Inrush Currents

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FionaZuppa

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AZ
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Part Time Electrician (semi retired, old) - EE retired.
quick Q. for branch circuits that have motors at end (not including NEC exceptions here for wires and motors), is the inrush current simply discarded when calculating OCD sizing? is it discarded if the inrush time period is less than time period for OCD to trip while over the rated Amps of the OCD ?? need to understand how NEC handles inrush currents when determining OCD size. i see where NEC defines certain types of motors, but that is in the exceptions section for wire sizing.

example, my testing shows ~30A inrush for about 1-1.5sec, but OCD is a 20A. is there a threshold where the OCD would be up-sized due to the inrush current magnitude and/or inrush period? or does NEC say the OCD should be sized off of the equipment nameplate only (thus NEC is saying "we trust what the manufacturer has put on the nameplate if its UL listed item")? what if its not UL listed equipment?
 

Jraef

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So it's a bit tricky to categorize the INTENT of the NEC on this.

The NEC requires that a circuit be protected from over current and short circuit. An OCPD, such as a fuse or circuit breaker, does both tasks.

But the reason there is an entire section (Article 430) devoted to motor applications is because of the issue you raise; motor starting current can sometimes cause issues with the more generalized rules of OCPD sizing for other types of circuits. So does the NEC "discard" (by which I think you meant disregard) the motor starting current? No, in fact to a great extent that is WHY Article 430 exists. In table 430.52, you select the rating of the Branch Short Circuit and Ground Fault Protective Device. If you look at the values allowed in that table, they are far higher than you could use elsewhere, specifically because of the issue you mentioned.

But ALSO in that article, you are required to have RUNNING thermal overload protection for the motor, either in the form of an separate thermal overload relay, thermal devices inside of the motor, or you CAN use the OCPD if it provides thermal protection. Table 430.52 is based on a circuit where there is some OTHER form of acceptable running thermal overload (over current) protection in the circuit.

If you chose to do the latter, i.e. the motor has no internal thermal protection and you do not have a separate thermal OL relay, so you want the OCPD to do BOTH functions, that's where it can get tricky. If you follow the rules for thermal overload protection, you find that it requires the OCPD to be sized much smaller than what is allowed in table 430.52 (which by the way is the MAXIMUM size, not minimum). The problem is that most fuses or circuit breakers, when sized to do the thermal OL task, will NOT hold in if there is any kind of significant starting current surge, and you get nuisance clearing. THAT is not the concern of the NEC, that is the concern of whomever is getting cheap on trying to do two things with one device.

The NEC gives you a remedy, it's up to you to decide you want to take a chance at trying to squeak one by.

Side note from a pedantic engineer: "Inrush Current", despite the common use of it to describe motor STARTING current (aka "Locked Rotor Current"), is not the same. "Inrush Current" is a specifically defined term referring to the in-rush of current in an electromagnetic coil when first energized, where there is no impedance yet established. It only lasts for a fraction of a second, but is much much LARGER than typical motor "starting" current, in theory as high as 2200% of motor FLA. Inrush happens first, when there is nothing but the natural resistance of the winding conductors slowing down the rise of current, then once the coils begin to interact with each other and provide mutual induction to create impedance, the current drops down to what it take to create torque (in a motor), the "Starting Current", then stays at between 500 and 600% of FLA until the motor accelerates to around 80-90% speed. You get "Inrush Current" in transformer windings and solenoid coils as well, same issue. But in those cases, there is no "Starting Current" afterward.
 

FionaZuppa

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AZ
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Part Time Electrician (semi retired, old) - EE retired.
yes, i do mean inrush and not locked rotor. i understand inductors and cap starting very well :)

but 430 seems to be that "exception" section written for ampacity table, thus allowing dedicated motor circuit to use the smaller wire. but using the smaller wire because why? i dont see account for inrush currents, or are these assumed? example, if inrush to a big motor was say 166A for 0.021sec would #14 still be allowed if the MCA of plate said 10A? 166A is fusing current of #14, but, within 0.021sec its not likely to get hot enough to melt. but for safety would i want such an allowable setup? this is a far stretching example, but i think you see what i am asking.
 

K8MHZ

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Michigan. It's a beautiful peninsula, I've looked
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Electrician
Side note from a pedantic engineer: "Inrush Current", despite the common use of it to describe motor STARTING current (aka "Locked Rotor Current"), is not the same. "Inrush Current" is a specifically defined term referring to the in-rush of current in an electromagnetic coil when first energized, where there is no impedance yet established. It only lasts for a fraction of a second, but is much much LARGER than typical motor "starting" current, in theory as high as 2200% of motor FLA. Inrush happens first, when there is nothing but the natural resistance of the winding conductors slowing down the rise of current, then once the coils begin to interact with each other and provide mutual induction to create impedance, the current drops down to what it take to create torque (in a motor), the "Starting Current", then stays at between 500 and 600% of FLA until the motor accelerates to around 80-90% speed. You get "Inrush Current" in transformer windings and solenoid coils as well, same issue. But in those cases, there is no "Starting Current" afterward.

Great info!!
 

K8MHZ

Senior Member
Location
Michigan. It's a beautiful peninsula, I've looked
Occupation
Electrician
yes, i do mean inrush and not locked rotor. i understand inductors and cap starting very well :)

but 430 seems to be that "exception" section written for ampacity table, thus allowing dedicated motor circuit to use the smaller wire. but using the smaller wire because why? i dont see account for inrush currents, or are these assumed? example, if inrush to a big motor was say 166A for 0.021sec would #14 still be allowed if the MCA of plate said 10A? 166A is fusing current of #14, but, within 0.021sec its not likely to get hot enough to melt. but for safety would i want such an allowable setup? this is a far stretching example, but i think you see what i am asking.

Fusing current has two dimensions, current and time. 166A is for 5 seconds or so. Probably a couple orders of magnitude higher for .021 seconds.
 

FionaZuppa

Senior Member
Location
AZ
Occupation
Part Time Electrician (semi retired, old) - EE retired.
The 2 cycle withstand for #14 copper without damage to the insulation is 1200 amps.

cant it withstand 1,000,000 amps for 0.000000000000000000001sec ?? the movement of charge is what causes the heat, and the longer it moves the hotter it will get.

whats the 2 cycle definition?
 

Barbqranch

Senior Member
Location
Arcata, CA
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Plant maintenance electrician Semi-retired
cant it withstand 1,000,000 amps for 0.000000000000000000001sec ?? the movement of charge is what causes the heat, and the longer it moves the hotter it will get.

whats the 2 cycle definition?

If I counted zeros correctly, that would be 6240 electrons making a mad dash (or mad drift current in reality) down the wire.
 
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