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

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FionaZuppa said:
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.
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Most equipment is tested at a minimum to 200% of nominal plus 1 kV so a 250 V plug should survive 1500 V. A lot of distribution or service entrance though gets much higher (BIL) ratings.
 
Yup, truly a classic line. From Mister Mom (1983)
With a lot of free time left over after taking care of the household and children, Jack Butler decides to become a do-it-yourselfer.

 
paulengr said:
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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At same time a 460 motor being supplied by 495 volts possibly getting too much???

IMO, the amps, efficiency, power factor, probably even speed marked on nameplate are only accurate if that motor actually has 460 volt input and is actually putting out nameplate power. Change actual input volts or mechanical load and everything else is going to change. But yes we do normally set supply to be higher so that hopefully we don't end up below equipment rating after any voltage drop encountered. On top of that the 460 motor rating still has an acceptable plus or minus tolerance.
 
paulengr said:
Most equipment is tested at a minimum to 200% of nominal plus 1 kV so a 250 V plug should survive 1500 V. ...
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This is fortunate. I can't count the number of times I've seen a 250-volt-rated relay socket connected to 480. (usually for a phase-loss monitor)

I've never seen one that had suffered an insulation failure, and often wondered if the 250-volt-rated and the 600-volt-rated components weren't exactly the same components. (except for the label and price tag)
 
paulengr said:
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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I have never seen 440 as a distribution voltage in 40 years. Of course, I don't get around much! Just work central Virginia or ships (an all together different animal, where they quote 450 as equipment voltage due to line loss).;)
 
kwired said:
At same time a 460 motor being supplied by 495 volts possibly getting too much???

IMO, the amps, efficiency, power factor, probably even speed marked on nameplate are only accurate if that motor actually has 460 volt input and is actually putting out nameplate power. Change actual input volts or mechanical load and everything else is going to change. But yes we do normally set supply to be higher so that hopefully we don't end up below equipment rating after any voltage drop encountered. On top of that the 460 motor rating still has an acceptable plus or minus tolerance.
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That’s 7.6% over name plate. Higher starting torque, higher current draw. NEMA has a nice chart.


Still in tolerance.
 
drcampbell said:
This is fortunate. I can't count the number of times I've seen a 250-volt-rated relay socket connected to 480. (usually for a phase-loss monitor)

I've never seen one that had suffered an insulation failure, and often wondered if the 250-volt-rated and the 600-volt-rated components weren't exactly the same components. (except for the label and price tag)
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Coils are a little different. Motor surge ratings are generally 1200-1750 V standard for instance. 200%+1 kV is wiring, bus bars, breakers. Transformers are designed for much higher since they often see distribution surges.

One of the most ridiculous situations was the Advantage starters with electronic coils that had lower surge ratings than what the contactors they were operating generated. AFCI electronics are so cheap that NEC mandates whole house surge protection.

But in general either it has to meet the 209%+1 kv rule.
 
Frank DuVal said:
I have never seen 440 as a distribution voltage in 40 years. Of course, I don't get around much! Just work central Virginia or ships (an all together different animal, where they quote 450 as equipment voltage due to line loss).;)
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Try North of Farmville! The utility gets pretty weak out there.
 
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