Selection OCPD for non standard motor HP

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cppoly

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How do you select a branch circuit OCPD for motors with HP that do not alight with the HPs listed in the motor tables? Do you just round up, down?

For instance 208V, 3 ph, 12 HP motor, how do I get the NEC FLC from table 430.250?
 
Is the motor itself actually marked "12HP" on the nameplate or is the info on the equipment itself ?

Some critters such as air compressors have some unique "tricks" in marking HP
 
cppoly said:
How do you select a branch circuit OCPD for motors with HP that do not alight with the HPs listed in the motor tables? Do you just round up, down?

For instance 208V, 3 ph, 12 HP motor, how do I get the NEC FLC from table 430.250?
Click to expand...

Non standard motors - use motor nameplate FLA in place of NEC table FLA.



Just curious, is your 12 HP motor an "air over" rated motor, if so it is probably build to about same standards as a 10 HP motor but can be loaded 120% as long as located in the air stream it creates. It will probably have similar "inrush" as a 10 hp motor also.
 
Do you know where the code says to use the nameplate FLA instead of table FLC for nonstandard HP motors?

The 12 HP motor I gave is just an example.
 
I think you can interpolate also.

10 hp = 30.8
15 hp = 46.2

Difference 15.4 then divide by the 5 increments in between 10hp and 15 hp

15.4/4 ~= 3.1

10hp= 30.8
11 hp = 33.9
12 hp = 37
13hp= 40.1
14 hp = 43.4
15 hp= 46.5 close enough
 
Dennis Alwon said:
I think you can interpolate also.

10 hp = 30.8
15 hp = 46.2

Difference 15.4 then divide by the 5 increments in between 10hp and 15 hp

15.4/4 ~= 3.1

10hp= 30.8
11 hp = 33.9
12 hp = 37
13hp= 40.1
14 hp = 43.4
15 hp= 46.5 close enough
Click to expand...
How about 15 and even 16 hp single phase motors?
I see quite a few of those anymore on farms.
Table only goes to 10 HP.

Another common one is 10 hp 480 volt single phase. No 480 volt motors in the single phase table.
 
I always go by the nameplate, get my 125% and round up to the next standard size OCP. If the motor's on a drive then
I go by the OEM specs. Hopefully that reasonably matches the NEC chart.
 
After reading things again I assume you are selecting the branch circuit short circuit and ground fault protection setting. Though it's maximum setting is still based on motor FLA and you don't have one to select from NEC tables - quite often you can get by with less then maximum allowed. 12 HP 208 volt would possibly have a table entry of about 32 amps 2.5 times that would be 80 amps - but like I said earlier many such motors I run into would be a 10 hp design but allowed to be loaded more if located in an airstream. It will still have similar starting current as a 10 HP, and would likely hold on 60 amp maybe even 50 amp breaker in many instances. Impedance of source/supply circuit is what makes or breaks you on this holding during starting, as it is the current level during those first few cycles after closing the circuit that are going to effect the instantaneous trip.
 
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