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| Subject - Unbalanced Load | |
| David Hyatt | OK, the neutral carrys the unbalanced load. It has to be sized accordingly. If you have two phase wires and one neutral and lose a phase it will have an unbalanced load. Or in the case of florecent lights it will carry current. It all looks good on paper, wish I had a good diagram to see how the current travels. Just in my mind I see a grounded conductor (neutral)carring current back to the panel, why is it not a dead short. Maybe this is easier, wiring lights in series, The current is traveling through the white wire as well as the black, this works when the resistance of the bulbs are in place, renome the bulbs and the sockets then you have a dead short. Just because of the resistance of the bulb it is not a short. I hope someone can understand my question, because I can't hardly understand it myself. |
| ricky | I don't understand the question.All of the voltage is dropped across the load.The same current flows in the hot and neutral,just at a lower voltage.The resistance or reactance of the load causes it not to be a short,as you said.The unbalance between phases would result from pulling more power on one phase than on the other.If your flourescent lights were say 120v,you would want to split them up on different breakers on differnt phases,so you have the same number on each phase.This would balance the load.Did I help? |
| David Hyatt | Ricky, what you said is right. But if you acually have voltage on the neutral which is bonded with the ground you would see a short. I know this is not true but why is it not true. Like if phase A with everything on pulls 50 amps, and phase B pulls 30amps then the neutral with everything on carries 20 amps. How would you explain this to someone in simple terms. |
| David Hyatt | Where is Scott and his fancy diagrams???? This is like saying if you take the neutral lose with a unbalanced load at the panel, put a voltmeter between it and the ground bar you will read voltage? Maybe its amperage that the neutral carries instead of voltage. I don't know now I am confussing myself? Help.
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| JimmyDee | Think of the service comming into your house as 2 series 120 volt transformers. (thats what they are) Now from one hot to the other there is 240 volts. Hook up 2 light bulbs (120 volt) in series and you will drop the same voltage accross each one. Don't need a neutral. Now if you were to hook a neutral to the point where the 2 light bulbs connect, (center point). You have done nothing to the circit and there will be no curent flow. Now screw in a light bulb in one socket that is half the size as the other one and you will have a different voltage drop accross each light if no neutral is there. (60 accross one and 180 on the other). Hook the neutral back up and both lamps will receive equal voltage once again because each will get 120 volts. The amperage on neutral will only be what the difference is between the lamp because the light are still series (sort of) and the current flow in the neutral is flowing in opposit directions, soooo they will cancle each other out except for the difference. Jim |
| ricky | Yes.I understand the question.Jim is right.And if a phase drops,all the current in the remaining phase will flow thru the neutral,which would be more than if the phase were up and currents were canceling each other.Right? |
| David Hyatt | So on a unbalanced load when the neutral is carring voltage, could you remove the neutral and read voltage between it and ground? |
| JimmyDee | quote: Yes, that is correct. Remember that if the neutral becomes disconnected, you now have a series circuit. The voltage drop will be different across each load and the one with the lower resistance will drop the least voltage (like maybe a toaster) and the one with the higher resistance (like radio, TV) will drop the most voltage. This is why when the neutral becomes disconnected, the smoke that is packaged in every radio or TV is let out. And as we know, when the packaged smoke is discharged for any reason, the equipment no longer works.  Jim |
| ricky | David,it sounds like you need to visualize the difference between voltage and current.I had a professor once describe voltage as water pressure,and amperage as water flow.Sitting there on your faucet(and outlets) all the time,is water pressure,say 50psi,but you don't get wet because there is no flow.If you turn on the faucet just a little,a little water flows out.The faucet is like a light dimmer which has a very high resistance to current flow.As you turn the knob,the resistance drops,and current flows.Now lets say you have this water flowing thru a pipe and it turns a paddle wheel.(lets assume it's a closed system so no water can escape)At the end of the pipe,after the paddle wheel,you put your finger over the pipe and stop the flow.the paddle wheel stops and pressure builds beneath your finger.This is like disconnecting a grounded conductor.When disconnected,there is no voltage drop across the load because there is no current flow,and you will measure 120 volts to ground throughout the circuit.Do you get it? OH,the lower voltage on the neutral I mentioned up above is very small,and is a product of whatever current is flowing and the resistance to ground.V=IR.Sometimes a couple of volts on old wiring. |
| JimmyDee | ricky, I guess I should have said where the neutral I was describing becomes disconnected. What I was describing is when the neutral feeding a service or on a three wire circuit becomes disconnected say at the panel. This would leave the two loads connected in series and just what I described in my previous post will happen just like that. What you are describing is when the neutral is feeding a load and it disconnects from the load. By all means, the current will cease to flow. However, if the circuit, just like a service feed, looses its neutral, current will flow from hot through a load (or several) back to the neutral point and then return back through another load fed by the other hot. You will have smoke if the loads are unequal. Jim |
| ricky | Yes Jim I knew what you were saying.I'm just trying to help him understand that he will get shocked thru the load by that white wire if he disconnects it and touches it.Thats all. |