Help with calculating MCA

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RexV

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Hi Guys,

I have a question. I am building a unit that has 4 motors in it and a 5000 VA transformer.

I know that if only motors are present in the system then MCA = (1.25 * Load1) + load 2
load 1 = largest motor
load 2 = sum of all motors

However, with this transformer, do i simply count it similar to that of a motor? I also have another formula that says when motors, electric resistance heaters, and other loads over 1.0 A are present (no compressors)

the formula is 1.25 ( load 1 + load 2 + load 3)

I would appreciate any clarification.

Thanks,
 
RexV said:
Hi Guys,

I have a question. I am building a unit that has 4 motors in it and a 5000 VA transformer.

I know that if only motors are present in the system then MCA = (1.25 * Load1) + load 2
load 1 = largest motor
load 2 = sum of all motors

However, with this transformer, do i simply count it similar to that of a motor? I also have another formula that says when motors, electric resistance heaters, and other loads over 1.0 A are present (no compressors)

the formula is 1.25 ( load 1 + load 2 + load 3)

I would appreciate any clarification.

Thanks,
Click to expand...
430.24

MCA = 125% largest motor + sum of other motors + 125% non-motor continuous load + non-motor, noncontinuous load
 
Smart, I almost agree with your post but suppose you have a box with 3 motors and a name plate that says
FLA = 100 amps. You don't know that the 100 amps is the sum of the 3 motors. You would size the
ckt at 100 x 1.25 = 125 amps. This is similar to the post but the code treats it differently.

MCA = 125% largest motor + sum of other motors + 125% non-motor continuous load + non-motor, noncontinuous load
Click to expand...
Should that not be MCA = 125% largest motor + sum of other motors + 100% non-motor continuous load
 
bob said:
Smart, I almost agree with your post but suppose you have a box with 3 motors and a name plate that says
FLA = 100 amps. You don't know that the 100 amps is the sum of the 3 motors. You would size the
ckt at 100 x 1.25 = 125 amps. This is similar to the post but the code treats it differently.


Should that not be MCA = 125% largest motor + sum of other motors + 100% non-motor continuous load
Click to expand...
See following:
2014 NEC said:
430.24 Several Motors or a Motor(s) and Other
Load(s). Conductors supplying several motors, or a mo-
tor(s) and other load(s), shall have an ampacity not less
than the sum of each of the following:

(1) 125 percent of the full-load current rating of the highest
rated motor, as determined by 430.6(A)

(2) Sum of the full-load current ratings of all the other
motors in the group, as determined by 430.6(A)

(3) 100 percent of the noncontinuous non-motor load

(4) 125 percent of the continuous non-motor load.

Informational Note: See Informative Annex D, Example
No. D8.

Exception No. 1: Where one or more of the motors of the
group are used for short-time, intermittent, periodic, or
varying duty, the ampere rating of such motors to be used
in the summation shall be determined in accordance with
430.22(E). For the highest rated motor, the greater of either
the ampere rating from 430.22(E) or the largest continuous
duty motor full-load current multiplied by 1.25
shall be used in the summation.

Exception No. 2: The ampacity of conductors supplying
motor-operated fixed electric space-heating equipment
shall comply with 424.3(B).

Exception No. 3: Where the circuitry is interlocked so as
to prevent simultaneous operation of selected motors or
other loads, the conductor ampacity shall be permitted to
be based on the summation of the currents of the motors
and other loads to be operated simultaneously that results
in the highest total current.
Click to expand...
I should also note there are some associated inconsistencies, but I won't get into that unless they are brought up. No need to add confusion to what already appears to be a confusing matter.
 
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