I ran into the following situation. One bx cable had 3 wires in it ( as in 12-3 with no ground- sheath being ground). One wire went to neutral bus, each of the other wires went to a seperate 15 amp breaker. This situation was uncovered when I ran into a problem in the upstairs duplexs, each duplex was on a different 15 amp circuit.. The problem was as follows: across the two wires I had 118 volts but from EACH wire to the metal box (ground) I had 25 to 30 volts…. not 120 from one and 0 form the other. So is this a problem inherent in splitting what is essentially a 220 line into 2 seperate 110 volt circuits. I think that is generally a poor idea anyway. Or, does this voltage on both wires indicate some other problem somewhere in each circuit. Generally, in the panel things looked good to me.
The neutral bus (bar?) was not grounded to the panel…. and the ground bus (bar?) was. I could also ask if Im the only one who is not licensed in electrical work to do these things… I’m a construction supervisor (MASS)…. thanks.
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Replies
You have a bad ground connection somewhere. Hot to ground should always measure within a gnats eyelash of the hot to neutral measurement.
What you descibe is a multi-wire circuit.
"The problem was as follows: across the two wires I had 118 volts but from EACH wire to the metal box (ground) I had 25 to 30 volts.... not 120 from one and 0 form the other. So is this a problem inherent in splitting what is essentially a 220 line into 2 seperate 110 volt circuits."
It is not clear what you are talking about. You have 3 wires, 2 hots and a neutral. So I don't know what "two wires" you are talking about.
Between the two hots you should have 240 volts.
What are you using to measure the voltage?
"The neutral bus (bar?) was not grounded to the panel.."
Where is the main disconnect? Where does the grounding electrode system connect?
Lets start at the beginning. At the panel measure Hot, Hot, each not to neutral bus and Neutral bus to Ground bus.
If the each of the two hots to neutral are not equal, if the hot to hot is not aprox 240, and the neutral to ground bus is not very near zero then you have a problem, potential very server problem, with the panel or service.
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
The two wires I'm referring to are as follows. Here's the situation. Each 15 amp breaker feeds one conductor in a bx cable, the third wire in the cable is a neutral wire. In the house, at a duplex on each circuit I had the following problem.... each wire in the metal box where a duplex is hot.... it has 25 volts between it and the metal box... I'm using a tester which cost 100 dollars but I can't remember the name of it.... Frug? Krug?.... thanks
The metal box isn't grounded.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
As Bill described, that is a Multi Wire setup. The AC into a home comes in two phases... while it is pulsing "up" on the red wire it is pulsing "down" on the black.
If you have a single device that uses both the red and black wires, it combines the voltage up to 240v
Theoreticly, since both the red and black wires are each pushing 120v out, there should be a total of 240v coming back through the white - right?
Nope! Because the red and black wires power is on an opposite phase, when they meet back on the white wire they cancel eachother out.
If there is an unequal draw on the two wires, say you have 2 100w bulbs burning on the black wire and 1 100w bulb burning on the red, then you only have the difference between the two draws flowing down the neutral.
If you don't have a full 120v going between a hot and a metal box, then the ground connection is flawed or missing. On an older home it may not even exist. Like mine.Rebuilding my home in Cypress, CA
Also a CRX fanatic!
".. I'm using a tester which cost 100 dollars but I can't remember the name of it.... Frug? Krug?.... "I am not talking about the brand. Rather the type. Is it a solenoid tester (wiggley), DVM, analog meter."In the house, at a duplex on each circuit I had the following problem.... each wire in the metal box where a duplex is hot.... it has 25 volts between it and the metal box.."Was it is?You need to be very accurate in your terms.Or you talking about from the hot wire to the box, the neutral wire to the box?But FIRST you need to run the tests that I described at the panel.For any meaningfull measurement at the box there has to be bonding of the neutral to the grounding electrode system someplace.Since it is not in the panel then the system is suspect. Now depending on all of the circumstances the bonding is sometimes done at other places. But them the panel is technically a sub-panel and should be feed by 4 wires..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Hi. Let's see if I can be more specific... My tester is a Fluke T5-600. When I pull the duplex receptical out of the metal box it has two wires attached to it. One should be hot, one should be neutral. The bx jacket should be ground. When I test the voltage from either of these wires (neutral, hot) to the metal box that the duplex was housed in, I get a reading of 25 volts. When I test accross the same two wires I get a reading of 118 volts. That is the problem with a duplex in one of the 15 amp circuit. I had the same situation in a duplex on another 15 anp circuit... in that case I got reading of 30 volts between "neutral" and the box and between hot and the box the houses the duplex.
