Unbalanced amperage

Discussion in 'Electrical and Wiring' started by Quatrix, Dec 20, 2018.

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  1. Dec 20, 2018 #1

    Quatrix

    Quatrix

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    Hello experts!

    I have a question about my circuit breaker panel. Could someone tell me why exactly the smarter than me individual(s) decided to put 180A on the left leg (sum of all circuit breakers) and 170A on the right leg? Would this be an unbalanced situation and overload the neutral?

    P.S. Circuit Breaker panel attached to this post.
     

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  2. Dec 20, 2018 #2

    Snoonyb

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  3. Dec 20, 2018 #3

    Quatrix

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    Why NO?
     
  4. Dec 20, 2018 #4

    Snoonyb

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    What you need to understand is that the breakers protect the circuit conductors, so you are not asking the right question, and even though because the value of the breakers appears to be unbalanced, you'll need to run a load test to ascertain the load on the two primaries which will undoubtedly be less then the ampasity of the primary service feed conductors.
     
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  5. Dec 20, 2018 #5

    afjes_2016

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    And from a bit of memory on balancing the load don't the two pole breakers cancel out each other?
     
  6. Dec 20, 2018 #6

    hornetd

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    The reason that the grounded current carrying conductor is sometimes a neutral is that the current on one half of the transformer's secondary winding balances out the current on the other half of the transformer's secondary winding. In a residential neighborhood that transformer is likely to be single phase. That means it takes it's supply for the primary winding, which is the one through which the utility's current flows, from only one of the three distribution wires. In most utility arrangements each phase has the same voltage relative to each other and to the neutral. Since the primary is a single phase conductor the secondary side is also single phase. There is only one winding on the each side of the transformer. On the primary side one end of the winding is connected to the distribution multi grounded neutral. On the secondary side the neutral's grounding connection is made to the center of the winding and not the end. The ratio of the turns in each winding results in a secondary voltage which is a total of ~230 volts. Since there is only half of that winding on each side of the neutral connection point there is only half of that voltage, ~115 volts, between the neutral connection point and each end of the winding. Since you have the same voltage between the neutral of the secondary and the grounded neutral the loads on either side of the neutral balance each other and behave as if they are a 230 volt load. It is only the difference and not the sum of the two total loads that travel on the neutral conductor. If the actual load on one energized conductor is 150 Amperes with 170 amperes on the other energized conductor then only 20 ampere difference in the two current flows will travel on the neutral.

    Warning: The connection arrangement I have described is only, by far, the most common for residential service to individual detached homes. It is not the only arrangement. There are several others which are much less often used.

    As others have indicated the circuit breakers in your panel are sized to protect the conductors used in the circuit they protect. The breaker can only respond to the current which is flowing through it on the energized conductor which is attached to that breaker. It cannot protect the neutral except when the neutral is one of only two conductors in the circuit. Even then it only protects the neutral from overload because all of the current flowing in that neutral has to come from that one breaker. If there are 2 energized conductors in the circuit then, like the neutral in the supply wiring from the utilities transformer, the neutral of that feeder or multi wire branch circuit will only carry the difference of current flow on each of the energized conductors. The present code requirements for the installation of Combination Arc Fault Circuit Interrupters prevents the use of Multi Wire Branch Circuits in new construction or major remodels because the presently available CAFCIs can only protect a single energized conductor. That is why four wire cable with 2 separate neutral conductors is now commonly available. It maintains most of the labor savings of a multi wire branch circuits without using the same neutral for two energized conductors.

    --
    Tom Horne
     
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  7. Dec 21, 2018 #7

    Quatrix

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    hornetd,

    I appreciate your detailed reply. Essentially, the safety ‘buffer’ we have is that loads must be running on both phases simultaneously in order to have the current through the neutral be minimal. In situations when loads on one leg consume more current than on the other would result in an unbalance. If current on that hot service entrance conductor exceeds the Main breaker capacity, it may trip, causing every circuit to lose power. Please correct me if I’m wrong.
     
  8. Dec 21, 2018 #8

    hornetd

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    Yes the Main Breaker; which the code calls the Service Disconnecting Means (SDM); will trip if the current exceeds the rating of the breaker. The closer the overload is to the breaker's set point the slower it will open. In general breakers open much more quickly on a fault or short circuit than they do on an overload. Most breakers contain both a thermal and a magnetic trip. The thermal trip will open on an overload but in the case of a small overload it will take some time to do so. In the event of a fault or short circuit the sudden spike in current flow trips the magnetic element of the breaker very quickly.

