Motor feeder calculation

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david luchini

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
Ingenieur said:
wrong
it is the equivalent of 430.22 for power conversion equipment 430.122
not for multiple devices, that is 430.24 (which is NOT modified by part X)
Click to expand...

Wrong.

430.122 doesn't say anywhere that it is the equivalent of 430.22. It tells you how to size feeder conductors supplying power conversion equipment. No where does 430.122 say that it applies only to a single motor.
 
kwired

kwired

Electron manager
Location
NE Nebraska
Wrong only occurs when you are trying to use smallest possible feeder conductors for the application;)

Quite often the feeder is sized larger then the minimum required and we have no issues in this area.
 
T

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
As the OP, thanks for all the discussion on this. I think it is fair to say the NEC could use some work in this area to make things more clear. For starters, if you take 430.122 at face value you would be applying 125% for every VFD which could make your feeder become wildly oversized.
 
Smart $

Smart $

Esteemed Member
Location
Ohio
texie said:
As the OP, thanks for all the discussion on this. I think it is fair to say the NEC could use some work in this area to make things more clear. For starters, if you take 430.122 at face value you would be applying 125% for every VFD which could make your feeder become wildly oversized.
Click to expand...
Certainly could use some work.


But what is your basis for saying the feeder would be wildly oversized? Say you have 10 VFD's on a feeder. Motor FLA ~= VFD input current rating. Motors operating at full load 24/7. Would this be any different than a continuous operating resistive load, sized at 125%.

430.24 makes the assumption that when multiple motors are involved, not all of them will be operating at the full load condition and not 24/7.

What we have in these requirements are probability vs. the extreme possibility. 430.24 reflects leniency for probability while 430.122 reflects the extreme possibility.
 
Unbridled

Unbridled

Senior Member
Location
Tampa, Fl
texie said:
I'm struggling for the correct MCC bus/MCC main breaker size for the load below:

2 each 100 HP 480 motors. Table value of 124 amps each. There is also 53 amps of other misc. calculated load fed by the MCC.
Each motor is VFD with a nameplate input of 156 amps. Each VFD has a 200 amp inverse time breaker in the MCC.

Method 1: 124 amps + 200 amps = 324 + 53 = 377 (400 amp breaker/bus).

Method 2: 156 amps + 200 amps = 356 + 53 = 409 (600 amp bus/450 or larger main breaker).

The real issue here is whether method 1 is NEC compliant as if not the the next standard MCC bus sizing is 600 and adds a great deal of cost.
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430.94 Overcurrent Protection. Motor control centers shall
be provided with overcurrent protection in accordance with
Parts I, II, and VIII of Article 240. The ampere rating or
setting of the overcurrent protective device shall not exceed
the rating of the common power bus. This protection shall be
provided by (I) an overcurrent protective device located ahead
of the motor control center or (2) a main overcurrent protec-
tive device located within the motor control center.
 
G

gr33n

Member
Location
Columbia, MD
david luchini said:
There's no language in 430.122 to suggest that the 125% applies only to the largest VFD.
Click to expand...

430.22 only applies for conductor ampacities (from controller (non-VFD) to the motor)
430.122 applies to branch/feeder conductors of a motor controlled by a VFD
430.62 applies to the protection of the feeder of multiple loads.
430.53 applies to the protection of "group motor" installations where only one branch circuit protection device is used for multiple loads.

When talking about the largest load, it refers to only one of the motors, even if two or more of them are of the same size.
 
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