Why are breakers 120, 240 but receps and plugs 125, 250?

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LarryFine said:
There are nominal supply voltages and there are equipment ratings.
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What I was told by a Hubbell rep regarding this a long time ago:
For a time in the early days of electrification, most residential distribution voltage standards were 110/220V. In the late 20s, design standards for equipment mfrs, like NEMA, came into being. Then in the 30s with the REA (a Depression era government program to provide electricity to farms and rural areas) it was bumped to 120/240V and made a national standard. So NEMA, having to now adjust their design standards to match, anticipated the distribution voltage standard possibly increasing again and they settled on 125/250V as a design standard so that they would not have to change again for a long time.
 
Flicker Index said:
The title says everything, you know so yeah. Why do certain wiring devices have the ratings 125v, 250v.
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Want more confusion - motors and appliances often marked 115 or 230.

For 480 volt nominal the motors are usually marked 460 volts, supply volts often is close to 500 (no load anyway) but devices are marked 480 or sometimes devices marked 600 volts are used or is only thing available that will work for the application.
 
kwired said:
Want more confusion - motors and appliances often marked 115 or 230.

For 480 volt nominal the motors are usually marked 460 volts, supply volts often is close to 500 (no load anyway) but devices are marked 480 or sometimes devices marked 600 volts are used or is only thing available that will work for the application.
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And lets not forget capacitors.
 
Jraef said:
What I was told by a Hubbell rep regarding this a long time ago:
For a time in the early days of electrification, most residential distribution voltage standards were 110/220V. In the late 20s, design standards for equipment mfrs, like NEMA, came into being. Then in the 30s with the REA (a Depression era government program to provide electricity to farms and rural areas) it was bumped to 120/240V and made a national standard. So NEMA, having to now adjust their design standards to match, anticipated the distribution voltage standard possibly increasing again and they settled on 125/250V as a design standard so that they would not have to change again for a long time.
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I wish we had gone to 250 volts. Nice, even, round number.
 
LarryFine said:
Isn't that what I said? :giggle:
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Yes, I was just elaborating on your response because it was an otherwise useless chunk of information rattling around in my brain... :p
I had asked a similar question to that Hubbell rep a few decades ago. This was the first time I saw value in it, I couldn't pass up the opportunity to let it out.

I have, over the 40+ years I've been with my wife, forgotten her birthday, or Valentines day, anniversary etc. But an offhand conversation with a Hubbell rep 30+ years ago? Stuck in there.
 
NEMA standard receptacles are 125 and 250. Think there are some though many never see them rated 208, I know there are some rated 277, otherwise they usually rated 480 instead of 500 at that nominal voltage level, in fact I don't recall ever seeing a 500. There are 600 volt receptacles also.
 
There is an ANSI standard. Everything is based off multiples of 120 V for distribution voltages and 110 V for utilization, but NEMA also requires equipment rated +10% / -15%. Transformers can be adjusted typically +/-5%

So it’s really 120/240 at the transformer. Allowing for drops (2% distribution, 3% branch) it will be down to 220/110 V at the load, in a perfect world. The transformer might be tapped up a little though since taps are in 2.5% increments. So between voltage drop differences and transformer taps it might run a little high at the receptacle. That 2.5% explains 125/250 V.

“200-250” is a very weird voltage class. In three phase equipment distribution can be 240 delta, 208 wye, or even 220 V under European
standards with similar single phase equipment. Equipment for these voltages tends to be wide range. At 480 all we have is 480 and 440.
 
That's one of the nice things about standards: there are so many different ones to choose from.

There's also a standard (!) answer for questions like this: They evolved independently, at different times and places, crafted by people who didn't collaborate and couldn't anticipate the future. (and sometimes, didn't really know what they were doing)
 
paulengr said:
“200-250” is a very weird voltage class. In three phase equipment distribution can be 240 delta, 208 wye, or even 220 V under European
standards with similar single phase equipment. Equipment for these voltages tends to be wide range.
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Its funny you say that because I find it to be so true. Most of the 240 volt stuff I buy almost always happens to be triple rated.


https://imgur.com/a/ODUH49r
 
220, 240, whatever it takes. ;)

Using 240v with an insulator that's rated 250v just gives you a feeling of safety factor.

Or look at it another way, the 250v rated insulator is probably good for 9+kV in std temp/pressure/atm, but running 250v on a rated 250v item just sounds like living on the edge. 10v(rms) is a good safety margin. 😵
 
Frank DuVal said:
What about 460? I haven't seen 440 on a motor since ones made in the 1930s....
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480 and 440 are the distribution (transformer) voltages. 460 and 420 are the equipment voltages. Just like you’d set a transformer to 600 in a lot of Canadian plants but the motors are marked 575. If they were marked 600 its like saying no voltage drop allowed.
 
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