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Comment by ethbro | Hacker News Reader

As far as I know... there's no documentation of home power systems in the US.

So the difference between "electrician installed" and "mysterious, unknown person installed" is only in the finished product.

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I had house that had a high quality ceiling fan that always made noise. It was high and hard to reach but after perilous ladder work I discovered it wasn't the fan; it had been wired up wrong, the variable speed control was wired up across the two separate 120v legs supplying the home with electricity. I was surprised that the fan even worked at all that way. I don't know who wired it up originally.

I'm not an electrician so correct me if I'm wrong, but for those unfamiliar with residential wiring in US homes, ordinarily homes are supplied with two out of phase 110VAC lines. Heavy appliances like electric clothes driers and electric car charging points use the 220VAC that I available between the two "hot" lines. Everything else in the house use either one of the "hot" lines (black wires according to the national electrical code). There are actually 3 wires going to each outlet, light, etc. though the house: one black (belonging to one of the two sources of 110VAC), one white (called the neutral line, approximately 0V above ground, and one green a separate green wire that is connected to ground around the point where power enters the house.

Correct, but they're 120/240V. Or at least, should be.

It's actually 120V/~208V. If you're feeling frisky, you can confirm this by going around between random pairs of outlets in your house and comparing the right slots from two different outlets until you find a pair of them that are out of phase from each other.

Fundamentally, the power grid is based on three phase power. Typically, the average telephone pole will have four power wires on it, and one or two thick bundles of phone line or maybe cable TV, I dunno. Three of those four power lines are the three phase electricity, and the fourth is the common neutral line that is ideally near 0V, but typically fluctuates a bit.

Only three out of those four lines enter your house. One is is the neutral line, the other two are just two of the three power lines. Those two power lines are not 180 degrees out of phase, they are 120 degrees out of phase. If they were 180 degrees out of phase, the math is simple: it's obviously just 240V. Since it's 120 degrees out of phase, you end up with a sine wave that looks something like 120\(sin(x)-sin(x+pi\2/3)). (only you need to adjust it for frequency) If you're better at your trig identities than I am, you can calculate that out exactly, but for simpletons like me I just plug it into my calculator and find a local maximum of 207.846 something.

(note: In the US, we use neutral and ground to refer to the common center of the three phase power, and the wire that is buried in the ground near your house, respectively. In the UK, I think they call these two wires ground and earth, respectively. Note that the term "ground" has a very different meaning (read: one of them can kill you, the other one keeps you safe) depending on the context.)

That's interesting, and I don't doubt that you have seen a system like that, but it sounds more like what could be expected in an industrial setting with 3-phase power.

The few houses that I was involved in wiring of in the states were definitely "split phase" systems, which work like the GP describes. In those, three current-carrying wires come from the transformer - two ends and a centre tap from one winding. Those are wired in the breaker box so that the house has two circuits that are 180* off from each other, giving 110V on either circuit, or 220V between the two "hot" legs. There's a better description at https://en.wikipedia.org/wiki/Split-phase_electric_power.

This is not correct for the majority of US households.

Two of the phases DO NOT run to the house, a typical US household service is split-phase. The primary winding in the transformer at the pole connects between a single phase and ground, and the secondary winding is center-tapped to provide a 'neutral', with 120v on either side of the neutral. The voltages are indeed 180 degrees out of phase.

Your test would show 220-240v in the vast majority of US-based residential situations.

The 120/208 you are referring to is when all 3 phases are fed in to a WYE transformer that has 3 120v secodary windings fed from a center point, which becomes the neutral. With a WYE transformer, each phase gets 120v to ground/neutral, and the phase-to-phase voltage indeed peaks at 208v due to the 120 degree rotation between phases. This is typically only found in large residential(600+ amps), and medium to large commercial and industrial buildings with 3-phase service.

Another method would be using a 'Delta' transformer, in which one of the phase-to-phase windings are center-tapped, similar but a little different than the single-phase residential service. The two phases on either side of that center-tap will be 120v to neutral, but the 3rd phase will be 208v to netural/ground. This is usually called the 'wild' leg and is often used to power lighting loads.

Source: I've been doing this for a long time.

Also https://www.doityourself.com/forum/electrical-ac-dc/549775-l...

And https://ctlsys.com/support/electrical_service_types_and_volt...

Indeed. It happens all the time that a homeowner does significant wiring projects in their own home. Everything you need is available at the local hardware store or Home Depot. Some do a good job. I've also seen things like lamp cord used in fixed wiring behind walls or in attics.

I inherited some piece of furniture - I don't recall what - that my wife's ex had attached a florescent lamp to. He used a telephone cord for the 120v power connection.

People who can't explain how current, voltage, and resistance relate to each other shouldn't be allowed near wiring.

A fluorescent lamp that one would attach to furniture probably only takes 10W. That's less than 100mA at 120V; a telephone wire can handle that. Also, telephone wires once carried as much as 50VDC for ringers, so I'd expect the insulation to be adequate, if only barely. The biggest problem, I think, is that because the insulation is relatively thin, there's a greater risk of mechanical damage to it.

One slightly obscure example is someone using a European (220V) light fixture or extension cord (with plug adapters) in North America. (110V).

The current for the same wattage will be roughly double in North America. So lamps, etc. intended for use in Europe will have wiring that is undersized for the current draw if used in North America.

> I've also seen things like lamp cord used in fixed wiring behind walls or in attics.

We found exactly that, lamp cords and extension cords, when we started our remodel a month ago. The former owner did some renovation ten years ago, and was an architect so we always assumed he did a decent job. That was one of a few questionable choices he made, so we're now re-wiring the entire house.

An old house that is rewired would have a permit on file if it were done properly, and includes the name of the contractor if there is one. But there is often no proof on site.

Electrical work can be done properly without a permit having been pulled. Depends on where I suppose but even contractors often don’t get permits.

In Australia all work carried out by an electrician is required to be certified. That is to say, the electrician must, on completion of the work, hand you a certificate stating that all work is to the relevant standard. They can't reasonably withhold the certificate, and it must be provided on completion not on payment.

An organisation called TechSafe[1] is charged with carrying out random inspections.

1. https://www.techsafe.com.au/

I include pulling a permit as part of doing a job properly.

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