240V across mains but not across bus bars?

Discussion in 'Electrical Forum discussion & Blog' started by ColdShower, Feb 17, 2012.

  1. ColdShower

    ColdShower New Member

    Messages:
    11
    Location:
    Oregon
    Hi, I'm trying to figure out why my 240V appliances aren't working. I measure 120V on both legs but 0V across them, at the appliance, the dual-pole breaker and across the bus bars themselves. But I do get 240V when measuring across where the service cables connect to the main breaker.

    Can a fault in the main breaker cause this? It seems to be making good contact and has never tripped. Thanks in advance if anyone can help me with this.
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,180
    Location:
    New England
    What do you get on the leads coming off the CB feeding the appliance?

    A dual-pole CB generally, can only be installed such that the two sets of contacts connect to opposite busses. If they both are connected to the same bus, you'd get what you're seeing. Normally, there's a tab or something to prevent you from installing the CB in a spot where that could happen, forcing it to be on both legs of the main.

    Is this new wiring, or something that had been working and stopped?
  3. Hackney plumbing

    Hackney plumbing Homeowner

    Messages:
    1,174
    Location:
    Alabama
    If your getting 240 across the service wires but not getting 240 out of the main breaker I would think the main breaker is the problem or the bars it connects to.
  4. ColdShower

    ColdShower New Member

    Messages:
    11
    Location:
    Oregon
    It's 0V across the leads coming off the CB too yeah. It's installed right and all...it's old wiring and quit working on its own.
  5. ColdShower

    ColdShower New Member

    Messages:
    11
    Location:
    Oregon
    That's what I'm thinking too but AC and these phases is a mystery to me and I'm learning as I go...plus would like to know which of those two it is before I go further.
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,180
    Location:
    New England
    Have you moved the CB, or is it in a position where it worked at one time?

    If you measure from one lead on the CB to neutral, get 120vac, and do the same on the other, get the same 120vac, but zero between them, then the bus connections of the CB are connected to the same bus...not one side on one, the other side on the other. This shouldn't happen if the panel is good and the CB is one of the acceptable versions - it's supposed to have a key or lockout on the back that prevents it from being installed in a slot where that can happen. But, if it's the wrong type, in the wrong slot, or the interlock tab is broken, it could happen.
  7. ActionDave

    ActionDave Electrician

    Messages:
    351
    Location:
    Colorado
    Sounds like something wrong with the main breaker to me.
  8. ColdShower

    ColdShower New Member

    Messages:
    11
    Location:
    Oregon
    Yeah it seems like it must be a problem in the main breaker or the buses underneath it...guess i'd like to hear a theory as to how a fault there could change the phase before I tackle that though.

    Jim, I get 120V between neutral and all the leads of the breakers. The design of the breaker panel is uhh non-standard from what I've been reading trying to figure this out. I should have some part numbers or a photo to show and explain but I don't right now. There are two buses but just one column of breakers and there aren't tabbed positions for them vertically...you can slide the breakers up and down the bus bars as long as they aren't completely filled with breakers. Every breaker has two contacts on each of the two buses and two switches so the source of the bus doesnt change when then the CB position is changed...hope this makes sense.
  9. jwelectric

    jwelectric Electrical Contractor/Instructor

    Messages:
    2,542
    Location:
    North Carolina
    Try resetting the main, turn it off and then back on.
    You have lost one leg and are reading through one of the 240 volt breakers such as the water heater
  10. ColdShower

    ColdShower New Member

    Messages:
    11
    Location:
    Oregon
    oops I intended to mention that I've cycled the main off and on a number of times and paused a bit while off but it didn't change anything. The main breaker feels solid and functional and not able to jiggle it or anything...not as if one of the two switches has failed that is...for what that's worth.

