310.14(B) Conductor Ampacity Calculation

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westgbazo

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Location
MI
Occupation
EE
I have an application where I'm sizing a feeder to an air compressor's industrial control panel (ICP). The ICP contains a large VFD for the 400HP compressor motor, (3) contactors for small fan motors, and the controller. Note that the nameplate below only indicates (2) small fans but there are (3).

1776092903066.png

Nameplate for the ICP lists a "total panel rating" of 482 A, but no minimum circuit ampacity (MCA) is provided. NEC 409.20 requires the MCA for the feeder to the ICP to be calculated using motor FLC's from the nameplate (125% of largest + 100% of other loads). Using the motor FLC's from the nameplate, I get: 774.25 A

An existing circuit breaker is available to serve this load but the max conductor size we can terminate on the breaker is (2) sets of 500kcmil, which is rated at 760 A using Table 310.16. The MCA is 14 A higher than the conductor rating. Based on the panel rating of 482 A, I'm not worried about this feeder handling the actual load.

Has anyone successfully calculated conductor ampacity, for an installation similar to this, using the formula provided in 310.14(B)?

1776093973209.png

I have the first three variables, but I'm not sure where to go next for Yc or Rca. Is it possible to determine values for these variables that are accurate enough to trust plugging into the 310.14(B) formula? Looking to see if this is a possible path to gown down before we go buy a larger breaker. Thanks in advance for any help you can provide!
 
I've not used that formula before, but I would fist ask how to square the total panel rating of 482A and the MCA calculation you are getting.

It would seem that the MCA would be something closer to 482A
 
ron said:
I've not used that formula before, but I would fist ask how to square the total panel rating of 482A and the MCA calculation you are getting.

It would seem that the MCA would be something closer to 482A
Click to expand...
Since the panel FLA is much lower, I'm trying to get the manufacturer to provide a more reasonable MCA for us. Then we can size our feeder based on that. Until then, I have to use 409.20.
 
I am having a hard time making out that namplate how are you getting that number?
They likely were adding 15% of the VFD input to get that panel rating?
Even if I add the missing 4th circuit, some small load is not shown possibly the control power,
you'll need to consult the manufacturer seems the nameplate is incorrectly filled out.
Here is my *guess* at a MCA:
Item​
Amps​
Circuit 1 VFD Drive​
400​
Circuit 2​
3.45​
Circuit 3​
3.45​
Circuit 4​
3.45​
? Control Xfromer?​
1.65​
Add 15%​
60​
Panel Rating​
472​
​
​
Item​
Amps​
Circuit 1 VFD Drive​
400​
Circuit 2​
3.45​
Circuit 3​
3.45​
Circuit 4​
3.45​
? Control Xfromer?​
1.65​
Add 25% of Largest Motor​
100​
MCA​
512​
 
tortuga said:
I am having a hard time making out that namplate how are you getting that number?
They likely were adding 15% of the VFD input to get that panel rating?
Even if I add the missing 4th circuit, some small load is not shown possibly the control power,
you'll need to consult the manufacturer seems the nameplate is incorrectly filled out.
Here is my *guess* at a MCA:
Item​
Amps​
Circuit 1 VFD Drive​
400​
Circuit 2​
3.45​
Circuit 3​
3.45​
Circuit 4​
3.45​
? Control Xfromer?​
1.65​
Add 15%​
60​
Panel Rating​
472​
​
​
Item​
Amps​
Circuit 1 VFD Drive​
400​
Circuit 2​
3.45​
Circuit 3​
3.45​
Circuit 4​
3.45​
? Control Xfromer?​
1.65​
Add 25% of Largest Motor​
100​
MCA​
512​
Click to expand...
Nameplate lists circuit 1 as a 400HP 602A motor. Just got photos of this motor and its actually 400kW/602A, not 400HP. It a confusing nameplate but the units are to the right of the numbers. I'm waiting to hear back from the manufacturer with a corrected nameplate.

I was getting 774.25 A for an MCA by the following:
(125% x 602A) + 7.25A + 7.25A + 7.25A = 774.25 A
 
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