motor currents...

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ToolHound

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Seeking some perspective on motor currents.

For electric motors for equipment such as drill presses or air compressors or such, would the starting current and the locked rotor current be of similar order of magnitude ?

Next, would you agree that the nameplate FLA more less means same as 'running current' ?

Next, for a regular, garden variety 20 amp circuit breaker, approximately how quickly is it designed to trip upon experiencing overcurrent ?

Next, I have a book that sez, quote,

"The next size up rule does not apply when fuses are used for overload protection".

Where if anywhere in Article 430 or anywhere in the Code is that rule stated ?

Next, outside of 15 and 20 amp lighting and receptacle circuits, multioutlet branch circuits supplying receptacles for cord and plug connected portable loads, situations where ampacity of conductors corresponds with the standard ampere rating, and outside of circuits over 800 amps, for what any scenario(s) would you say the next size up rule for OCPD devices is not applicable ? (in other words, what is any scenario(s) that would be an example(s) of when the next size up rule is not allowed to be followed ? ).

Thanks.

 
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ToolHound

Senior Member
Motor currents...

Motor currents...

Seeking some perspective on motor currents.

For electric motors for equipment such as drill presses or air compressors or such, would the starting current and the locked rotor current be of similar order of magnitude ?

Next, would you agree that the nameplate FLA more less means same as 'running current' ?

Next, for a regular, garden variety 20 amp circuit breaker, approximately how quickly is it designed to trip upon experiencing overcurrent ?

Next, I have a book that sez, quote,

"The next size up rule does not apply when fuses are used for overload protection".

Where if anywhere in Article 430 or anywhere in the Code is that rule stated ?

Next, outside of 15 and 20 amp lighting and receptacle circuits, multioutlet branch circuits supplying receptacles for cord and plug connected portable loads, situations where ampacity of conductors corresponds with the standard ampere rating, and outside of circuits over 800 amps, for what any scenario(s) would you say the next size up rule for OCPD devices is not applicable ? (in other words, what is any scenario(s) that would be an interesting example(s) of when the next size up rule is not allowed to be followed ? ).

Thanks.
 

jim dungar

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Staff member
Location
Wisconsin
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PE (Retired) - Power Systems
For electric motors for equipment such as drill presses or air compressors or such, would the starting current and the locked rotor current be of similar order of magnitude ?

Next, would you agree that the nameplate FLA more less means same as 'running current' ?

Next, for a regular, garden variety 20 amp circuit breaker, approximately how quickly is it designed to trip upon experiencing overcurrent ?

Yes, for all intents, starting current and locked rotor current are simliar during the normal starting period.

Again, for all practical purpose FLA is equal to running current, the motor loading has a major effect on this.

For the most part, a 20A breaker and a 20A fuse are going to behave similarly up to roughly 500%. There will be some big differences at very high currents like you might have with a bolted fault.
UL tested protective devices will carry 135% for a surprisingly long time (easily reaching the 15 minute mark), they often carry 600% for 10 seconds, and they will usually open 'immediately' once you get into the 10X area.
 

ToolHound

Senior Member
Hmmm...

Hmmm...


Next, I have a book that sez, quote,

"The next size up rule does not apply when fuses are used for overload protection".

Where if anywhere in Article 430 or anywhere in the Code is that rule stated ?


Hmmm, maybe 430.32(A)(1), quote:
- - - -

430.32 Continuous-Duty Motors.
(A) More Than 1 Horsepower.​

Each motor used in a continuousduty application and rated more than 1 hp shall beprotected against overload by one of the means in430.32(A)(1) through (A)(4).
(1) Separate Overload Device.​

A separate overload devicethat is responsive to motor current. This device shallbe selected to trip or shall be rated at no more than thefollowing percent of the motor nameplate full-load currentrating:
Motors with a marked servicefactor 1.15 or greater125%Motors with a markedtemperature rise 40?C orless125%
All other motors 115%

Modification of this value shall be permitted as provided in430.32(C). For a multispeed motor, each winding connectionshall be considered separately.
Where a separate motor overload device is connected sothat it does not carry the total current designated on themotor nameplate, such as for wye-delta starting, the properpercentage of nameplate current applying to the selection orsetting of the overload device shall be clearly designated onthe equipment, or the manufacturer?s selection table shalltake this into account.​

FPN: Where power factor correction capacitors are installedon the load side of the motor overload device, see​
460.9.




 
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