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07-20-2009, 06:39 PM   #1

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## Tripping breakers,neutrals,120 volts and 240 volts OH My!!!

It has be requested that we discuss some commonly asked questions about 3 wire 120/240 volt single phase residential electrical systems. So that is what we will do and hopefully learn a little along the way.

Quote:
 1) If a black (hot) wire touches the electrical box and then travels on the grounding wire to "ground", how does this trip the breaker in the box? I don't understand how that surge would go to the box and trip the breaker that the light is on...or does it cut all power?
If we all only had a dollar for everytime this question is asked....

The secret is it (current) doesn't travel to ground (earth) it travels to the transformer.....

Lets use the first diagram below for reference ......

I'm going to primarily reference a 120 volt circuit but the principle is the equal on a 240 volt branch circuit. The branch circuit starts with an ungrounded conductor connecting to a circuit breaker that protects that conductor from over current. These breakers are typically called the OCPD or over current protective device. Breakers are thermal magnetic in nature for residential applications...thermal being the overload protection and magnetic to handle over current faults such as an ungrounded conductor (hot wire) contacting a metal electrical box. Included in the branch circuit will be a grounded leg.... aka neutral....and an equipment grounding conductor (EGC). These are typically the white (neutral) and bare copper wire (EGC) in a cable like romex. The black of course is the hot wire or the ungrounded conductor.

The hot wire of a 120 volt branch circuit is connected at one end to its circuit breaker and travels out to the outlets where power will be utilized. For example a light or a receptacle that are points of power utilization. So voltage (120 volts) is supplied to all these points so long as the circuit breaker is turned on. Nothing really happens until you turn on a light on or plug in a kitchen mixer and turn it on. However for anything to operate we have to have a complete circuit back the the source. The source is the transformer serving the home out on the pole or on a pad mount. To complete that circuit we use the white wire or grounded leg (neutral). The grounded leg (white ) connects to the appliance or light bulb and then returns unbroken to the load center where the circuit breakers are located. It is then terminated to the neutral bar in the load center. The circuit is completed at this point because the utility neutral (service grounded conductor) connects to the neutral bar also and provides the low impedance/resistance path back to the (source) transformer center tap.

Finally we are getting to the meat of the question as we have one more wire to talk about. That's the equipment ground or the bare wire that parallels the other conductors of the branch circuit. It has a single purpose.... to bond all metal together that is likely to be energized if a fault occurs. So the equipment grounding conductor bonds the switches, light fixtures, metal receptacle yokes, metal boxes all together. This is done along the entire branch circuit and then that equipment grounding conductor terminates to the neutral bar along with all the other neutrals and equipment grounds. It is very important to note that the neutral bar in the main breaker load center (service equipment) is the only place where grounded legs (white wires) and equipment grounds are joined (bonded). The egc and grounded legs are never bonded load side of the service equipment panel. To do so will allow neutral current to flow on the equipment ground wire and all the metal it bonds on the branch circuit creating an possible shock hazard to humans. The equipment ground is never to carry system current. It's single purpose is to carry fault current to open a circuit breaker in the event of a ground fault. At all other times it never carries any current. It is distinguished from the white wire aka neutral aka grounded leg in that it is a non current carrying conductor of the branch circuit and has nothing to do with the electrical system working properly. Its sole purpose is human protection from electrical shock.
It does this by providing what the NEC calls the 'effective ground fault path'. Which is simply completing the circuit for unwanted ground faults just like the grounded leg completes the circuit for system operation of your homes appliances and other devices utilizing power. Remember all current (fault and system) seeks the source (transformer). This is why we bond the equipment ground, grounded legs, neutrals with the neutral bar at the service equipment because the only low impedance path back to the source (transformer) center tap at that point is over the service neutral.

The breaker trips because a fault to ground is not regulated as to how many amps is to be used like a kitchen mixer or light bulb. These loads are bypassed during a ground fault and the ungrounded conductor simply becomes nearly a loop of continuous wire of little or no resistance/ impedance beginning at the transformer and ending at the transformer allowing massive amps to flow on the faulted circuit. All that current that flows during the fault must pass through the circuit breaker. These amps overwhelm the breaker and the magnetic trip mechanism opens the contacts in the breaker de-energizing the branch circuit and clearing the fault.

The second diagram shows the the fault current path during a ground fault to a metal electrical box resulting in the opening of the circuit breaker.
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Last edited by Stubbie; 07-20-2009 at 07:08 PM.

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07-20-2009, 06:54 PM   #2
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Quote:
 Lets start with a few questions from hellothere 123 Quote: 1) If a black (hot) wire touches the electrical box and then travels on the grounding wire to "ground", how does this trip the breaker in the box? I don't understand how that surge would go to the box and trip the breaker that the light is on...or does it cut all power?
I will offer a less sophisticated explanation.