    In the absolute worst case scenario of an unbalanced load, were the load on one energized conductor is nothing and the load on the other energized conductor is is near it's maximum ampacity, the neutral will only carry the same current as the energized conductor which is carrying all of the current that is coming in to the home. There is very little danger of exceeding the capacity of the neutral as long as it is only getting current from an identically loaded conductor. If however the neutral has to carry the current from 2 circuits which are not on different halves of the transformer secondary winding it can end up carrying a load that is the sum of the two energized conductors loads. That will overload the neutral and since the neutral does not have over current protection, except that which is installed in the energized conductors it is properly paired with, it will overheat, fault, and possibly develop an arc which would kindle a fire almost immediately.

    --
    Tom Horne
     
  9. Dec 21, 2018 #9

    Eddie_T

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    Great explanations Tom, balance would be fleeting at best since a home has many intermittent loads. When I wired my house I didn't give it any thought but the heaviest loads are 240v anyway. My heaviest 120v loads would be microwave, electric iron, vacuum cleaner, space heater and shop tools. At a NASA tracking station I worked at they balanced loads at least once annually to minimize neutral current (most loads were constant from electronic equipment). I have thought it might be nice to have a current loop on my home neutral as a curiosity item but I am not sure if it might be a code violation to do it the way I was considering.
     
  10. Dec 21, 2018 #10

    hornetd

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    If by a current loop you mean a current measuring transformer I do not know of any reason you could not have one. They do not actually connect to the conductor which they are measuring the current in so I cannot see anyone having an objection to it. Some of them are quite bulky so you would need to be careful not to overfill the wiring trough of the panel's cabinet.

    It is quite common for utilities to use current transformers to measure usage in properties with large loads or multiple buildings. In farm properties it used to be common practice to have current transformers on the service drop from the last utility owned pole to the premises owned Yard Pole. The Service Disconnecting Means would be located at the Yard Pole and from there separate feeders would run to the individual buildings. Sometimes the Current Transformers and the meter that read the flow through them would be mounted at the last utility owned pole to assure reading access to the meter.

    --
    Tom Horne
     
  11. Dec 21, 2018 #11

    bud16415

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    Where I worked for 43 years was a huge industrial plant we had our own coal fired power plant and could co generate power on and off the grid to fully power the facility. We had in the hay day 16,000 employees and huge inductive loads throughout the plant and power factor correction was a major concern. It is somewhat similar to what we are talking about here but instead balancing the power factor.


    We had a compressor house as part of one building that supplied compressed air underground for the whole plant. The compressors were gigantic and they were powered with special motors that introduced a capacitive power factor that would correct the inductive factor and losses.


    The end result was that 100% of the compressed air needs was done at zero cost in electricity as the installation saved more money than it cost.


    The really remarkable part of this it was all done around 100 years ago and has ran 24-7 for all that time with hardly anything except some preventive maintenance once a year during the two week plant shutdown.
     
  12. Dec 21, 2018 #12

    nealtw

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    Okay maybe I am dense, I understand that you want to lower the current in the neutral by matching likely users like fridge and freezer.
    But I don't understand overloading the neutral to melt wires, the ground will take half the load so the load going back to the meter will never be more than the main breaker. Correct me? Is that not what the ground is for, or at least one reason.
     
  13. Dec 22, 2018 #13

    JoeD

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    The ground does not ever carry current except in a fault. At least it is not supposed to.
    Current on the neutral in a standard home installation is always going to be less than the current on the hot unless all your loads are on the same leg of the service. This is next to impossible to accomplish. The current on the two hots cancel each other out on the neutral. The neutral only sees the difference in current.
     
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  14. Dec 22, 2018 #14

    nealtw

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    The question was at the top of the page was about over loading the neutral when out of balance. That would only be the neutral to the road. The ground and neutral are tied together at the main. ???
     
  15. Dec 22, 2018 #15

    Eddie_T

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    It is correct that the neutral and the earth provide parallel current paths to the center tap of the pole transformer. However the current through the earth requires a voltage to push it. The available voltage at the panel would the voltage drop of the neutral to the pole, 3/0 SE cable would have a negligible resistance compared to the resistance of the ground connection and its earth path to the pole ground connection. Most ground connections are not tested so probably don't meet stated code resistance value. I saw a ground test by Mike Holt on youtube where a 50' ground rod had 68 Ω resistance. My ground rod is 10' about the same as the pole ground so any earth path current would be negligible compared to the neutral.
     
  16. Dec 23, 2018 #16

    nealtw

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    Okay, that is a little deep but I will buy but could not explain it to anyone.
    Why are ground and neutral connected?
     
