expect any splices in the run but could be one near the HVAC unit - there could be others but unlikely.

I was taught that any current taking a shortcut is a short(ened ) circuit, and different from a dead short.

It’s not an open circuit because current is flowing.

 

[FredWanaker]

using your logic, It was an open circuit because no current was flowing at times. It was not a shortened circuit because when it was working the current was always taking the full path. At other times it was a corroded circuit which falls under a partially open circuit - like a cold solder or loose solder joint in a piece of electronics. That said - it was a loose wire causing it and I am glad he found it. The voltage he measured at times was the return voltage from the other leg. It may be that most of the time only the 120V devices were working. I hope there is no other damage and he can put it behind him.

 

[jskeen]

Sounds like you got this one nailed down now, but for future possibilities, and general consideration. If you want to test the wire run to a particular device, try the following. Disconnect both legs from the breaker and from the system at the other end. test voltage to ground at both ends. Should be zero, but check both ends. then, connect the two legs together at one end (yes, that's a short, but that's why we make sure there are zero volts on the conductors first) Any voltage at either end of the wire indicates there are other connections somewhere, and that needs to be solved. Then go to the other end and read the resistance (that's the ohm setting on your meter) you should get zero or very close to it between the two conductors. since all you should be reading is the internal resistance of the copper conductors from on leg through the connection you created and down to the other. anything over a few miliohms indicates a problem with that particular run of wire somewhere.

 

[wyliesdiesels]

I was taught that any current taking a shortcut is a short(ened ) circuit, and different from a dead short.

It’s not an open circuit because current is flowing.

Totally wrong…

 

[sparky 1971]

I picked up a new CB and will install it tonight, but I wanted to ask about another thing in the meantime.

At the device (in my case a furnace @ the incoming terminal block), could the poor circuit breaker connection cause it to read 120v on each leg individually, but read no voltage when checking both legs together at the same time...or would only a polarity issue cause this condition?

There have been times that I would get 120v on each leg when tested individually, but when I would put my leads on both legs at the same time I would get 0volts, instead of 240v.

Yep. You were reading the same 120 leg on both terminals. The 120 that you did have was feeding through the furnace and back to the terminal strip and that is why you were reading 120 to ground on both. It takes two opposite 120's make 240. I have a friend that is an HVAC tech and I can't get that beat into his head. I don't know how many times I've told him on the phone to read phase to phase and to please forget about anything to ground.

 

[Uncle Ben]

Makes sense. Thanks for clarifying.

I replaced the breaker and the AC is working again and continuing to run, and of course 240v at the furnace terminal block

 

[FredWanaker]

good work. You listened to suggestions and found the fault.

 

[Hohn]

Totally wrong…

ok, I'm not an electrician and I'm not saying I'm right. Just saying that's what I was taught.
Conceptually, it seems to me that open and short are two extreme ends of a spectrum, and neither term is quite correct for the case where resistance is neither zero nor infinite.

What *is* the correct term for this in-between state?

 

[Hohn]

ok, I'm not an electrician and I'm not saying I'm right. Just saying that's what I was taught.
Conceptually, it seems to me that open and short are two extreme ends of a spectrum, and neither term is quite correct for the case where resistance is neither zero nor infinite.

What *is* the correct term for this in-between state?

@Innovate1 @wyliesdiesels

https://www.khanacademy.org/science/electrical-engineering/ee-circuit-analysis-topic/circuit-elements/a/ee-circuit-terminology

Logically, nothing can be "partially open" just as a woman cannot be partially pregnant. A door is open or closed. If it's open 1mm, it's open.

Short Circuit

Short Circuit Definition: 1. A load that occurs when at ungrounded conductor comes into contact with another conductor or grounded object. 2. An abnorman connection of relatively low impedance, whether made intentionally or by accident, between two points of different potential. Related Videos...

electricalschool.org

The correct term for a leak of current-- I mean "an abnormal connection of relatively low impedance, whether made intentionally or by accident" is a short circuit.

QED

 

[FredWanaker]

he had an intermittent open. That would be how I would describe it. You can also have an intermittent short. A nail driven thru the romex when my house was built in 1979 created an intermittent short in the AC circuit that I did not find until the house was re-sided in 2003 or so. Periodically, only on extremely hot days, the AC would pop the breaker for no reason. The reason we found was the nail caused arcing due to heat expanding things, which in turn caused the wire to short thru the nail. Prior to discovering it when we removed the siding, we thought the AC was just overloaded from being run so much on hot days. It only happened 3 or 4 times in 12 years.

 

[dave*99]

Looking for a word that describes multiple electrical failures? How’s this:

fault: A partial or total local failure in the insulation or continuity of a conductor.

It was copied from an IEEE engineering standard document.

 

[Uncle Ben]

Sounds like you got this one nailed down now, but for future possibilities, and general consideration. If you want to test the wire run to a particular device, try the following. Disconnect both legs from the breaker and from the system at the other end. test voltage to ground at both ends. Should be zero, but check both ends. then, connect the two legs together at one end (yes, that's a short, but that's why we make sure there are zero volts on the conductors first) Any voltage at either end of the wire indicates there are other connections somewhere, and that needs to be solved. Then go to the other end and read the resistance (that's the ohm setting on your meter) you should get zero or very close to it between the two conductors. since all you should be reading is the internal resistance of the copper conductors from on leg through the connection you created and down to the other. anything over a few miliohms indicates a problem with that particular run of wire somewhere.

Smart...that makes sense. Thanks for this explanation.