In the panel, there are two 15 amp breakers (among others). The breakers for these two circuits above described have hot wires connected to them that each feed into a single bx cable that leads from the panel. The third wire in the bx cable connects to the neutral bar in the panel. That's the situation.
Thanks , in advance, Bill and others for your thoghts.
The box is not grounded.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
Yes, the box is floating with respect to the neutral.But you need to go back to the panel to see if that is also floating or if you have a bad EGC between the panel and the boxes..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
>>The neutral bus (bar?) was not grounded to the panel.... and the ground bus (bar?) was.
Which would be appropriate IF 1. this box is a sub-panel (i.e., doesn't have the main disconnet breaker/fuses) AND 2. there is a line from this neutral to the neutral in the main panel (that is, there should be 4 lines coming from the main panel: 2 hots, a neutral and a ground) The ground might be BX ifd it's an older installation.
If both of those considtions exist, then this is called a floating neutral and is how sub-panels are supposed to be wired.
What made the teaching of Jesus different and apparently so hard to accept then as now, was that it required a critical reassessment of the structures and values and attitudes of human society as his listeners and followers shared in it.
- Monika K. Hellwig
from Jesus: The Compassion of God (The Liturgical Press, 1983)
IF this is the only cable that is bad,
AND both circuits are showing the problem
THEN remove the neutral from its' buss and reinstall it.
THEN check the BX sheath ground to ground buss connection.
IF that doesn't correct the problem, THEN check the BX sheath to Outlet box connection.
IF the problem is still there,
Plug a known good three conductor extension cord into a known good outlet and carry the female end over near the bad outlet.
I'll call the suspect outlet conductors HotB, NeutralB and GroundB and the extension conductors HotX, GroundX and NeutralX.
Voltage readings shoud be
* = These are the readings that will tell you the most, if one is bad that B condutor is bad or has a bad connection somewhere.
SamT
Guys that don't do things correctly the first time.....then argue that they did nothing wrong.....if made to agree to fix the problem, rarely put the time and effort into truely doing it properly. they'll just look for the quickest fix to appease you and get their money. JDRHI 84310.51
I might be blind but I haven't seen any body ask if the hot leads are on separate legs or the same leg if the breakers are not next to each other the it's possible they are on the same leg. If hots are on same leg will you get 240 across both hots?
Nope.
Will see zero volts difference.SamT
Guys that don't do things correctly the first time.....then argue that they did nothing wrong.....if made to agree to fix the problem, rarely put the time and effort into truely doing it properly. they'll just look for the quickest fix to appease you and get their money. JDRHI <!----><!----> 84310.51
thanks so far, I'm going back to the site next week..... both breakers for these two lines are on the same bus, by the way. O.K., Bill, what's a EGC? and thanks for all those tips on testing the system from a known good outlet with an extention cord. And I'll remove and reinstall the neutral from it's neutral bus. and I'll check the bx sheath connections at that box and others along the line. Thanks; I'll check back in.