Amps = currrent flow. Only x amperage will flow thru a wire before it gets hot, melts the insulation and eventually melts.

The breaker only allows 15 amps to flow thru a typical residential lighting circuit.

A light bulb draws only a fraction of an amp thru it's tiny filiment.

A toaster draws maybe 4 amps thru it's more beefy element.

An unprotected dead short draws as much as it can before the wire burns in half.

As far as how a breaker works? I think it's some kind of voodoo.

07-21-2009, 06:50 PM   #3

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Hellothere123's second question

Quote:
 2) Neutral wire (white) : At a electrical outlet, the black wire provides the power...is the neutral wire also carrying this same power, basically is it as hot as the black wire? Or is it just there to return unused power back to ground once an appliance is plugged into the outlet.
Use diagram below for reference

First a bit of terminology clarification.

As you saw in the last diagram and discussion the term neutral was used to describe the white wire in a 120 volt branch circuit. Technically this is incorrect. The white wire is really a grounded leg not a neutral. The NEC recently changed it's definiton for neutral to neutral conductor...that being a conductor connected to the neutral point of a system that is intended to carry current. Neutral point being the center tap of a transformer winding or the point where the vectorial sum of the nominal voltages of all phases within the system creat a potential of zero. The thing I think that is important to understand is that in order to be a neutral conductor the current it carries in a residential single phase supply must be the unbalanced current from two or more ungrounded conductors. So examples would be the service neutral (carries the unbalanced current of the entire dwelling) or the white wire serving as the neutral to a 120/240 volt rangewhich is carrying the unbalanced current of the two hot wires serving the range. We can discuss this further if necessary.
My point being a 120 volt branch circuit only has one ungrounded (hot) conductor and therefore the white wire is not a neutral but a grounded leg that carries all the current of the branch circuit not the unbalanced current because there isn't any.

As to the question presented.... power is consumed by the load as current passes through it. In households the most common unit of power is watts. It is simply the product of volts and amps. A 7 amp mixer on a 120 volt branch circuit uses 840 watts of power. A 120 volt branch circuit protected by a 15 amp breaker will provide 1800 watts of power for household use. Power does't flow on the conductors.... current does... and is forced thru the load by voltage. All power is consumed by the appliance or whatever load is being served.

So in speaking of a 120 volt branch circuit the white wire is a grounded leg with a potential of 0 volts. It's a grounded leg because it is a circuit current carrying conductor bonded to the service neutral which is intentionally earthed at some point.. Any conductor that is connected to or is connected to a conductive path to earth is considered grounded. So in a single phase system the only wires that are not grounded are the hot wires and they are appropriately called 'ungrounded conductors'....

An ungrounded conductor and a grounded conductor (aka neutral) in a 120 volt branch circuit carry the same current flowing thru the household loads but different potentials (voltages). As the diagram shows the load is 5 amps so the ungrounded (hot) wire carries 5 amps as does the grounded conductor. The ungrounded conductors voltage is 120 volts and the grounded leg is zero so the voltage drop accross the load is 120 volts. Never consider the white wire a conductor that can't harm you... it is ever bit as lethal as the hot wire. Getting in series with the grounded leg or a neutral will be quite a shock....

Power is not returned it is used. Current is returned to the source (transformer). One of the biggest myths you can believe is that current returns to earth over the grounding system via grounding electrode conductors to the ground rods and metal water pipes and then earth. Current always seeks the source it came from and it will take all paths to get there and yes it will use the earth if and only if it is the only path available to it. But if we provide a low impedance/resistance path back to the source as in a completed circuit vitually all current will take that path.
As an example the NEC requires a ground resistance for a ground rod to be 25 ohms or less. If we plug that into Ohms' law we get 120=(I)(25)....I equals 4.5 amps...so what is important to understand is that if that was the path we relied on to get back to the source we could never get a breaker to trip on a ground fault. 4.5 amps is all that 120 volts could push thru the earth and a 15 amp breaker would never trip out. As an aside remember that we bond the grounded legs carrying current at the neutral bar where we are also bonding to the grounding electrode conductors which go to earth (ground). Essentially we have intentionally provided 2 paths for current to follow one is to earth and one is to the transformer over the service neutral. Obviously the lowest impedance/resistance is the wire path and almost all the current takes that path.

Hope the diagrams are helping all who are interested understand....I'm doing my best to get it into words.... Others will certainly chime in from time to time and it is certainly possible I may make an error ...I am human... though my wife sometimes asks me what planet I'm from.
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Last edited by Stubbie; 07-21-2009 at 06:55 PM.

