mbrooke
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Why are capacitors listed 380 and 440 volts? What permits them to be used on a 480 volt supply? 
380 and 440 volt capacitors
- Thread starter mbrooke
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ptonsparky
Tom
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Nominal voltage?Why are capacitors listed 380 and 440 volts? What permits them to be used on a 480 volt supply?
Why are capacitors listed 380 and 440 volts? What permits them to be used on a 480 volt supply?
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mbrooke
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Huh? That makes no sense. 480 volts is the nominal voltage.
mbrooke
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380 and 440 aren't nominal.
Beaches EE
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370 v and 440 v capacitors are commonly found in single phase condensing units and heat pumps in start and run applications. The voltage rating is the maximum working voltage and there is no harm in using a higher-rated part than the original. Critical is the capacitance, which should not deviate by more than +/- 5%.
mbrooke
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So what do you put in 480 volt units?
FionaZuppa
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There are caveats to what you say, to be aware of.
1) the higher V rated item is often physically bigger, so it may not fit as a replacement.
2) the higher V rated item uses better (but usually just thicker) insulation between the wraps, which means heat dissipation will be less.
FionaZuppa
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A cap rated for more than 480v(rms). Caps are peak V items, when the diff charge on plates gets too high, it will punch right through the insulation, arc flash, and many times explode. Something like a 750v rated cap, or one that says "rated for 480v(rms)".
All caps should be marked MAX volt, but some mark using "rms" or "nominal".
380(nom,rms) is a 537max
440(nom,rms) is a 622max
480v(rms) is 678max
Certainly also depends on how the cap is used. Directly across 480v lines, 380 and 440 will pop.
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ptonsparky
Tom
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I should have asked how you are using it.
In motors the counter emf generated in the motor windings while it is running is the problem. We can replace the 370 with a 440 but not the other way around. It may not even start the first time with the wrong voltage.
In motors the counter emf generated in the motor windings while it is running is the problem. We can replace the 370 with a 440 but not the other way around. It may not even start the first time with the wrong voltage.
FionaZuppa
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If you mean using a 370 where a 440 was, it would only be the case of no-start is the cap blew up, given the MFD was the same, etc.
mbrooke
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ptonsparky
Tom
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Single phase 480? What is the back EMF?
Beaches EE
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OP, what is the application in a 480v circuit? Do you have a schematic?
A 440 V starting cap is for single phase. It is usually an electrolytic design and NOT continuous duty. The fluid will boil and it will vent or explode after about a minute. They are only intended for 10 second operation.
Surge testing is normally 200% of rated on most equipment so I would expect it to easily survive 480 V, for a LITTLE while. And since a starting cap will run $20 while a continuous duty power factor or filtering cap will run a few hundred at the same size it’s easy to make a mistake. Capacitors can be one of the more baffling parts of electrical work.
Surge testing is normally 200% of rated on most equipment so I would expect it to easily survive 480 V, for a LITTLE while. And since a starting cap will run $20 while a continuous duty power factor or filtering cap will run a few hundred at the same size it’s easy to make a mistake. Capacitors can be one of the more baffling parts of electrical work.
mbrooke
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mbrooke
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Frank DuVal
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You have a 480 volt single phase fan motor???
Frank DuVal
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480 x 1.41 = 676.8 peak volts. I would suggest a 1kV capacitor.....
ptonsparky
Tom
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We just saw a single phase 480 volt motor not too long ago. First for me. IDR caps on it.
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