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Long story short, I did a bit of a career change, I'm now doing electrical engineering with an architecture firm instead of embedded systems. So I've jumped from the simple world of DC to the land of AC electronics and it's definitely different considering I never took a phasors class. Anyway, I'm really having difficulty understanding the differences between "phases" and "poles." I'm the type that needs a more concrete explanation than being tossed a copy of the NEC and told to read it (though I'm told that I'm supposedly doing fine and everybody else started from the same place or even worse).

I made this little table below based on what was dictated to me (not sure if it's accurate), but I'm really not understanding it. In embedded-land we're taught that red is hot, black is ground, and life is simple. Now everything is flipped; black is hot, ground only serves as a last-ditch protection mechanism, and "neutral" is used instead as a means of returning voltage to the source which might not be needed sometimes if powering a motor (i.e. HVAC equipment, water fountain chillers, hand dryers, etc.). Myself being me, I feel like a burden grilling into these questions because I was the kid in college who'd ask questions at nauseam.

The internet so far as been an OK resource explaining the basics between 3 phase and single phase. Things like the center-tap neutral to get 120V between hot and neutral and 240 between both hots make sense to me. I get the 120 degree phase differences in a 3-phase system, but again, nobody online seems to want to talk about polarity, and I'm totally lost on that. I THINK it has to do with the number of wires running though the cable (#poles/hots + neutral) but I really need a thorough explanation and the NEC is just a bunch of legalese at the moment.

---- Edit ----

By poles, I mean circuit breaker poles, not magnetic poles. And I didn't mean to say that neutral returns voltage, of course it returns current. I was rushing.

enter image description here

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  • \$\begingroup\$ Looks like you're in N America, right? \$\endgroup\$ Jan 26 at 18:25
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    \$\begingroup\$ Just to be clear, "polarity" is not related to the number of poles. It usually is used in electrical engineering when discussing magnets (north and south poles) or in motors (again north and south poles or alternating poles in AC systems). Can you edit your question to clarify? \$\endgroup\$
    – Transistor
    Jan 26 at 18:36
  • \$\begingroup\$ @user_1818839 Correct, USA \$\endgroup\$ Jan 26 at 18:44
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    \$\begingroup\$ I too think you mean poles (as in circuit breaker poles). I think "polarity" in your question is making things confusing. \$\endgroup\$
    – Tyler
    Jan 26 at 19:07
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    \$\begingroup\$ Might be what you are after is "Wiring Diagrams for NEMA Configurations", search for that, many examples on line. Or if someone in your office has an "Ugly's Electrical Reference" \$\endgroup\$
    – Tyler
    Jan 26 at 19:37

4 Answers 4

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Concepts

The first thing you have to get used to is that GND/Vss doesn't exist. There's a completely unrelated thing called earthing or safety grounding which is (as best possible) referenced to the dirt around your structure. It is a safety shield and never, ever carries current (except during fault conditions obviously).

Here are 2 more concepts:

  • AC power circuits are wired as an isolated system. Both sides of the loop are wired, with a dedicated return wire just for that circuit. It's disconnected from safety ground in all respects.
  • Very important concept here: Neutral is just another live/hot wire for all practical purposes. It's handled a little bit differently because it is less unsafe than other live wires, but that's only for penny-pinching, there's not a fundamental difference.

Tying down the float

The problem with isolated systems is they can "float" to unreasonable voltages. Your 120/240V split-phase supply might come in at 4000V, 4120V and 4240V. That would break down insulation. So we also use the earthing system to "bias" the system to a sensible voltage above earth. You could use a car battery or a 1-volt transformer to create a bias, but a scrap of wire is cheap and has a 0-volt bias. You pick a live to be thus bonded to earth.

Once you do, you call it Neutral.

So "neutral" is a made-up concept.