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 07-22-2009, 11:25 AM #4 UAW SKILLED TRADES     Join Date: Jan 2007 Location: Kansas Posts: 5,341 Rewards Points: 2,652 Well I think maybe the title scared every DIY away..... Most of what I post is common knowledge to electricians so for those DIY searching the forums these diagrams should be useful. Feel free to copy them if they are of interest. The remaining questions were going to be about 120 volts needing a neutral and 240 volts not needing a neutral. so I've drawn some diagrams to show what these look like. I'll just post them for all to reference and use if they like. Main thing to understand is in order to create a 120 volt voltage drop in potential across a load in a 3 wire 120/240 volt single phase system is we must have a grounded leg or neutral connected to the load and complete a circuit to the center tap (midpoint) of the transformer winding. The NEC calls this the neutral point or point of zero potential. The 240 volt wave diagram shows the two legs of the circuit one yellow one red depicted by E1 and E2. What I'm trying to show here is the 180 degree offset of the two legs of the service. We don't really have 2 phases on a 3 wire 120/240 volt single phase system. What we have is a single phase that has been offset providing for a 240 volt potential between the ends of the transformer winding. Attached Thumbnails         __________________ " One nice thing about the NEC articles ... you have lots of choices" Stubbie Last edited by Stubbie; 07-22-2009 at 11:56 AM.
 07-22-2009, 12:03 PM #5 UAW SKILLED TRADES     Join Date: Jan 2007 Location: Kansas Posts: 5,341 Rewards Points: 2,652 For the heck of it I'm posting a diagram of a multi-wire shared neutral branch circuit. The meter and breaker box are not shown but typically you would use a double pole breaker connected to both legs in the breaker panel and share one neutral between the two legs. The second diagram is showing a typical multiwire application to a detached building like a garage. This is an excellent option as it effectively brings two 120 bolt branch circuits to a detached garage where only small loads will be required like hand tools and lights. If you keep the branch circuit at 15 or 20 amps you do not need to install any grounding electrodes at the garage. Attached Thumbnails     __________________ " One nice thing about the NEC articles ... you have lots of choices" Stubbie Last edited by Stubbie; 07-22-2009 at 12:41 PM.
 07-22-2009, 03:02 PM #6 yeah, right   Join Date: Jan 2009 Location: California Posts: 142 Rewards Points: 75 The sine wave diagram you posted is misleading. The RMS voltage is 120V, but the peak voltage is 170V. To show an AC voltage sine wave, the vertical scale indicates true voltage and the horizontal scale indicates time. The diagram shows one cycle, or 1/60th of a second. Attached Thumbnails   __________________ Honey, does this tool belt make me look FAT?
 07-22-2009, 03:44 PM #7 UAW SKILLED TRADES     Join Date: Jan 2007 Location: Kansas Posts: 5,341 Rewards Points: 2,652 Only misleading to an engineer possibly. The RMS (root mean square) voltage is what you say but I could just have easily stated the diagram showed RMS voltage. I don't think a homeowner cares about rms only the principle behind the waveform. However your correction is duly noted....in a professional format we could make this far over the head of many DIY that visit this site. V-peak is actually 169.7 volts..... Just poking fun....the object of a discussion is to to get information that aids in the understanding for the benefit of all here. The diagram is not 100% accurate as you have correctly stated...I made it this way to simplify the overall concept of the sine wave. If I had put my diagram on my circuit analysis class test I would been looking at a big red check mark when I got it back...... __________________ " One nice thing about the NEC articles ... you have lots of choices" Stubbie Last edited by Stubbie; 07-22-2009 at 04:16 PM.
07-22-2009, 05:56 PM   #8
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Quote:
 Originally Posted by Stubbie but I could just have easily stated the diagram showed RMS voltage.
I knew you were going to say that. But if you did, the voltage should be shown as a straight line, not a sine curve.

I don't think there's an easy way to accurately convey the information. A very simplistic view is that AC voltage actually varies from 0V to a peak voltage. If you "average" the voltage over 1/2 a cycle, that's going to be the RMS voltage.

169.70562748477140585620264690516V, but that's just being picky.

I'm pretty sure if you scoped 240V (from E1 to E2), you'd see a sine wave peaking at 339.41V.
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 07-22-2009, 06:00 PM #9 Electrical Contractor     Join Date: Jun 2004 Location: Newnan GA Posts: 6,815 Rewards Points: 224 Oh Nooo!!! I thought I was finished school! I think you are doing a great job stubbie, and I also think that maybe this post could be made a sticky, so years from now it will live on. __________________ "The problem isn't that Hillary Clinton lies. We all know she lies. The problem is that her supporters don't seem to care"
07-22-2009, 08:00 PM   #10