    I'm going to try removing all the breakers except the main and see what it measures across the bus bars before I try removing the main, some of them are broken and not even in use...maybe one of those is making contact across the buses and causing this? It'll be a few hours before I can make it to town to buy more flashlight batteries and try it though...
  11. ColdShower

    ColdShower New Member

    Messages:
    11
    Location:
    Oregon
    well it seems the main breaker is ok other than about 20V drop on one leg...I get about 200V across the bus bars with no breakers installed. And the breakers are all working ok. Seems I have a couple of problems in the house circuitry. If I turn on the circuit for the kitchen outlets I only get 120V across the buses and if I turn on the dual circuit water heater it then drops to 0V. So I'm at a loss again. Any ideas appreciated.
  12. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,874
    Location:
    Cave Creek, Arizona
    You have gone to a lot of trouble without finding out the real problem. Apparently one leg of the main breaker is allowing a small amount of current which gives you the 210 reading, but as soon as you turn on a load, the "backfeed" overpowers it and then you have no power reading through that leg. Put everything back together again. Then turn on the main breaker. Test each leg between the in wire and the out wire. The bad side will indicate 240 volts, the good one will be zero. I am not sure what kind of main breaker you have, but you probably do NOT have a disconnect between it and the meter so you will be working with a "live circuit", so unless you know what you are doing, you may want a professional to do it.
    Last edited: Feb 18, 2012
  13. ColdShower

    ColdShower New Member

    Messages:
    11
    Location:
    Oregon
    hmmm sorry I don't quite understand where the measurements are to be made at that would give 240V or 0V on a single leg or circuit. It may be something to do with the one bus bar...the suspect circuits are on that bus. I've just discovered that with no load on that bus at all it measures about 80V. And the loaded bus stays at ~120V. If I turn on just the kitchen outlets then no change but nothing is running there except a clock I think. If I turn on the water heater the bus jumps to 120V where it should be (like the other). But still the 0V across the two of them. It's not even really loading the thing with the water heater on...I mean it's not drawing any current. (I have a clampmeter). Ack it's a mess I think...stupid boxcar.
  14. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    When you have an open somewhere, you get readings like you are talking about, because just putting the meter in the circuit provides a path for current flow.

    You have to either in your head, or on paper, have an excellent picture of the complete circuit and all loads, and from that you can interpret the readings you have. But for sure, something is an open circuit somewhere.
  15. ColdShower

    ColdShower New Member

    Messages:
    11
    Location:
    Oregon
    ok thanks...I'll read up on how to troubleshoot for that and start with the water heater...and hope it stops raining outside where the access panel for that is.
  16. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,874
    Location:
    Cave Creek, Arizona
    Measure voltage from the top/incoming of each breaker to the bus below it. IF the breaker is good the voltage will be zero, if bad it will be something other than zero. forget about the water heater or anything else. If you have 240 to the main breaker and anything other than 240 AFTER it, the problem is in the breaker or its connections. You are making your problem WAY too difficult. All you have to do is make FOUR measurements and you will know where the problem is. Feed to feed, bus to bus, feed to bus1 and feed to bus2.
    Last edited: Feb 18, 2012
  17. cacher_chick

    cacher_chick Test, Don't Guess!

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    Location:
    Land of Cheese
    Just a note that some loads should be on when you do this test. If there is no load on the side of the panel being tested, you will not get an accurate reading of the voltage drop.
  18. ColdShower

    ColdShower New Member

    Messages:
    11
    Location:
    Oregon
    Well I still don't understand...maybe I should have a picture of my panel...or take a look at a standard one. My breakers connect directly to the bus bars...thats the "incoming"...and there are two contacts on each breaker (two breakers really on each unit)...and each one makes contact with a different bus. So there's like two buses above and below each breaker and two leads coming off.

    I'm not sure if this house was ever getting more than ~200V combined.
  19. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,180
    Location:
    New England
    Look at it this way...the CB are just smart disconnect switches. A switch connects the input to the output. There should be effectively a direct short between the input and the output. If you were to measure with your meter on one end of the bus bar to the other end of the same bus bar, with the meter you likely have, you'd measure zero volts, since the bus bar is acting like a short connecting the two ends. A CB is similar. So, if you measure from the inlet to the outlet across the CB, it should read zero volts since the switch inside should be closed (or shorting the inlet to the outlet). If you measure some voltage, that means that the switch isn't working properly and is acting like a load - dropping some voltage inbetween.

    It can get a little messy depending on the load, if any, and sometimes you need a load to get an accurate reading.
  20. cacher_chick

    cacher_chick Test, Don't Guess!

    Messages:
    3,250
    Location:
    Land of Cheese
    Is is a Federal Pacific panel?
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