residential wiring theory question

[ms136]

I have tried for years to find an answer to these questions and just when I think I have, I find additional info that contradicts it. Maybe I should not get so hung up on theory, but I think knowing the theory will help me troubleshoot down the road.

I understand that in order to have a working circuit there has to be a source, resistance, and a complete path. Is the secondary of the supply transformer considered the source in residential wiring circuits? Let's say I have a two wire circuit (black hot wire, white neutral wire, and ground wire) going to a light bulb. The current comes in through the black wire and through the bulb then out through the neutral wire. But, the neutral wire is connected to earth ground at the panel, so why is this not an open circuit? How does current return to the source (the secondary of the transformer)?
Also, won't the neutral (white) wire become the "hot" wire and the black ("hot") wire become the neutral when the sine wave alternates and causes a reversal in the current?

I understand DC theory just fine , but most residential wiring books and even courses do not explain the theory so that I can understand it.

 

[infinity]

the neutral wire is connected to earth ground at the panel, so why is this not an open circuit? How does current return to the source (the secondary of the transformer)?

Because it is also connected to the the transformer secondary. Since this connection has a very low resistance most of the current travels through it.

Also, won't the neutral (white) wire become the "hot" wire and the black ("hot") wire become the neutral when the sine wave alternates and causes a reversal in the current?

The neutral doesn't become "hot" because it's grounded. The relation to the word hot is ungrounded. We tend to think of "hot" as something that has a potential to ground. A simple 120 volt circuit will have a grounded and ungrounded conductor. One will have a potential of 120 volts to ground the other will have a potential near zero to ground because that conductor (grounded conductor) is physically connected to the ground.

 

[jdsmith]

Also, won't the neutral (white) wire become the "hot" wire and the black ("hot") wire become the neutral when the sine wave alternates and causes a reversal in the current?

Since the neutral is grounded its voltage to ground should always be close to zero. When making measurements ground is the most common reference point - where you put the black lead on the meter. If you hooked up a scope to watch the sine wave you'll see positive voltage for half of the cycle and negative voltage for the other half. It's a premise of AC terminology that we recognize the amplitude of the wave relative to a reference point (ground and usually neutral) and we do something to indicate the phase of the signal. In residential phase is easy - you get two ungrounded conductors that are 180 degrees out of phase with each other.

Jeremy

 

[72.5kv]

This two wire circuit would still work even if the ground (earth) was remove from the equation.

http://www.mikeholt.com/instructor2/img/product/pdf/1080685312sample.pdf

take a look at this link

 

[ms136]

Thanks to all of you! I have spent a small fortune on textbooks looking for these answers. Also, thanks to Mike Holt & company for this forum.

 

[charlie b]

Also, won't the neutral (white) wire become the "hot" wire and the black ("hot") wire become the neutral when the sine wave alternates and causes a reversal in the current?

Essentially, yes, but we need to be a bit careful with the use of the words here. During one half cycle, current travels from the source along the black wire, through the bulb, and back to the source along the white wire. During the other half cycle, current travels from the source along the white wire, through the bulb, and back to the source along the black wire.

Planet Earth is not part of the current paths, even though the white wire is connected (via the N-G bond in the main panel, and via the Grounding Electrode Conductor, and via the Grounding Electrode itself) to dirt. That is because by the time the current has made its way to the N-G bond point, it will have made its way back to its source. There is no driving force to compel the current to move from the source into the dirt.

 

[LarryFine]

Is the secondary of the supply transformer considered the source in residential wiring circuits?

Absolutely. That's why local earth leakage current is only (mostly, really) trying to get back to the local transformer's neutral, and not necessarily the earth at large (meaning both the planet and our system earth). I say 'mostly' because the earth at large is bonded to our local system neutral.

Let's say I have a two wire circuit (black hot wire, white neutral wire, and ground wire) going to a light bulb. The current comes in through the black wire and through the bulb then out through the neutral wire. But, the neutral wire is connected to earth ground at the panel, so why is this not an open circuit? How does current return to the source (the secondary of the transformer)?

Because you need to think of the neutral as the system circuit conductor that happens to be grounded, and not as a ground conductor that happens to be (here's the part you didn't realize) connected to the transformer secondary.

The system must not ever depend on the earth as part of a current-carrying conductive path (aka a circuit). Every conductor that is part of the circuit must be fully sized for the intended load, while EGC's may be smaller.

Look at Table 250.122. Notice how the EGC may be reduced above 30a circuits? A circuit conductor certainly can't. Neutrals of feeders may be reduced only to the extent that line-to-line loads will not contribute to the neutral load current.

Also, won't the neutral (white) wire become the "hot" wire and the black ("hot") wire become the neutral when the sine wave alternates and causes a reversal in the current?

No, because "hot" and "neutral" have nothing to do with voltage polarity. DC has a positive and a negative, while AC alternates polarity twice per cycle, or 120 times per second. Which conductor is polarity changes, but which conductor is "hot" or "neutral" does not.

I understand DC theory just fine , but most residential wiring books and even courses do not explain the theory so that I can understand it.

Okay, here's a little ditty I posted a while back. It got a compliment or two, so with your DC background, it should make sense.

http://forums.mikeholt.com/showpost.php?p=708650&postcount=4

 

[coulter]

... In residential phase is easy - you get two ungrounded conductors that are 180 degrees out of phase with each other. ...

Jeremy -
I'd say there are two ungrounded conductors "in-phase" with each other. If L1 and L2 were 180 out of phase, then the V(L-L) = 0.

carl

 

[rattus]

Here we go again:

Here we go again:

Jeremy -
I'd say there are two ungrounded conductors "in-phase" with each other. If L1 and L2 were 180 out of phase, then the V(L-L) = 0.

carl

I always think of L1 and L2 being measured relative to the neutral bus. If you probe L1 and L2 with a properly synched scope, you will see that the voltages are 180 degrees out of phase; then the line to line voltage is 240V.

Note that I did not call this a two-phase system because it's not!

 

[coulter]

rattus -
I probably would choose vectors.

carl

 

[coulter]

or maybe like this using sources.

carl

edited to add: This sketch doesn't exactly contain any phase information

 

[rattus]

Backwards:

Backwards:

rattus -
I probably would choose vectors.

carl

Think about it.

V1 = 120 @ 0 = 120 + j0
V2 = 120 @ 180 = -120 + j0

V1 + V2 = 0
V1 - V2 = 240 = V12

 

[rattus]

Still backwards:

Still backwards:

or maybe like this using sources.

carl

edited to add: This sketch doesn't exactly contain any phase information

V1 and V2 should be measured relative to neutral as in a 3-ph wye. Then your first sketch would indicate a phase reversal, and

V1 - V2 = 240

And, the second sketch would indicate no phase reversal, and

V1 - V2 = 0