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Thread: 240V across mains but not across bus bars?

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    DIY Junior Member ColdShower's Avatar
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    Default 240V across mains but not across bus bars?

    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.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    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?
    Jim DeBruycker
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    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    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.

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    DIY Junior Member ColdShower's Avatar
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    Quote Originally Posted by jadnashua View Post
    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?
    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.

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    DIY Junior Member ColdShower's Avatar
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    Quote Originally Posted by Hackney plumbing View Post
    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.
    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.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    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.
    Jim DeBruycker
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    Electrician ActionDave's Avatar
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    Quote Originally Posted by ColdShower View Post
    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.
    Sounds like something wrong with the main breaker to me.

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    DIY Junior Member ColdShower's Avatar
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    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.

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    Electrical Contractor/Instructor jwelectric's Avatar
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    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

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    DIY Junior Member ColdShower's Avatar
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    Quote Originally Posted by jwelectric View Post
    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
    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...

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    DIY Junior Member ColdShower's Avatar
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    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.

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    Moderator & Master Plumber hj's Avatar
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    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 by hj; 02-18-2012 at 10:13 AM.
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    DIY Junior Member ColdShower's Avatar
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    Quote Originally Posted by hj View Post
    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.
    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.

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    Plumber jimbo's Avatar
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    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.

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    DIY Junior Member ColdShower's Avatar
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    Quote Originally Posted by jimbo View Post
    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.
    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.

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