"O.K., Bill, what's a EGC?"Equipment grounding conductor. That is the ground (wire, metalic metal sheathing or conduit) that runs from a metal box, the ground one a device, and/or a hardwired load back to the point where the neutral is bonded to the grounding electrode conductor (wire to the ground rod(s), qualified metalic cold water pipes, ufer ground, etc)."and thanks for all those tips on testing the system from a known good outlet with an extention cord. And I'll remove and reinstall the neutral from it's neutral bus. and I'll check the bx sheath connections at that box and others along the line. Thanks; I'll check back in."THAT IS ABSOLUTELY MEANINGLESS UNTIL YOU VERIFY THAT THE PANEL IS SELF IF PROPERLY GROUNDED.WHILE USING AN EXTENTION CORD TO A "KNOW GOOD RECEPTACLE" IS GOOD TECHNIC WITHOUT VERIFYING THAT THE PANEL IS CORRECTLY WIRED IT IS NOT ONLY MEANINGLESS. BUT ALSO CAN BE CONFUSION AND/OR HIDE A DANGEROUS CONTIOUS.YOU NEED TO TEST THE PANEL FIRST!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!.
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
After you and Bob had already said that the panel had to be checked first, I figgered I didn't need to.
We still don't really know if this is even a sub or main, we just know that it sounds like a sub.SamT
Guys that don't do things correctly the first time.....then argue that they did nothing wrong.....if made to agree to fix the problem, rarely put the time and effort into truely doing it properly. they'll just look for the quickest fix to appease you and get their money. JDRHI <!----><!----> 84310.51
Bill and Casey and Sam, Thanks for all the input. The panel is a main panel, not a sub panel as far as I know,,,, it is in a duplex.... so I'll check it out for sure. So, first I check out to see that the neutral bar is not bonded to the panel but grounded to the grounding bar. and then see that the grounding bar or panel are properly grounded to a qualified water pipe, or grounding rod. If all this checks out then switch one of the two breakers that share a common hot bus to the other hot bus. Then check out the EGC (bx sheathing) to see if it's grounded to the metal boxes.... on each box in the two circuits. From a theory point of view I can see how sharing the common neutal can overload it (with up to 30 amps of power) but I can't understand how current from one hot could ever return down the other hot, minimizing the load on neutral. This is a very useful website. Or should I say I enjoy learning from you guys who are willing to teach .Thankks
but I can't understand how current from one hot could ever return down the other hot, minimizing the load on neutral
The theory behind Kirkhoffs Law is a little esoteric, but you can think of it as if the electrons from hot A start down the return neutral conductor and hot B says " Hey guys, I need you to come this way", so the neutral doesn't see them.
In that sense the current is only "Returning" until the common point of the neutral circuit, then it starts "Feeding."
Fer instinks, assume a three wire split phase circuit, 2 opposing hots and 1 neutral. Connect 2 pigtails to the neutral and run them and the hots to two outlets with, say, 10 amps on each.
When you use your clip-on ampmeter to measure the current flow, you'll see 10 amps on each hot, of course, and 10 amps on each neutral pigtail. But the neutral itself will not show any current.
If you use a magical ampmeter that shows instantaneous direction of flow you'll see current flowing down the A neutral pigtail from the A outlet towards the common neutral. But on the B pigtail it will be flowing towards the B outlet. But only for that 1/2 cycle of AC power. If you unplug the load in the B outlet, it won't be calling for current and the electrons from the A outlet will go home down the common neutral.
According to Kirkhoff, if you had an even more magical meter that could identify each electron, it would see 10 amps of electrons, each labled with a little "A" flowing down the common neutral towards the panel and another 10 amps with little "B"s flowing up towards the B outlet for a total common flow of zero.
All this changes every 1/2 cycle of AC.
