I always thought AC current requires 2 wires to carry the current both ways just like DC. Recently I read a post on this sub saying neutral wires go to ground. What am I missing?
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No, power always wants to try to go back to the source. It will use a ground path, but in the case of AC power, it’s trying to find its way back to the transformer, a la neutral. Neutral is just the center point (tap) on the transformer.
Ground is only there in your AC circuits to protect the equipment in case of a fault. It’s there to provide the fastest, least resistant path back to your panel to facilitate tripping of the breaker/fuses as QUICKLY AS POSSIBLE. Poor grounds mean slower tripping or not at all.
Source: NEC, Mike Holt, https://youtu.be/mpgAVE4UwFw -1:00 in
You are missing fundamentally what current is. Current is not something that goes through a wire into your machine, current is the flow itself.
Imagine a tube of water. You can put a turbine into that tube that will be connected to power a mill or whatever other machine. If the water moves, the machine moves, if it doesn’t then nothing moves. It doesn’t matter in which direction the water moves to move the machine, the power isn’t the water, its the movement of water. You can design your machine to work when the flow of water goes in only one direction (DC) or when it oscillates between both directions (AC).
The wire is your tube, electrons are the water and current is the flow of water. So current doesn’t really go to your machine, it goes through your machine and comes in the “other” wire. It really isn’t another wire though so much as the other side of the same circuit (the water has to go somewhere!).
The third cable is something unrelated to the current being AC or DC. Its more like another tube that is there in case your machine is spilling water that could hurt you if you touch it.
The current does need to return to the source. The neutral wire is connected to ground for 2 reasons.
1. For safety. The ground wire is attached to the metal chassis of the device. If for some reason the chassis becomes energized the current has a low resistance path back to the source. This will trip the breaker quickly.
2. To stabilize the voltage. It gives a reference point of 0 volts, the earth.
The ground and neutral can only be connected like this in one place. Where the power enters the building – the main distribution.
the earth acts as the 2nd wire, connecting the neutral from your home to the neutral at the power station.
so the important thing here is that voltage doesn’t really exist in a vacuum,
Voltage is the difference in electrical potential between two points, in DC its typically between the positive and negative terminals of a power supply/battery, in AC its typically the live and the neutral. The Neutral is connected to earth somewhere upstream of where the supply is being used (substation, power plant…) to give a 0v reference (the potential of the ground at that point), against which the live can have a potential difference of whatever your local mains voltage is relative to that potential (because the earth is so big, the potential effectively dissapates down to nothing, so its safe to consider it to be 0v),
The Earth is a special type of cable that does a different job to the live and neutral, in normal operation, its not supposed to carry any current, its just supposed to be there to provide a nice, easy path to earth in any sitatuion where a fault with a wire/component causes things to become live that you don’t want being live (like the case of a device, or a water pipe), the idea being that electricity is always trying to take the easiest path to ground, its better that that path is down a wire than through a human, or a gas main
Voltage is always a ‘difference’ between two electrical points. Your height is a good analogy, it’s always the difference between the top of your head and the bottom of your feet regardless of wether you’re standing on the ground or standing on the roof of your house (your clothes will fit fine either way).
All electrical appliances will work fine as long as the difference between ‘hot’ and ‘neutral’ is 120V (US), but for safety we make sure the voltage between ‘neutral’ and ‘ground’ is 0V (which makes them electrically the ‘same’) Just like your clothes will fit you regardless of where you’re standing, it’s safer if you change clothes while standing on the ground rather than standing on the roof of your house.
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