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why is the US not Metric
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Kleinstein:
The US are at least using metric inches already for quite some time.  That is an inch is exactly 25.4 mm and not the measured widths of the thumb or whatever body part of a local chief.
joeqsmith:
Because 402.336 meters just sounds stupid.   So does 304.8 meters. 

I was taught the metric system in grade school.  Throughout my professional career, I have mostly used metric.   I suspect like most countries we use a mix of different units.   
vad:

--- 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.

240V is a two-phase system with a neutral wire and two hot wires running at 180 degrees phase shift.
Monkeh:

--- 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.


--- Quote ---240V is a two-phase system with a neutral wire and two hot wires running at 180 degrees phase shift.

--- End quote ---

No.. it's definitely single-phase centre-tapped.
wraper:

--- Quote from: blueskull on October 26, 2019, 12:20:54 am ---
--- Quote from: Monkeh on October 26, 2019, 12:16:00 am ---
--- Quote ---240V is a two-phase system with a neutral wire and two hot wires running at 180 degrees phase shift.

--- End quote ---

No.. it's definitely single-phase centre-tapped.

--- End quote ---

What makes "phases" must be 120 degrees apart from each other?

--- End quote ---
It does not need to be necessarily 120o as you can use phase count other than 3. But 180o makes little sense because it would be not better than single phase with double voltage.
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