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Borrego
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# Posted: 2 Mar 2013 08:36pm
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Help, you electricians out there. So I'm a licensed GC, have worked on electrical for years, even wired several houses from the pole down...I realized lately that I really don't understand how it all works. I looked it up online and could not find an understandable answer!
So....
1) I know a neutral wire carries current, right? To complete the circuit?
2) When connecting the neutrals and grounds at the box, they should now be separated? But most older boxes use the same bus bar....
3) IF the neutral carries current (half the time?) then why doesn't it make the ground wire hot in the box? And why can you touch a white wire and not a black?
Can anyone explain in simple terms how simple house current works?
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maine_island
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# Posted: 3 Mar 2013 06:18am
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1) yes the same current that flows thru the black "hot" wire returns thru the white wire. It does this all the time, not just half the time as you say in your point 3.
2) neutrals (white wire) and grounds (bare copper) should be separately connected together - i.e. all neutrals tied together, and also all grounds tied together. also, all the grounds are connected to actual earth ground for example a metal rod driven into the ground outside the house.
finally, SOMEWHERE in the wiring system, AT ONE PLACE neutral and ground are connected together. this is what makes the neutral wires "neutral", i.e. they are at a very low voltage compared with ground.
as you noticed, the current practice is to connect these together in one place only, whereas the older practice was to connect them together haphazardly whenever the chance arose.
3) since the neutral white wire always carries exactly the same current as the hot black wire, and since the neutral wire has some resistance - however small - the point where the neutral wire is connected to ground will be at zero volts with respect to ground, but the other end of the neutral wire (for example, at an outlet) might be at 1 or 2 volts with respect to ground.
when you touch any one of these wires you are completing a circuit with your feet (for example) at ground potential, and your hand at the potential of the wire.
-any ground wire should be at exactly 0 volts with respect to ground, and so touching ground wire is perfectly safe.
-since white neutral wire is usually only a volt or two different from ground, naturally you will not feel this as a shock. so that is safe too.
-black or hot wire is at 120V with respect to ground - you will definitely feel that.
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jg@jeffgreefwoo dworking.com
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# Posted: 3 Mar 2013 04:05pm
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The neutrals should always be isolated from grounds everywhere in the system except at the service enclosure (meter box) where the utility comes in. If the grounds are not connected to the neutral here, they have no low impedance path to the transformer, and so cannot cause the breakers to trip in the event of ground fault current.
The neutrals do not necessarily carry equal current to the hot wires. In any given single phase circuit the amperage in both wires will be the same, but voltage is reduced in the neutral. But since the two phases of a 110-220 system cancel out, no return is required on the neutral for 220 circuits. A 220 circuit uses no neutral, just two hot phases. So, from a subpanel, the neutral could have no current in it at all if current on all circuits of both phases is perfectly equal (a rare event). But because some of the current is cancelled most of the time, the neutral can be de-rated (made smaller) in some cases. But don't do this unless you understand all the rules.
You wanted simple rules for household systems-
1-Ground all metal components of everything associated with the electrical system, and lead the grounds back to the service panel.
2-Bond the neutrals and grounds at the service only, never in subpanels, never anywhere else.
3-Drive a ground rod at the service and any subpanel in a separate building and connect with an unspliced GEC (grounding electrode conductor) sized to 250.66 in the NEC.
With those basics in place you will have 90% of your safety features intact.
It's best to understand the principles behind electrical installations to know how to do it right. A good book is Soare's book on grounding. Hard to read, but good info.
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jg@jeffgreefwoo dworking.com
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# Posted: 3 Mar 2013 04:39pm
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You asked- wouldn't bonding the neutral to the grounds cause the grounds to be charged? The answer is yes, and that's why you bond only at the service.
The current in the neutral is doing one thing and one thing only- trying to get back to its source, the transformer up on the pole out in the street. It can do that one of two ways. If bonded to the grounds, it can go to the ground rod, through the earth, to the ground rod at the pole, and up the wire on the pole to the transformer. But the earth is a lousy conductor, so very little current takes this path. A far easier path for the neutral to follow is through the neutral service entrance conductor, the neutral wire on the swinging wires coming into the house. This is a low impedance path back to the transformer.
If you bond the grounds and neutrals in a subpanel in the house, say, 50 feet away from the service where the service conductors are, you just gave the neutral a whole bunch of other possible ways to try to find a way back to the transformer, through any one of the ground wires in the subpanel that are attached to all the circuits and equipment tied to that panel. That neutral current is trying to get to the neutral bar in the service, cause that's the fast path back to the transformer. If it can get to the service, say, through the ground wire attached to the washing machine, it will do so. This would only happen if the washer had some other connection to the service, like a water pipe. This is called a parallel path for return current. The neutral current is returning to the service both through the neutral feeder that feeds the subpanel, and through the ground wire to the washer, to the water pipe, to the ground wire attached to the water pipe, to the service.
Electricity takes the PATHS of LESSER resistance, not only the path of least resistance, so in this example most of the current returns via the neutral subpanel feeder, but some via the washer which has some metallic connection to the service, like a water pipe. The small current returning through the washer can be enough to arc and cause fire, or enough to shock and kill you. 4 milliamps in your heart will stop your heart.
So, if the grounds and neutrals are separated in the subpanel, no neutral current from the subpanel can ever get to the washer via the grounds.
But, you say, since the grounds and neutrals are connected in the service, the neutral current could travel from the service BACK to the subpanel through the feeder ground wire, into the washer circuit ground wire, then on to the service as in the example above.
It could, but won't, because neutral current at the service is far closer to the transformer, so the path to the transformer there is far less in resistance, much easier. The path via the washer is now so far away in comparison that only the slightest, if any, current will go that way.
Make sense?
