430.6(A)(1) New Exception

xptpcrewx

Power System Engineer
Location
Las Vegas, Nevada, USA
Occupation
Licensed Electrical Engineer, Licensed Electrical Contractor, Certified Master Electrician
Exception No. 4: For motors rated less than those shown in Table 430.250, the actual current rating marked on the motor nameplate shall be permitted to be used to determine the ampacity of conductors or ampere rating of switches, branch-circuit short-circuit and ground-fault protection.

Thoughts?
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
My thought: What's the concern? Smaller replacement motors will have smaller current differences than larger motors will.

We're told to normally use the tables instead of nameplate numbers because replacement motors may have different ratings.
 

xptpcrewx

Power System Engineer
Location
Las Vegas, Nevada, USA
Occupation
Licensed Electrical Engineer, Licensed Electrical Contractor, Certified Master Electrician
My thought: What's the concern? Smaller replacement motors will have smaller current differences than larger motors will.

We're told to normally use the tables instead of nameplate numbers because replacement motors may have different ratings.
I agree with the idea behind using FLC values (provided they are available), but anything below a 1/2 H.P., 3-phase motor is not listed in the table.
 

xptpcrewx

Power System Engineer
Location
Las Vegas, Nevada, USA
Occupation
Licensed Electrical Engineer, Licensed Electrical Contractor, Certified Master Electrician
I would suggest contacting NEMA, to see how common a motor less than 1/2 hp 3 ph are
Came across a few at a generating station for an aqueous ammonia forwarding pump application. First time, I've had to look up the FLC for something this small three-phase...
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
Exception No. 4: For motors rated less than those shown in Table 430.250, the actual current rating marked on the motor nameplate shall be permitted to be used to determine the ampacity of conductors or ampere rating of switches, branch-circuit short-circuit and ground-fault protection.

Thoughts?

430.6(A)(1) mentions that the table values "shall be used to determine the ampacity ... instead of the actual current rating marked on the motor nameplate." I'm not sure about the rationale behind this, but my guess is that the table values represent a conservative high end of the range of full load currents in different motors with a given horsepower rating. For example, at the link below there is some variation between the currents of different motors that have the same horsepower rating, even when the RPM is the same. The table ratings seem to often be 15% to 25% higher than the typical rated current. I'm thinking that if a replacement motor is chosen, using one with the same horsepower is expected but duplicating the current rating may be impractical or just likely to be overlooked. And so having a table value that represents the high end of the range of possible currents would cover the variations that exist between different motors of the same horsepower.

Another thing to note is that the currents in the motors at the link below and also in Table 430.250 don't tend to go down with horsepower as fast as would a linear relationship. And so, for example, assigning a 1/4 HP motor with one half of the table current listed for a 1/2 HP motor would not be appropriate.
A suggestion is that your proposed exception specify 1.25 times the motor nameplate current for those motors with a HP less than those listed in the table. Or whatever multiplier might be more suitable.


https://s3.amazonaws.com/s3.supplyh...tors-General-Purpose-D1P2D-Submital-Sheet.pdf
 
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