  17. Dec 23, 2018 #17

    hornetd

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    To give the current flowing to ground from a lightning strike to the overhead power lines a less destructive path to get there is one reason. The National Electric Code puts it this way: 250.6
    (A) Grounded Systems.
    (1) Electrical System Grounding. Electrical systems that are grounded shall be connected to earth in a manner that will limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operation.

    --
    Tom Horne
     
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  18. Dec 23, 2018 #18

    hornetd

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    Nealtw

    Just apply Ohm's law to the normal situation and you will see what is going on. The National Electric Code (NEC) requires that a driven electrode have a resistance to ground of 25 Ohms or less OR that a second ground rod be installed at least 6 feet away from the first. Once I acquired the tester to do it easily I got in the habit of measuring the resistance to ground on all the ground rods I installed and all the ones at the existing installations I worked on. It takes about 5 minutes. I learned that just about every electrician didn't bother measuring because the resistance to ground of the single rod was never, in my experience of measuring them, lower than 25 Ohms. Even with the second rod the resistance still did not go below 25 Ohms. In nearly all installations, that I measured, the resistance to ground at the house was over 50 Ohms. The other end of that return path would be the Grounding Electrode at the transformer with the earth as the conductor in between.

    Power Utilities are not regulated using the NEC rules. The code that the state regulators apply is the National Electrical Safety Code (NESC). Thus the Grounding Electrodes that they use are sometimes similar to those in the NEC but often very different. One common power utility transformer grounding electrode is a spiral of bare copper wire which is stapled to the but of the pole before it is set. Another is to cover the first foot of the but of the pole with copper sheathing. Whenever I could measure the utility's electrode without tampering with the installation I did so. That wasn't very often because the Utility's Grounding Electrode Conductor is often covered by a wood or plastic molding for the first 10 feet from the ground and the Linemen staple the GEC to the pole very tightly. If you cannot get the transformer jaw of the Ground Loop Impedance Tester around the GEC you cannot take the measurement.

    Impedance is the complex resistance which at 60 Hz is always quite close to the resistance alone in value and is calculated in the same way. So with 50 Ohms at the Service Disconnecting Means; read main panel; and 70 or more at the pole the total Impedance of the Ground Return Pathway is 120 Ohms. The earth is so large that the Earth's effective resistance in this pathway is Zero. If the Neutral conductor of the Service Entry Conductors is intact then the voltage over that entire pathway is very low. Lets take the worse case. An old service in which the Service Disconnecting Means consist of a fused pull out and one of the fuses has opened for whatever reason. That way the surviving fuse, which is the only source of current flow, loads the neutral with all of that current. Now all of that current is flowing on the neutral and the Ground Return Pathway, in proportion to their resistance, as well as on the energized conductor with the surviving fuse. There is no opposing current from the other energized Service Entry Conductor to cancel out a portion of the current and reduce the load on the neutral.

    For example if the current through the intact fuse is 120 Amperes. The resistance of say 100 feet of #2 American Wire Gauge Aluminum Clad Steel Reinforced (ACSR) overhead conductor; because that is what the utility is likely to have used; is only .519 per 1000 feet. So over my example of 100 feet would have a resistance of 0.0519. Ohms law says the Current times the Resistance will equal the voltage. Thus E = I X R. E is the Electro Motive Fore measured in Volts, I is the current in Amperes. R is the Resistance in Ohms. So in our case the Voltage Drop; which is the voltage reduction caused by the resistance of the conductor; is 0.0519 Ohms of Resistance X 12o Amperes Of current = 6.228 volts over the utilities Service Conductors between the Main Bonding Jumper; in the panel cabinet that contains the SDM, and the center tap of the utility's transformer. That is the only voltage that the Ground Loop Pathway is subjected to. So with only 6.228 volts to "push it" and 120 Ohms of resistance the Ground Loop will carry about 50 Milliamperes of current. Yes I know that I took some shortcuts by not calculating the parallel resistance of both pathways first but any difference is outside the measuring accuracy of the available instrumentation.

    So out of all of the 120 Amperes of current the Ground Loop pathway carries only 50 Milliamperes of current. No where near half.

    --
    Tom Horne
     
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  19. Dec 24, 2018 #19

    JoeD

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    The ground and neutral are connected for one reason. That is to give the current a return path to trip the breaker if the hot should come in contact with any metal parts of the system or anything plugged into it with a grounded plug.
     
  20. Dec 24, 2018 #20

    hornetd

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    JoeD

    I suspect that he was asking about the Grounding Electrode Conductor rather than the Equipment Grounding (Bonding) Conductor (EGC).

    I could be wrong of course.

    --
    Tom Horne
     

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