Sizing the feeder protection device

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dblake

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Sacramento, CA, USA
Here is the existing setup. A feeder circuit supplies three branch circuits. Each branch supplies a 20 HP motor.
  1. Each branch has a 27A FLC and a 70A SCPD (27 * 125% = 68).
  2. The feeder load is 87.75A (27*125% + 27 + 27).
  3. The feeder conductors are sized 3 AWG (100A for 75C wire at 30C ambient).
  4. The feeder conductors are protected with a 110A SCPD (<= 70 + 27 + 27 = 124).

An engineering change to the process line is eliminating one of the branch circuits. Do I have to resize the feeder protection device in step 4, above?
  1. Per NEC 430.62(A), must it now be downsized to 90A (<= 70 + 27 = 97)?
  2. Per NEC 430.62(B), can it now be sized "based on the ampacity of the feeder conductors"? If so, what is that basis?
 
jumper

jumper

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3 Hr 2 Min from Winged Horses
dblake said:
Here is the existing setup. A feeder circuit supplies three branch circuits. Each branch supplies a 20 HP motor.
  1. Each branch has a 27A FLC and a 70A SCPD (27 * 125% = 68).
  2. The feeder load is 87.75A (27*125% + 27 + 27).
  3. The feeder conductors are sized 3 AWG (100A for 75C wire at 30C ambient).
  4. The feeder conductors are protected with a 110A SCPD (<= 70 + 27 + 27 = 124).

An engineering change to the process line is eliminating one of the branch circuits. Do I have to resize the feeder protection device in step 4, above?
  1. Per NEC 430.62(A), must it now be downsized to 90A (<= 70 + 27 = 97)?
  2. Per NEC 430.62(B), can it now be sized "based on the ampacity of the feeder conductors"? If so, what is that basis?
Click to expand...

Reread 430.24. You do not have to change the feeder OCPD if you do not change the feeder size.
 
david luchini

david luchini

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Location
Connecticut
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Engineer
jumper said:
It does not. 430.62(B) references it for the feeder size (yes, you know that, I am just thinking out loud here).

We dropped a load, kept the feeder size, but need to reduce the feeder OCPD? I need a few minutes to figure this out.
Click to expand...

Silly, isn't it.
 
Smart $

Smart $

Esteemed Member
Location
Ohio
david luchini said:
I don't believe 430.24 addresses overcurrent protection.
Click to expand...
No... but 430.62 (A) & (B) do. They both reference 430.24, but your OCPD-change assessment is correct under 430.62(B). Under 430.62(A), 94.5A would be the maximum OCPD rating where the feeder is sized per 430.24.
 
david luchini

david luchini

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Smart $ said:
Under 430.62(A), 94.5A would be the maximum OCPD rating where the feeder is sized per 430.24.
Click to expand...

Under 430.62(A), 97A would be the maximum OCPD rating where the feeder is sized per 430.24, as pointed out in the OP. Of course, 90A
would be a standard OCPD size that would comply with 430.62(A) in this case, but if one could find a non-standard 97A OCPD, that would comply as well.
 
Smart $

Smart $

Esteemed Member
Location
Ohio
david luchini said:
Under 430.62(A), 97A would be the maximum OCPD rating where the feeder is sized per 430.24, as pointed out in the OP. Of course, 90A
would be a standard OCPD size that would comply with 430.62(A) in this case, but if one could find a non-standard 97A OCPD, that would comply as well.
Click to expand...
Correct... but with the minimum feeder ampacity for the 2 motors now being 27A × 125% +27A = 61A, the #3 is no longer sized per 430.24, so the feeder thus qualifies for OCPD rating under 430.62(B)... which permits 100A OCPD, provided the feeder is not derated below 91A.
 
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