We gotta back up the train here a bit. I understand the purpose of the neutral and ground in the house. If we have a short, we want that current and voltage to go to the path of least resistance, hopefully back to the breaker, to trip it
OK, let’s try to go back to some basics.
I assume you know how a transformer works; I suggest you grab some paper and draw the circuits out for yourself as you follow along; visual reinforcement is a great learning aid and along the way you might have the “Ah Ha” moment where this becomes clear.
Vin / Vout = Turns in / Turns out and the current capacity is more or less determined by winding size
If you have a 120VAC primary transformer with a 24VAC secondary, you get 24VAC output
Suppose you put this transformer on a metal chassis like old radios etc. and you plug it into the wall with a standard grounded 3 pin plug so that L1 is fused and connected to either wire of the primary and the neutral is connected to the other lead of the primary with the ground connected to the chassis. All is well with the circuit here; the primary is a completed circuit via the neutral and L1.
If one of the primary windings anywhere along the coils faults to the chassis, the ground that previously carried zero current suddenly carries the fault current back to the house breaker panel where it completes the circuit via the ground to neutral tie I showed in the previous post and the fuse should blow.
At this point of the discussion, the secondary winding is completely isolated from the primary. Let’s call them X1 and X2. You could connect either lead to the chassis (which is grounded) and nothing would happen; no current would flow. The other lead of the secondary would then be 24VAC to the chassis, often called chassis ground in this case which is also referenced to earth ground via the 3 prong plug.
Let’s assume neither X1 nor X2 secondary winding are connected to the chassis for this paragraph. If you take two 12 volt lamps of the same wattage in series and connect them to X1 and X2, they will both light equally. They have the same resistance. If you connect the midpoint between these two lamps to the chassis, nothing bad will happen, it is still isolated and, current will not flow to the chassis. Now, connect a 5 watt and 10 watt both 12volt lamps in series and connect them to X1 and X2. The 5 watt lamp gets very bright in a hurry and goes poof; the 10 watt lamp barely glows until the circuit goes open with the failure of the 5 watt lamp; this happens the same if the midpoint is connected to the chassis because it is isolated.
Now let’s change things a bit and connect a wire to the center of the secondary windings, a true center tap I’ll call C. Nothing on the secondary is connected to the chassis at this point.
X1 to X2 is still 24VAC
X1 to C is 12VAC
X2 to C is 12VAC
Now, connect a 5 watt and 10 watt both 12volt lamps in series and connect them to X1 and X2 BUT with the midpoint connected to C.
They both light with their respective brightness and wattage; the 5 watt one doesn’t go poof! The unbalanced extra 5 watts used by the 10 watt lamp is carried back to the transformer center tap C. The 5 watt lamp and half of the 10 watt lamp’s current are the same as two 5 watt lamps in series without the midpoint connected to the transformer; the imbalanced 5 watts for the 10 watt lamp returns back to the transformer via the center tap.
Now if we connect the center tap C to the chassis (which is referenced to earth ground as previously stated) there is no harm, no foul and no current flow between C and the chassis.
And when we repeat the experiment, the results are the same. The wire from the midpoint of the lamp back to C still carries the 5 watts; chassis ground is just a reference.
The transformer at the pole though....you said, "For instance, if you had 30 amps load on the 120volt side from L1 to neutral and 20 amps load on the other side from L2 to neutral, the neutral conductor would carry the 10 amp difference back to the transformer."
Yes, that is true as in the lamp scenario above; but to carry it further: It would be very helpful for you to draw this on paper.
Suppose you have another transformer connected at the primary exactly as above but with two 12VAC secondary windings, completely isolated from each other. So as not to confuse this with the above the first winding leads will be labeled X3 and X4; the other will be X5 and X6.
If you connect your 5 watt 12V lamp to X3 and X4 it will light; circuit completed. If you then connect the 10 watt lamp to X5 and X6 it too will light; a completely separate circuit. With both lamps on it you have 4 wires to the transformer secondary
With the above 4 wire scenario in mind, if you connect X4 to X5 there will be 24VAC between X3 and X6; and the X4&X5 connection is the center tap and behaves EXACTLY the same as the single winding with the center tap mentioned above. Only three wires are needed for your two lamps and the center tap only carries the imbalanced current.
On a side note, not relevant to this discussion, if you connect X3&X5 as well as X4&X6 both making two parallel windings (they have to be wound the same direction or phasing if you prefer) you will get 12VAC at double the current capacity.
