Upsizing breaker for motor load, necessary?

[petersonra]

But that leads me to look at it from a different perspective, why do they go through the trouble of keeping the magnetic trip the same across several ranges current ratings? :?

UL had a guy that was a prophet. Years ago he foresaw the Internet before there were even computers and arranged to put odd things into all kinds of UL specs so that forums like this would have something to talk about.

NFPA also had one, which is why there are so many curious things in various NFPA standards.

Due to budget cuts however, both organizations have had to get rid of their prophets so future generations will probably have to live with more rational codes and standards and there will be less to talk about on the Internet. This could be a very serious problem as people will have to start working all day to fill in their time instead of spending it on the Internet. This will probably cause mass unemployment as employers find they need fewer employees. Likely to be the cause of the next Great Depression.

 

[Mgraw]

That's true, but if I increased a 15amp breaker to a 25 amp breaker for an AC unit the magnetic trip threshold would remain the same. But that leads me to look at it from a different perspective, why do they go through the trouble of keeping the magnetic trip the same across several ranges current ratings? :?

From what I see the only time the max time is the same is at 80 times and above and then that is at 1 cycle. I could certainly be looking at it the wrong way. Another issue is magnetic trip times are determined by each breaker manufacturer and not dictated by Nema. Thermal trip times do have some max values dictated.

 

[mbrooke]

UL had a guy that was a prophet. Years ago he foresaw the Internet before there were even computers and arranged to put odd things into all kinds of UL specs so that forums like this would have something to talk about.

NFPA also had one, which is why there are so many curious things in various NFPA standards.

Due to budget cuts however, both organizations have had to get rid of their prophets so future generations will probably have to live with more rational codes and standards and there will be less to talk about on the Internet. This could be a very serious problem as people will have to start working all day to fill in their time instead of spending it on the Internet. This will probably cause mass unemployment as employers find they need fewer employees. Likely to be the cause of the next Great Depression.

There are some sharp tacks out there, but I doubt this is a conspiracy.

As for the trip curves themselves they aren't based on a UL standard, in fact UL does not even require breakers to trip magnetically. Interestingly at some point 20 years a go manufacturers for unknown reasons began lowering the trip values on single pole version to around 10x which would make sense considering these breaker generally do not see motor loads.

I would venture to guess something happened along the lines here too, but the fact they go through the trouble of keeping the magnetic trip the same leads me to believe magnetic tripping of smaller motors is not an issue.

 

[mbrooke]

From what I see the only time the max time is the same is at 80 times and above and then that is at 1 cycle. I could certainly be looking at it the wrong way. Another issue is magnetic trip times are determined by each breaker manufacturer and not dictated by Nema. Thermal trip times do have some max values dictated.

Do you know what currents a 1 cycle inrush pulls on average? Theoretically Id guess it would always be below the magnetic curve because the motor would always trip the breaker? Manufacturers do set the magnetic trip (and I am curious to know Square D's reasoning) however Id be curious to know why the NEC makes an allowance to size up to 250% when all manufacture trip curves seem to speak to the contrary.

 

[Mgraw]

Do you know what currents a 1 cycle inrush pulls on average? Theoretically Id guess it would always be below the magnetic curve because the motor would always trip the breaker? Manufacturers do set the magnetic trip (and I am curious to know Square D's reasoning) however Id be curious to know why the NEC makes an allowance to size up to 250% when all manufacture trip curves seem to speak to the contrary.

My guess would be they are allowing some cushion on the magnetic trip because the breaker is only being used for shorts and grounds. The current on inrush would vary by each motor would be my guess.

 

[GoldDigger]

But that leads me to look at it from a different perspective, why do they go through the trouble of keeping the magnetic trip the same across several ranges current ratings? :?

Just as a guess, it is easy to change the thermal trip rating while keeping the same time shape of the curve, just by changing the resistance of the thermal element. Testing for different values is easy.
But for magnetic trip, the trip point is pretty much set by the mechanical arrangement of the parts, and changing it would involve changing springs or number of turns in the trip coil, which may require more testing and cost more money.
This way the same base unit can be used for a range of values, with just one component being changed. Easier stocking and quick change of the production mix.

 

[mbrooke]

Just as a guess, it is easy to change the thermal trip rating while keeping the same time shape of the curve, just by changing the resistance of the thermal element. Testing for different values is easy.
But for magnetic trip, the trip point is pretty much set by the mechanical arrangement of the parts, and changing it would involve changing springs or number of turns in the trip coil, which may require more testing and cost more money.
This way the same base unit can be used for a range of values, with just one component being changed. Easier stocking and quick change of the production mix.

That does sound like the most likely scenario. But that then somewhat changes the validity of the NEC's 250% rule.

 

[kwired]

That does sound like the most likely scenario. But that then somewhat changes the validity of the NEC's 250% rule.

I don't know where 250% comes from but can tell you other factors make a difference - like source impedance as well as line impedance between source and the motor. I have seen same motor, with same breaker series in two different locations, yet one will not hold during starting without upping the setting.

Magnetic trip is going to trip immediately (from what human eye can tell) if it is going to trip on a motor start. If it trips after a long enough delay that you notice it, the magnetic field has already built up enough that the current being drawn has dropped off pretty significantly. The current is still high until the rotor accelerates, but not nearly as high as it is the first few milliseconds of across the line starting. Reduced voltage starting methods are mostly about lowering the amount of current those first few milliseconds, though the time until transfer to across the line may be as long as a couple seconds, but for many larger loads full voltage is needed as soon as that initial coil magnetizing is done so there will be sufficient power available to accelerate the load.