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why is the US not Metric
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CatalinaWOW:
I think what he is trying to say is that there is no benefit to motors from having a 180 degree phase.  Which is the only area I am aware of that benefits from 120 spacing in any meaningful way.  That one is however a big benefit.
james_s:

--- Quote from: MagicSmoker on October 25, 2019, 12:34:07 pm ---Sigh... Until American children are taught metric the Imperial system will persist, simple as that. It's not about what is better, it is about what is familiar, and in the US the Imperial system is by far the most familiar one.

--- End quote ---

We were taught metric in school, and contrary to what many think, quite a few Americans are familiar with metric units, anyone in engineering, science, firearms, automotive, etc is going to encounter metric measurements.

The USA has a mix of metric and imperial units, just like Canada and the UK which are the two other countries I've visited. Of course which things are metric and which are imperial is different in each of them. UK even uses some esoteric old units like stones when referring to a person's weight. Doesn't really matter, you learn what the units are and it's easy enough to convert, especially with modern technology.

If I were starting now, metric would be an easy choice. With >200 years of using imperial units there's a lot of expense and challenges in changing now, so a good deal of inertia. Eventually I suspect metric will get more and more common as everything gets globalized.
james_s:

--- Quote from: CatalinaWOW on October 26, 2019, 12:44:30 am ---I think what he is trying to say is that there is no benefit to motors from having a 180 degree phase.  Which is the only area I am aware of that benefits from 120 spacing in any meaningful way.  That one is however a big benefit.

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Another very big benefit was iron transformer xray machines. With a single phase machine, the anode voltage follows the half wave rectified sine and is only near the desired kVp for a brief moment at each peak. Outside of that time the anode voltage is lower resulting in lower energy photons that contribute more to radiation absorbed dose and less to creating an image on film.

With a 3 phase machine the sine peaks overlap and the voltage never drops near zero so you can use a much shorter exposure and much lower absorbed dose for the same image.

The modern stuff is all going to inverter drives that give the tube clean DC from a high frequency SMPS, so with these the only real benefit of 3 phase is the much smaller bulk filter capacitors required.
vad:

--- Quote from: Monkeh on October 26, 2019, 12:16:00 am ---
--- Quote from: vad on October 25, 2019, 11:36:49 pm ---
--- Quote from: tooki on October 25, 2019, 08:34:46 pm ---
--- Quote from: vad on October 25, 2019, 08:18:50 pm ---My point is - majority of the devices in a typical modern home are low-power low-voltage devices. Having, let say 12VDC / 10A branches in my home, in addition to existing 120VAC and two-phase 240 VAC circuits makes a lot of sense for powering such devices.

--- End quote ---
I understand the appeal, but a 12V/10A circuit is just 120W, and you would easily bog that down with just a few lights and things. (For context, a mere 3 foot segment of LED tape is 1A.) I think people don't realize how much current many of their low-voltage devices draw! A cellphone charging is around 12W (which can be 2.4A at 5V, or sometimes less current at higher voltage), another amp at 12V. Little LED desk or night table lamp? Another half an amp. Next thing you know, your "low power" circuits are overloaded.

What does make some sense is to have low-voltage AC->DC power supplies that can handle multiple devices. I've been doing this, by replacing power strips full of individual USB chargers with 6- and 10-port chargers.

P.S. US 120/240V circuits aren't two-phase, it's a split-phase system. The 120V circuits are created from a single 240V phase, by using a center-tapped transformer.

--- End quote ---
In the US we typically have a separate 15A branch circuit per bedroom. So 120W low-voltage circuit per bedroom is plenty enough for LED lights - with 100 lpw devices you can light up bedroom with 12,000 lumens - equivalent to 15 x 60W incandescent bulbs - far more than anyone would need.
--- End quote ---

And once again, 15A at 12V requires a stupidly large cable - unless you put the 12V supply at the load. Which has already been done for decades - you too can go out and buy 12V constant-voltage drivers and lamps.


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No, it does not. 10 AWG would be sufficient for 10A for residential wiring.
10 AWG copper wire has resistance of 0.9989 Ohm per 1000 feet at room temperature. 10A through 75-feet long circuit branch gives 1.5V voltage drop both ways, which is acceptable.
Monkeh:

--- Quote from: vad on October 26, 2019, 12:59:58 am ---No, it does not. 10 AWG would be sufficient for 10A for residential wiring.
10 AWG copper wire has resistance of 0.9989 Ohm per 1000 feet at room temperature. 10A through 75-feet long circuit branch gives 1.5V voltage drop both ways, which is acceptable.

--- End quote ---

And 10AWG is ridiculous for a 120W circuit! Nor is >10% drop acceptable by any reasonable standard. How on earth is this better than putting a 12V supply at the point of load, as you can already do? It's more expensive, harder to work with, and less efficient!
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