Often people get the misconception that current is supposed to return to the transformer via the ground rods. Not true. The ground rods exist only to direct lightning to earth, and to equalize any stray potential in your electrical equipment, from whatever source, with the earth so you can't get shocked by touching a charged piece of metal while standing on the earth.
Ground wires in your circuits function for those two purposes also, but do another equally important function- clear fault current. When your washing machine fries and the hot wires touch the metal frame, charging it, you can get shocked by touching it. The hot current passes through the washer, you, the earth, the pole ground rod up to the transformer. That is one path for current back to the transformer. Because the earth has high impedance (resistance), that circuit might only develop 5 amps or so, not enough to trip the breaker but plenty to kill you.
But, if the washing machine frame has a ground wire on it, when the machine fries and the hot wire touches the frame, current immediately goes to the ground wire, back to the service, directly to the neutral (cause they are bonded there) and directly to the transformer. This is a very low impedance circuit and the amperage will immediately spike up as high as the wires involved will let it. Its basically a kind of short circuit. This causes the breaker to trip immediately because the current is now way over the 20 or 30 amps of the breaker. this is called clearing fault current, and cannot happen if the grounds at the service are connected only to the ground rod, and not to the neutral.
My fingers are tired
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toyota_mdt_tech
Member
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# Posted: 3 Mar 2013 04:52pm
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Nice post JF. Thanks for taking all the time too. 
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Just
Member
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# Posted: 3 Mar 2013 05:01pm
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jg@jeffgreefwoo dworking.com
I think I actually learned something ,,
.thanks
Jeff
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rayyy
Member
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# Posted: 3 Mar 2013 05:34pm
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You ever notice on all electric poles,there are always two wires spread apart from each other up on top of the poles.These are the two wires they send out two electricity circuits on.They are much higher voltage in order for them to pump out the voltage that far away from the generator.You then see these two wires going to a transformer which reduces the voltage befor they come into your house.Each wire feeds the left and right side of your electrical panel.from there through the breaker it goes to your outlets.The weird thing about electric is that once you plug in somthing and use it,it has to flow back to the generator in which it came from to complete the circuit.It comes to you on the black wire and leaves you on the white wire on your outlet.They dont bother using a third wire to do that they just hook it up to a ground wire out on the pole that the transformer is on.they use the earth to return that voltage.Your ground is set up to do the same way.It goes to the earth too .Ground is there to provide a path for current to flow in the event of a short circuit(you know when it sparks out and trips the breaker)It did it's job.By the way all those other wires much further down on the poles are phone,cable and other low voltage stuff.They have to be at least 4 feet away from those electric lines up above.
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Borrego
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# Posted: 3 Mar 2013 06:56pm
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Whew! Ok, think I got it, at least a lot better. Thanks!
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jg@jeffgreefwoo dworking.com
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# Posted: 4 Mar 2013 07:52am
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I must politely disagree, rayyy. The ground rod is never used as part of the return current circuit in normally operating systems. When you get a shock via the earth, the shock circuit can go through ground rods, but this is a fault circumstance.
All current in your household system returns to the transformer via the neutral. The circuit breaker trips in the event of fault current because the grounds are bonded to the neutral, not because they are connected to the ground rod. The path through the earth is too high impedance (too much resistance) to allow the current to get high enough to trip the breakers.
The two high voltage wires that feed your transformer don't need a third wire to return current, because they are two different phases. Just like your 220 oven or electric dryer, no neutral is required when you operate across two different phases.
When you plug in a circuit, the power for that does not come directly from the high voltage wires on the pole. It comes from the transformer on the pole. The power is not directly returning to the generator 100 miles away, it is returning only to the transformer itself. On a single leg circuit (110), the return is via the neutral. On a 220 circuit (across two legs) the return is across the two legs.
Power to the transformer on your pole comes, itself, from another transformer at a substation somewhere in your neighborhood. That transformer knocks extremely high voltage to medium high voltage for local transmission to your little transformer on your pole (which serves 6-12 houses). The transformer on the pole does not return its current to its feeder transformer via the ground rod and the earth. It simply works across phases.
Any and all transformers will continue to work if the ground is disconnected, they will just be less safe, not protected from lightning or imposed voltages.
It's important to understand that the earth is never part of the return current path in normally operating systems because if you believe so, you may fail to bond the grounds and neutrals in the service. This is what causes the breakers to trip in the event of fault current, not the connection to earth.
Read Soare's Book on Grounding.
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rayyy
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# Posted: 4 Mar 2013 04:42pm
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That transformer is grounding that nutral to the earth,right?Instead of using a 3rd conductor?It just saves them a lot of money and trouble running another ,third conductor.
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toyota_mdt_tech
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# Posted: 4 Mar 2013 04:55pm - Edited by: toyota_mdt_tech
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That neutral needs to go back to its source. I think it would be like trying to get a light to work on a 12V battery, taking 2 batterys, using the + from one battery then the ground from another 12V battery, it wont work. On AC, both line and line or line and neutral have to come from the same source.
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jg@jeffgreefwoo dworking.com
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# Posted: 5 Mar 2013 05:20pm
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A high voltage line on a power pole serving local transformers will have either two or three wires. If two, one is hot and the other is neutral. If three, two are hot and one is neutral. The earth is never the neutral. The neutral in the transformer is always grounded via a ground rod at the base of the pole. This is for safety purposes, not to complete the circuit back to the 'mother' transformer.
The earth is never a return circuit path in a normally operating system, but is often the path of fault current that can shock you. That's one reason some people argue against having grounded systems at all, it creates a path for fault current that can shock you. But the majority judgement, in the US anyway, is that the safety features outweigh the dangers, so we ground our systems for safety alone, not to make them function for normal loads. The earth cannot function to complete normal loads because it has too high resistance.
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