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Motor Circuit MOPD questions + discussion #180765 09/08/08 11:27 PM
Joined: Oct 2000
Posts: 2,721
Scott35 Offline OP
Broom Pusher and
Member
Hello all,

I wish to discuss scenarios involving a Refrigeration Condenser Unit, and the Branch Circuit OCPD, Feeder (Branch Circuit), plus Disconnecting means at the Equipment.

Our example CU will be as follows:

  • 460V 3Ø;
  • FLA = 78.2 Amps;
  • MCA = 93.2 Amps;
  • MOPD = 125 Amps;
  • Compressor(s) are _NOT_ Hermetic Refrigerant Motor-Compressor: Open frame Motor drives Compressor via external shaft;
  • Across-The-Line Motor Starting;
  • Circuit length less than 100 feet from Service Equipment;
  • CU runs less than 180 Minutes at any given time (non-continuous load).


What are your thoughts on the following Four (4) examples?:

------------------------------------------------------------
Example #1:

Nameplate references MOPD (Maximum Overcurrent Protection Device) as "Maximum Fuse Or HACR Circuit Breaker" = 125 Amp.

A: Branch Circuit Conductors = Three (3) #2 THHN CU, + 1 #6 CU EGC (Equipment Grounding Conductor).

B: 200 Amp, 3 Pole, 600V Fusible Disconnect Switch at CU, with Three (3) 125 Amp RK5 Fuses.

C: Branch Circuit Breaker = 200/3

Is this example (Example #1) NEC Compliant?
Is it a "Good Design"?
Please contribute your thoughts.

------------------------------------------------------------

Example #2:

Nameplate references MOPD (Maximum Overcurrent Protection Device) as "Maximum Fuse Or HACR Circuit Breaker" = 125 Amp.

A: Branch Circuit Conductors = Three (3) #2 THHN CU, + 1 #6 CU EGC (Equipment Grounding Conductor).

B: 200 Amp, 3 Pole, 600V _NON-FUSIBLE_ Disconnect Switch at CU.

C: Branch Circuit Breaker = 125/3 (HACR)

Is this example (Example #2) NEC Compliant?
Is it a "Good Design"?
Please contribute your thoughts.

------------------------------------------------------------

Example #3:

Nameplate references MOPD (Maximum Overcurrent Protection Device) as "MAXIMUM FUSE" = 125 Amp.

A: Branch Circuit Conductors = Three (3) #2 THHN CU, + 1 #4 CU EGC (Equipment Grounding Conductor).

B: 200 Amp, 3 Pole, 600V Fusible Disconnect Switch at CU, with Three (3) 125 Amp RK5 Fuses.

C: Branch Circuit Breaker = 225/3

Is this example (Example #3) NEC Compliant?
Is it a "Good Design"?
Please contribute your thoughts.

------------------------------------------------------------

Example #4:

Nameplate references MOPD (Maximum Overcurrent Protection Device) as "Maximum Overcurrent Protection Device" = 125 Amp. (no references to "Fuse" or "HACR Circuit Breaker")

A: Branch Circuit Conductors = Three (3) #2 THHN CU, + 1 #6 CU EGC (Equipment Grounding Conductor).

B: 200 Amp, 3 Pole, 600V Fusible Disconnect Switch at CU, with Three (3) 125 Amp RK5 Fuses.

C: Branch Circuit Breaker = 225/3

Is this example (Example #4) NEC Compliant?
Is it a "Good Design"?
Please contribute your thoughts.

------------------------------------------------------------

I pose these examples for Two reasons:
1: To stir up some discussions on the subject;
2: To verify past designs I have dealt with.

Looking forward to your replies.

Thanks in advance.

Scott


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
2017 / 2014 NEC & Related Books and Study Guides
Re: Motor Circuit MOPD questions + discussion [Re: Scott35] #180881 09/15/08 12:14 AM
Joined: Oct 2000
Posts: 2,721
Scott35 Offline OP
Broom Pusher and
Member
No takers???

This is not a loaded post!

Scott


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
Re: Motor Circuit MOPD questions + discussion [Re: Scott35] #180894 09/15/08 12:23 PM
Joined: Nov 2000
Posts: 2,148
R
resqcapt19 Offline
Member
I think the first point of discussion must be: "What code article applies, 430 or 440?". That being said, it is my opinion that 430 applies and that we can't use the minimum circuit ampacity and maximum OCPD rating on the nameplate as there are no provisions for that in 430.


Don(resqcapt19)
Re: Motor Circuit MOPD questions + discussion [Re: resqcapt19] #181039 09/20/08 12:41 AM
Joined: Oct 2000
Posts: 2,721
Scott35 Offline OP
Broom Pusher and
Member
Don,

I also believe that NEC Article 430 applies to these scenarios.

The CU Rack will have Several Motors & Other Loads being fed from it:

* Two (2) 15 or 20 HP Compressor Pump Motors,

* Four (4) 3/4 HP Condenser Fans @ the CU Rack,

* Four (4) Air Handlers (Evaporators) with Two (2) 2 HP Motors each,

* Each A.U. (Evaporator) has 25KW Electric Defrost (30.08 Amps @ 460V 3Ø), which is Non-Coincidental with the Blowers of a given unit.
One A.U. will go into Defrost at a time. The remaining A.U.s will continue running during Defrost.

* There is _NO_ Re-Heat in this application.

Scott


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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