All voltages are then referenced from neutral. 120/240V split-phase has neutral in the middle. So is it "+120V, 0V and -120V"? You might very well think that; I couldn't possibly comment. People will throw tomatoes at you if you try to label AC power with + and -. (even though it's right half the time, which is a better average than the tomato throwers LOL).

Usually, neutral is pegged in the middle (to minimize voltage of any wire). But on 3-phase "Delta" installations there isn't really a middle. So either a) a middle is synthesized; b) a corner is grounded making phase L1 neutral; or c) neutral is set halfway down a leg, typically in 240V "delta". Giving 120/240V split-phase down one leg, and a third "wild leg" which is 208V to neutral!

Grab any 2 wires

If you need to power something of a certain voltage, grab any two wires that have that voltage.

For instance if you are trying to power a water heater. In US split-phase you'd grab L1 and L2 (240V across them). In the EU you'd grab any Line and neutral (230V across them).

The nuts and bolts of physical installation have a book of rules (NEC in North America). A key rule is that if a wire is neutral, it doesn't need a circuit breaker and you don't (need to) switch that wire. (however if you do, it must have common trip with the live wires). That's pure economics - it's cheaper - but it depends on another rule: that every neutral serve only its partner non-neutral wire(s), and never any others. Thus, neutral is not like system common/GND on electronics.

And then you have the Philippines, where the old US installations have the neutral wire simply deleted by anti-colonial edict. There is no neutral. Every conductor needs circuit breaker protection.

What are poles, then?

Whth 120V/240V split-phase, the phases are 180 degrees apart so they're really just the same phase with a center-tap. Since they're not actually different phases, some people will throw tomatoes at you if you call them "phases". So we call them "poles".

That's it.

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  • \$\begingroup\$ Thanks for the very thorough explanation. So if I'm understanding correctly, "neutral" is tied to earth back at the distribution panel. But at the same time it's still live? \$\endgroup\$ Jan 26 at 21:39
  • \$\begingroup\$ @audiFanatic It's a "live" that is usually close to earth voltage most of the time. If everything goes according to plan. Ask any Code electrician and they'll say "don't count on it". That is why we insulate the neutral wire. LOL once I had a neutral-ground bond fail (that tie you mention) and separately, L1 shorted to ground. Neutral was 120V to ground. L2 was 240V to ground. Yet, everything worked! Found it while testing... could've been that way for months. All the other work was correct, so no boom-boom. \$\endgroup\$ Jan 26 at 22:05
  • \$\begingroup\$ Now by "earth voltage" we don't necessarily mean 0V as I would have said in digital-land; we mean whatever voltage the earth decides to be at on a given day. And the "bond" you speak of, that's what "bonding" is then, correct? I didn't want to misspeak (again, Andy is already angry), so I said tied instead. \$\endgroup\$ Jan 26 at 22:13
  • \$\begingroup\$ @audiFanatic Just think of split-phase as a center-tapped transformer located at a nearby power pole. The center tap runs down the pole and is grounded into the earth there at the bottom. Meanwhile, all three wires are brought over to the home, where again the center tap is re-grounded one more time. There is significant resistance between the two locations if this shared ground wire breaks (not desired) and if any current flows between these grounded lines at the two locations then the earth (dirt) at one location will have a voltage difference between itself and the power pole's nearby dirt. \$\endgroup\$
    – jonk
    Jan 27 at 4:29
  • \$\begingroup\$ @audi the correct phrase is "neutral-ground equipotential BOND". Grounding and bonding could be a book on its own. But yes, in a shoving match between an AC power system and natural earth, earth sometimes loses. \$\endgroup\$ Jan 27 at 4:58
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Typically Delta is used for 3 wire distribution and 4 wire Star is used for industrial to have both 120, 208V single and 3 phase power loads. Residential uses split phase 120/240V sometimes misnomer called dual-phase but more appropriately called dual-line L1, L2 and Neutral.