Where are these loads coming from? Some place in the transformer, or in the house? Why would there ever be an excess of current? A 60 watt bulb will always draw 60 watt, even if you have it hooked to a 200-amp breaker. So how could you ever have an excess of current? Even if you were to directly wire hot to neutral or hot to ground, if neutral and ground are at 0V, wouldn't there be an instant volt drop where they connected and a lot of heat generated?
The loads mentioned are hypothetical. Every time you turn on a light or 120 volt appliance the situation changes. Think of the transformer as the source of your power however you want to think about it; volts, amps, watts etc. If you have a 60 watt bulb connected from L1 to neutral and you turn on your 1200 watt 120V toaster connected from L2 to neutral, there will be an imbalance. The 200 amp breaker doesn’t care unless it sees an overload on one or both legs L1 and/or L2. The breaker doesn’t see the neutral, nor does it have to as the neutral amps can never be greater than either L1 or L2 in normal circumstances.
If you directly wire L1 or L2 to neutral or to a properly connected ground it will go boom. If you can stand them, Electroboom’s YouTube videos are happy to demonstrate this so you don’t have to.
And to answer your question, there would be heat generated but it would be governed by the available voltage and current as well as the time before something (like the breaker or wire vaporizing) breaks the circuit. If you turn off the breaker and connect line to neutral, the breaker should pop back off before any real damage is done and you won’t experience any real heating.
You would be better not thinking of ground and neutral as zero volts but as a reference point. Zero volts is like absolute zero temperature, unattainable in real life but a good reference point. Calling it ground or neutral is a lot more specific, purposeful and doesn’t mess with your head as much.
I had an old camaro and the ignition went bad. I had to hot wire it. I'd use the fuse panel. I'd hook to a 12V source, then use a wire to tie into the fuse for the Ignition. Then I'd hook into the 12V source and tie into the starter, but just long enough to get running. One day, I was a little careless and grounded that 12V source. The wire evaporated in my hand and left a white indent. At first it didn't hurt, then it hurt real bad. But there wasn't no excess current that just traveled safely back to the cells in the battery. Wouldn't an excess current be called a short?
I guess you found out the hard way why it is called “hot wiring” a vehicle. FYI a “short” is basically any unwanted return to source. In your case the wire you were holding made a circuit from the +12VDC source (basically the battery or unfused path to the battery + terminal) to the engine block or chassis which are very well bonded to the – terminal of the battery. A good car battery is easily capable of producing over 1000 amps at about 10 volts for a short time. That is 10 kilowatts available but as your wire that is the weak part of the circuit heats up, its resistance goes up significantly which probably saved you from being burned a lot worse and / or welding something you didn’t want welded. In the Camaro case the current wasn’t excess, it was just available from the source. Your wire was the same as a filament in a headlamp as far as the battery was concerned; it was connected between the battery poles and the current took the path of least resistance in the newly made circuit. The wire you used did have resistance, all wire does. If you had used a really big wire, you could have blown up the battery.
Also, how come our neutrals and grounds in the house are tied together? Why can we not have it like the transformers on the poles where neutral is seperate from ground? Actually, I probably could, couldn't I? Outside, there's a grounding wire that's buried in the earth that hooks to the panel. If I separated the ground and neutral buss-bars in the breaker panel, and made sure my ground buss-bar went to the grounding wire outside, and the neutral went back to the pole, I'd essentially have the same setup as the pole. Someone in this thread mentioned in their house they'd never have the ground tied to the neutral. But in every home I've seen, this is how it's always setup. Is there any dangers to doing what I just described?
Neutrals and grounds are tied together for safety. I think we have already discussed this. In many cases water lines go through the ground, houses are built on slabs of concrete that when damp are pretty good conductors in terms of life safety. If the neutral were not grounded, and wire does have some resistance as do wiring connections, there can be potential on the neutral conductor. Grounding the neutral mitigates the possible severity of a person coming into contact with a neutral while grounded.
The transformer on the pole for residential services in the USA almost always has the secondary neutral bonded to earth ground as near to the transformer as possible. It is usually a fairly small (#8) bare wire that goes down the pole to the bottom. The bare wire on the 3 wire drop from the pole to your house is the neutral conductor.
WHY on earth would you want to separate the ground form neutral in your house? It would be a clear violation of the code. It could put you and your family at a greater risk of shock. The National Electric Code is revised every 3 years, is based on a lot of history of mishaps and contains the best reasonable ways to mitigate problems. Look up ebay item 401538337928 it is a 2008 copy of the official NEC handbook for $18. Just buy it, read through it and keep it for future reference. The hardcover handbook has a lot of the code explained in real person English
Grounding and specifically proper grounding is a huge field in itself; there are power engineers that have made a career out of nothing but grounding.