Are you totally confused yet? If not I can relate the theory of "Hole Flow." LOLSamT
Guys that don't do things correctly the first time.....then argue that they did nothing wrong.....if made to agree to fix the problem, rarely put the time and effort into truely doing it properly. they'll just look for the quickest fix to appease you and get their money. JDRHI <!----><!----> 84310.51
On most panels the panels the the hot bus bar connection zig-zag so that two adjacent full size breakers on on oposite legs. You want to meausre the voltage between the two breakers where the leads to the loads connect.If they measure aproximately 240 volts then they are on different legs and proper.If you measure about zero then they are on the same leg (or one of them is open and you have a 240 volt load on it).If you measure something else then one of them is open."From a theory point of view I can see how sharing the common neutal can overload it (with up to 30 amps of power) but I can't understand how current from one hot could ever return down the other hot, minimizing the load on neutral"http://en.wikipedia.org/wiki/Split_phaseThis shows how the US single phase 120/240 supply works. It has a center tappped transformer.If you have a load (120) connected from V1 to N then the current flows from V1 through the load to N and returns to the transformer.Of if on the other leg a from V2 to N.A 240 load would be connected between V1 and V2 with NO CONNECTION to N. Current would flow from the transformer to V1, through the load to V2 and back to the transformer.The two legs are 180 degress out of phase with respect to N so that when V1 is at is positive peak (from N) and V2 is at it's negative peak. So at THAT INSTANT current would be flowing from V1 to a 120 volt load to N. And for a load on the other leg current would be flowing from N through the load to V2 (as it is negative). So the two different currents in N are in oposite direction and they cancel so that it is only the difference is currents." The panel is a main panel, not a sub panel as far as I know,,,, it is in a duplex..."Where is the panel with respect to the meter or more accurately where the power enter the structure. The main disconnect has to be "near" (upto the inspector and local rules, but usually with 1-3 ft). If not there will be a separate disconnect, basically single big 2 pole breaker). Some locations always requirement on the outside no matter where the panel is. If there is a disconnect then the panel treated as a sub-panel..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Bill,
There may be more issues and information lacking.
He has stated that it is a main panel. He has also stated that the neutral and ground busses are NOT bonded.
Poet,
These two statements are usually self contradictory.SamT
Guys that don't do things correctly the first time.....then argue that they did nothing wrong.....if made to agree to fix the problem, rarely put the time and effort into truely doing it properly. they'll just look for the quickest fix to appease you and get their money. JDRHI <!----><!----> 84310.51
WOW! ...I've got a degree in Electrical Engineering and you guys are starting to really confuse me!
How can you be an EE and not already be cornfused? I know that was my state when I got outa skool.SamT
Guys that don't do things correctly the first time.....then argue that they did nothing wrong.....if made to agree to fix the problem, rarely put the time and effort into truely doing it properly. they'll just look for the quickest fix to appease you and get their money. JDRHI <!----><!----> 84310.51
I don't care what anyone else says ...those sconces are wired in series....!
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sconces.
Is that like , you know, more than 1 scone?
If so, does that make all them tea drinkers' pinkies parallel to a tangent of the earths circumference at that point? Level, you know.SamT
Guys that don't do things correctly the first time.....then argue that they did nothing wrong.....if made to agree to fix the problem, rarely put the time and effort into truely doing it properly. they'll just look for the quickest fix to appease you and get their money. JDRHI <!----><!----> 84310.51
I probably tried to install a scone or two . but I have a weakness for Napoleons.however,using an elongated octagesimal divisor, I found that you can feed upwards of darn near 8 people with one very large scone.
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"He has stated that it is a main panel. He has also stated that the neutral and ground busses are NOT bonded."That is why I went in the issues of a service disconnect.To a lot of people if they don't see another BIG panel then this must be the MAIN panel.Typically I answer a question based on the assumption that they know what they are talking about and only need the specific answer.But once I start seeing that either they don't understand what is going on or what the describe is very far off what it should be (eitheer because some else screwed it up or they aren't accuratingly understanding the situation) then I ASSUME NOTHING IS CORRECT AND WANT EVERYTHING TO BE VERIFIED..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Edited 1/27/2007 8:19 pm by BillHartmann
"both breakers for these two lines are on the same bus, by the way."If you have both "hot" wires sharing a neutral wire with both hot wires being on the same bus in the main or subpanel, then you have an additional problem - you have an illegal and dangerous pair of circuits. Hot wires on the opposite busses in the main panel can share a neutral because the current is out of phase, so the current from one hot returns down the other hot, minimizing the load on the neutral. With both hots on the same bus, the current from both hot wires must return by the single neutral wire, overloading it and possibly causing overheading and worse. When you get your current problems sorted out, you need to switch one of the hot wires to the opposite buss in the panel.