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Quote:
 Originally Posted by zpm I knew you were going to say that. But if you did, the voltage should be shown as a straight line, not a sine curve. I don't think there's an easy way to accurately convey the information. A very simplistic view is that AC voltage actually varies from 0V to a peak voltage. If you "average" the voltage over 1/2 a cycle, that's going to be the RMS voltage. 169.70562748477140585620264690516V, but that's just being picky. I'm pretty sure if you scoped 240V (from E1 to E2), you'd see a sine wave peaking at 339.41V.
Yep I would agree with that and I'll take your word for the straight line. That does seem logical to me but my brain is way to rusty having finished college some 30 yrs ago and once I realized that I preferred being in the field I strayed away from those darn sine waves...made my head hurt... so I put my jeans and work boots back on..... Learning is a marvelous thing so I'm glad you entered the thread with your knowledge.
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 07-22-2009, 08:30 PM #11 Member   Join Date: Feb 2008 Location: Atlanta, Ga/Hamilton, Al Posts: 2,487 Rewards Points: 2,350 The root mean square of an AC sine wave should produce a sine wave with the RMS voltage as peak. I could be wrong, but I don't think that the peak sine wave is just "chopped" off at 120 V. There should be two sine values: one is the "real" sine wave of 169.7 V peak, and the other is the effective RMS sine at 120 V peak. In other words, one is a real sine that represents the path of the generator shaft against time, and the other is a composite sine that is a "DC equivalent" voltage found by taking the integral with respect to time.
07-22-2009, 09:47 PM   #12
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## Satirical explanation (by 220/221) of how a breaker works

Quote:
 Originally Posted by 220/221 I will offer a less sophisticated explanation. Amps = currrent flow. Only x amperage will flow thru a wire before it gets hot, melts the insulation and eventually melts. The breaker only allows 15 amps to flow thru a typical residential lighting circuit. A light bulb draws only a fraction of an amp thru it's tiny filiment. A toaster draws maybe 4 amps thru it's more beefy element. An unprotected dead short draws as much as it can before the wire burns in half. As far as how a breaker works? I think it's some kind of voodoo.
I wish there would be some Voodoo in the FPE and Bulldog breakers. Then we wouldn't have so many fires (Pity on the latter for being taken off the market. I really loved them.) (No matter what) Don't Drink and Drive!!!

07-22-2009, 10:55 PM   #13
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Quote:
 Originally Posted by InPhase277 The root mean square of an AC sine wave should produce a sine wave with the RMS voltage as peak.
The way I understand it, RMS is a calculated number from the waveform, not a waveform itself.

Quote:
 I could be wrong, but I don't think that the peak sine wave is just "chopped" off at 120 V.
If you're referring to the crude graphic I made, it was to visualize the relationship between the actual sine wave voltages and RMS voltage. Saying that RMS voltage is calculated as the "average voltage" over a 1/2 cycle (and ignoring the real math) is an easy way to comprehend it. The left side shows the peak, anything above 120V is the darker blue. The right side shows the darker blue filling in the areas that were less than 120V. This allows you to visualize an "average voltage" of 120.

Quote:
 There should be two sine values: one is the "real" sine wave of 169.7 V peak, and the other is the effective RMS sine at 120 V peak. In other words, one is a real sine that represents the path of the generator shaft against time, and the other is a composite sine that is a "DC equivalent" voltage found by taking the integral with respect to time.
You lost me with the last sentence. Maybe that applies to generator engineering, but for this I'm still going to go with RMS voltage being a calculated number.

Wiki defines it pretty well.
http://en.wikipedia.org/wiki/Root_mean_square
RMS is the square root of the mean of the squares of the values.

If you step through the degrees (D) from 0 to 179, calculate the voltage for each degree (V = sin(D) * 169.7), sum the square of the voltage (S = S + V * V), then divide the sum by 180 and take the square root (RMS = SQRT(S / 180)), you'll get real close to 120V. My Q&D program to do this came up with 119.996.

Interestingly (or not so much), 120V occurs at 45 and 135 degrees along the curve.
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07-22-2009, 10:56 PM   #14
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Quote:
 Originally Posted by Stubbie I strayed away from those darn sine waves...made my head hurt...
yeah, this is starting to give me a sine-us headache. yuk yuk.
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07-22-2009, 11:21 PM   #15
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Quote:
 Originally Posted by zpm yeah, this is starting to give me a sine-us headache. yuk yuk.
Maybe we should all just sine-off, but let's not get off on a tangent, cos that's the last thing we need... Highhhh Ohhhh!

What I meant by "real" and "composite" sines is that there is a real sine wave that peaks out at 169.7 V. However, while the RMS value is a calculated value, if you plotted several points on a graph against time, it would take the shape of a sine wave. So while it isn't "real", it has the shape and periodicity of a sine wave when viewed on a graph. Now we are talking way out in left field for DIY, but it is still fun.

Just remember: "Secant you shall find"

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