Why is there 8vac between one wire on an AC cable, and the air?
As it's AC, shouldn't both line and neutral do this?
Huh? "and the air"? Please describe better. Meter has 2 leads - what are you connecting them to?
Your meter is acting like an antenna.
Probe connected to one side of the AC line, the other not connected = 8v
Probe connected to other side of the AC line, the other not connected = 0v
The amount of electromagnetic energy/noise you pick-up depends on the input impedance, the length of the wires, and the position/location of the wires... It's very-variable and unpredictable. The meter is not designed to work that way and readings are meaningless.
Put a 1K resistor in parallel with the meter and it will go to zero.
Probe connected to one side of the AC line, the other not connected = 8v
Probe connected to other side of the AC line, the other not connected = 0v
The meter is measuring a difference in voltage between the two leads. Obviously there's a bigger signal on the AC hot lead than the neutral, but what's unknown is what the "air" meter lead is picking-up and the phase of that air signal in relation to the AC signal.
"Obviously there's a bigger signal on the AC hot"?
As they just switch polarity, aren't they the same?
I'm thinking of an alternator output. Hot or neutral are the same - just a label really. What am I missing?
The conductors aren't identical. One of the two conductors is grounded, and the other is not grounded. The grounded conductor, called the "neutral," will have the same, or nearly the same, potential as many of the surfaces in your home. The other conductor, called the "hot," "phase," or "ungrounded" conductor, will have a voltage that swings positive and negative relative to the grounded conductor. Almost nothing in your house is at that potential, other than other wires and their terminations.
You can probably prove this by measuring the voltage between each of the conductors and something that's connected to a cold water pipe, or even to the exposed screw on the coverplate for a receptacle or switch. For newer construction in the US, those screws are typically grounded. In the US, you'd typically see 120 volts between one conductor and a grounded surface, and a few tens or hundreds of millivolts between the other conductor and a grounded surface. If your house is of older construction, without grounding conductors for each circuit, or you're somewhere else, you might get different results.
As DVDoug says, the voltage that you might read with one probe isolated is unpredictable, and largely meaningless. I can't guess why you read 8V. But, the reason that the conductors behave differently is that they are, in fact, different.
tl;dr:
A typical US installation is powered by a 240-volt center-tapped transformer, with the center tap grounded. The two ends of the transformer each provide 120 volts relative to the center tap, and those voltages are 180 degrees out of phase with one another. A typical household circuit has two conductors, with one connected to one end of the transformer, and the other to the grounded center tap, and it provides a nominal 120 volts between the two conductors. A high-power household circuit, serving maybe a stove, oven, or water heater, will have two hot conductors, with 240 volts developed between them.
That's for the US. Other countries might have a similar installation, with a different voltage level - like 240 volts between a hot conductor and ground.
I'm thinking of an alternator output. Hot or neutral are the same - just a label really. What am I missing?
No, not at all. On ordinary AC devices like an ac motor or contactor, yes it doesn't matter, but on any AC wiring in a building , whether residential or commercial, electrical code has strict guideline regarding the wiring of the "hot " (called 'Line' in the USA) and Neutral . As an example, typically in the USA, since the AC is 120V instead of 240V like the UK, that voltage is one of three phases of a 3-phase transformer to Neutral. With 3-phase , the voltage from any phase to Neutral is the Phase-to-Phase voltage divided by SQRT(3) (1.7320). Thus, if the wall socket voltage is 120Vrms, the phase to phase voltage of the transformer it is coming from is 120 VACrms * SQRT(3) = 208 V.
The load for the building is evenly divided across the 3 phases, so each phase delivers power to 'n' circuit breakers but the sum of the total load of all the circuit breakers when added up does not exceed the rated load for one phase of the transformer. Since all the circuits for the building are spread across all three phases (to Neutral), the transformer dissipates heat evenly. In the USA, the Neutral must be connected to Earth GND at the transformer and I think at the main point of entry (POE). Every outlet box must be grounded and every appliance must be grounded. If a wire comes loose in any outlet box or appliance, it shorts out to the box which is earth ground, tripping the breaker, so it is impossible for the end user to be electrocuted if they touch the box or appliance housing. So , no, Hot and Neutral, are NOT just labels, they are distinctly different because the Neutral comes from the GND whereas the HOT comes from the phase (A, B, or C). For example, 3-phase devices (like motors etc) don't even have a Neutral, because Neutral, by definition , is the return for each phase for 120V house wiring , since the only devices in the house that use all three phases are the 3-phase appliances (if there are any). As you can see from the schematic, the 120 V is measured from each phase to Neutral, which , you can see is GROUNDED to Earth GND.
raschemmel:
For example, 3-phase devices (like motors etc) don't even have a Neutral, because Neutral, by definition , is the return for each phase for 120V house wiring , since the only devices in the house that use all three phases are the 3-phase appliances (if there are any). As you can see from the schematic, the 120 V is measured from each phase to Neutral, which , you can see is GROUNDED to Earth GND.
You seem to have illustrated a three phase motor winding which does have a neutral! 
That is a 5 wire motor but I posted that to show the relatinship between the phase to phase voltage vs the phase to neutral but yes, you are correct. If you can find a link for a 208V transformer I'll replace it. For tgd purpose of this post it didn't seem to matter but I suppose the contradiction cojld be confusing to someone not familiar with it. I can remove that schematic if you think it's necessary.
I did find this though which has a lot of information but doesn't have a transformer schematic.
Does this help ?
I found that on the net but we happen to have one of those 10kva 208 3-phase transformers at work. (that we aren't using at the moment)
In my experience, in the US, most residences and small commercial facilities have 240 volt single-phase center-tapped service, with the center tap grounded. Three-phase service to reasonably-sized residences is rare enough that utilities often don't extend all three phases of their feeders into residential areas.
In my experience, in the US, most residences and small commercial facilities have 240 volt single-phase center-tapped service, with the center tap grounded. Three-phase service to reasonably-sized residences is rare enough that utilities often don't extend all three phases of their feeders into residential areas
That makes sense.
"There's more than one way to skin a [xxx]" (noun deleted for political correctness ..ha ha)
You probably know more about it than I do.
Basic point made was that ignoring "hot " and "neutral" labels and treating them as the same ok for low voltage ac devices like solenoids, contactors, single phase motors etc but once you get into line operated devices powered from residential outlets then the two wires can no longer be treated as equal and the
"what's in a label ?" attitude is not acceptable.
How the voltage gets from the 480V 3-phase feeder to wall outlet does not seem to be relevant here other than to point out that the reason there is a difference is in that process of distributing the power, the neutal gets connected to earth ground so theoretically you could take any neutral wire and touch it directly to the outlet box and nothing would happen because it's already connected at the POE.
Try that with the "hot" or "Line" and you could wind up in the hospital .