how does this circuit work?

IDK why a house anywhere would have 3 phase power. But there is a wire for the light, a wire for the fan, two neutrals, so what's the issue?

Sorry, the diagram was wrong. I mistakenly labelled ground as neutral. It is now corrected.

When both fan and light are on, both the blue and black wires are energized.

I didn't think it matters. I'm in Myanmar if this helps.

So, again, you have a hot for the light, a hot for the fan and a neutral. What's the issue?

Is the equipment ground being used as the neutral? It must be tied to the neutral upstream somewhere. That's illegal at least in my world.

first lets clarify your picture

does a represent TWO blacks wires and only one is energized when the fan is on? what about the other black wire? are the two black wires connected together?

does b represent TWO white? are they connected together?

and c looks like it is ONE blue wire that is on when the light is on?

Translating 'downlight' into common English is a start. I assume you are talking about a switch rather than a small, recessed spotlight. Assuming the switch is configured so both black and blue are hot in one position, why wouldn't they both work?

.... it has a fan .. and a light.

John T.

That's what I understood it to be too. And it has more that enough wires to run both. All you need is two switched hots, a neutral and ground. So, I still don't understand what the question about how both the light and fan can work at the same time is all about. issue is.

I can answer that for one location.

In a housing development at what is essentially a campus, we fed 3 phase power to individual houses because of going to 208V airconditioners. I think we got a better price on a bulk buy of the units.

What we found is that everybody complained their dryers didn't work right and their ovens took too long to warm up. Their appliances expected 240.

For a straight thermal load like an over or dryer, that drop from 240 to 208 is actually pretty messy.

At first glance you'd think that 208/240 = 0.8666.., or about 13 pct, so to first approximations 13 pct longer to heat up.

However, wattage is amperage times voltage. And both of these [a] are dropping by that 13 pct.

So the final wattage is.. 0.86... times 0.86... = 0.73.

So more than 25 percent longer.

As a side note, we installed a "240v" air conditioner i a NYC building that had three phase, so it was being fed at the equivlanet of 208.

We very specifically asked the sales rep AND the manufacturer if this would be a problem. They said it was fine.

It wasn't.

Wow, that's amazing. Hard to imagine that a price difference on AC units would push anyone to use 3 phase in a new development, especially given that as you point out, you then have issues with the 240V appliances a typical house has. You'd think the power company would say, "what, are you nuts?" too. But it does go to show you that you can find something odd somewhere.

In the NYC area it's common for both houses and apartments in, well, apartment buildings, to be wired up this way.

The transformer has three hot legsplus neutral tapped off of it. A hot neutral (which is what a typical outlet would have) yields 120VAC.

A hot hot, which would be used for larger air conditiers, a clothes dryer, and maybe an electric stove, would yield the equivalent of 208.

So... each apartment (or house) would get two hots out of the three, and a neutral

Apartment #1 would have "hot" "a" and "b", apt #2 would have "b" and "c", apt #3 would have "a" and "c".

Rinse, lather, repeat.

All three hots, i.e. all three phases, would run to the elevator motor and a rooftop air conditioner.

Note that there are other ways of wiring the transformers so that hot hot gives a true 240, but that isn't the case in lots of the NYC area.

I can see that making sense in apartment buildings in the city, where you need 3 phase for heavy eqpt in the common areas of the building. I'm in NJ and have never seen it in homes here, it's all split-phase to homes. And the big disadvantage to using 208 as you point out is that it only delivers 73% of the power that you'd get at 240V. I hate waiting for my over to heat up even with 240.

Without using another transformer, getting 240 instead of 208 for the high voltage, or line to line loads, means the low voltage, or line to neutral loads get something like 177 - too much for "non-universal" devices and light bulbs, at any rate.

The phase angle mandates that.

If line to line is set at 240 and an additional transformer is employed (can even be a simple center tapped autotransformer to provide the "central" neutral) then everything works as designed - with 120 line to neutral. The center leg of the autotransformer needs to be grounded for safety.

Now, *now*, you tell us...

Where were you 15 years ago?

If they really wanted 3p equipment, they should have ordered 3p center tapped delta. Then they would still have 120/240 available. This is actually fairly common in mixed use industrial bays.