  • Anything we can say is 0V and is low impedance is called "ground" which is the Protective Earth or even a floating DC power supply like the car chassis.
  • This is also Neutral at the transformer but not quite 0V at the drop point due to unbalanced Neutral currents.
  • Phases go positive in sequence and if any two were swapped , 3 phase motors would go in reverse.
  • line drop of 5% is normally specified for residential from the distribution transformer (DT) and 5% from source with auto or manual tap-changing as required. I have rarely seen an exception. in North America where power quality is high. This shows your 3rd example with RMS and Vpp Voltages and Peak and average powers for an unbalanced load.

enter image description here

Knowing the trigonometry equations for 3x 120 deg phase differences , you can look at this simulation. with Vmax = sqrt. (2) =1.414 times Vrms. to make sense of the voltage and power being the products of Line voltage and current relative to Neutral.

To avoid redundancy Wikiwand shows a full clean description.

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Difference between phase and polarity?

Polarity is a specific subset of phase. If two sinewaves are out of phase by 180° then they have opposite polarity; one is peak positive when the other is peak negative.

For 3 phase systems and voltages, the phase angle between sinewaves is 120° and it would be unwise to use the term polarity although, we can use the term opposite polarity if we want to invert one of the phase voltages by using a transformer (for example).

Now everything is flipped; black is hot, ground only serves as a last-ditch protection mechanism, and "neutral" is used instead as a means of returning voltage to the source which might not be needed sometimes if powering a motor (i.e. HVAC equipment, water fountain chillers, hand dryers, etc.).

  • Forget about colours - too unreliable and too different from one country to the next.
  • Ground is usually best called "earth" but, I also see it is called "ground" in different parts of the world so, be clear about ground/earth and expect as many people to call it earth as ground
  • Earth is a protection mechanism but it's not last ditch - it's there all the time should a live wire come into contact with the chassis case (that is earthed)
  • Neutral returns current to the source (not voltage) and, in some 3 phase situations, there may be very little current to return because the load is balanced; current returns on the other two phase at any one instant in time.
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  • \$\begingroup\$ Thanks for some of the clarification. I wasn't entirely clear, as other users pointed out. So I edited the question if you wouldn't mind giving it a second run through. Thanks \$\endgroup\$ Jan 26 at 19:31
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    \$\begingroup\$ @audiFanatic that's not how things are done here. Once an answer is given, any amendment to the question is frowned upon unless it is agreed by those who have made an answer. I'm rolling back your question to where it was and please don't do this again. This is a Q and A site and not a progressive learning experience or talking shop. If you had got the question written down incorrectly AND made no sense, I wouldn't have answered but, as it happened, it DID make sense and now, you wish to move the goalposts so, listen and learn; this isn't how things are done around here. \$\endgroup\$
    – Andy aka
    Jan 26 at 19:37
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Your table seems to be mostly correct. The breaker requires one pole for every hot wire. The word "polarity" doesn't apply and is not used to describe the number of poles on a circuit breaker. Poles and polarity are two entirely different things. The word poles is used for the number of magnetic poles in a motor, but polarity is not. The word polarity is used to describe which end of a winding is connected to which terminal in some situations.

208/240V, single phase is usually wired with a neutral in order to make 120V hot-to-neutral available. 208/240V is used primarily in residential and commercial occupancies and rarely in industrial occupancies except for in office areas. Some appliances have 4-wire plugs with 2 hot prongs, a neutral and a ground. In an appliance like a stove, 208 or 240 volts is connected to cooking elements and 120 volts is connected to lights and timers etc. A 240 volt baseboard heater may have just 2 wires because there is just a heating element with no electric timer. In that case it might be wired with a 2-wire plus ground cable that has one black and one white wire with both hot. I believe that is permitted by code as long as the white wire is painted or otherwise marked black at the ends.

208, 240 and 480 volt circuits need a neutral if they are feeders to boxes that provide single-phase 208/240-120 or 277 volt branch circuits. Equipment needing single phase line-to-neutral from a three-phase supply would be expected to be quite rare.

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