Motor Branch Circuit: MOPD vs Ckt. - questions and discussion

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scott thompson

scott thompson

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Anaheim, California
*** PREFACE ***

I have cross-posted this message to this area (NEC area) and to the Electrical Calculations / Engineering area, as the content may be covered in both areas.

Additionally, I have this posted at ECN, in the NEC area.

Mods: Please let me know if cross-posting should not be done.
If so, please feel free to remove a post, and let me know of the infringement.

TIA!

Scott
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Message of Post...

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 3?;
  • 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.,, that is good questions you ask here and if you want my answer please do read on :cool: .,,,


But you did not mention the HP size so by reading the amprage you provide here and let say we have 60 HP ODP or TCFC motor so it may change the answer a little.,,



Answer for question #1

It is ok with me and the 200 amp breaker is very common as long they are rated for HCAR useage.

For the starter it will be DOL size 4 NEMA starter.
Otherwise you can use the IEC starter { the size will varies depending on the manufacter itself }


Answer for question #2

It is ok and it is the most lowest price for the package. However with nonfused disconnect switch make sure it is HP rated unless stated by manufacter of that device.

For the question #3 and #4

Both are ok as long the the breaker can hold up during start up so keep in your mind some breaker may not handle hevey inrush current.

And let me add one more item .,, really two anyway .,,

• Reverse Phase protection relay { the Piston compressor are not that serious affected with this but scroll compressor yeah }

• Time delay relay if the power briefly dipped or tempory outage and when the power come back on very fast. the time delay relay will give the compressor a brief pulse period { 5 to 15 minutes } to let the system stableized otherwise the unit will have heack of time to started again when the system is " loaded ".

Merci,Marc
 
They all meet NEC requirement. The #2 method is cheapest among them. But I would change the wiring to 3#4AWG(CU)+#6G(CU) to make it more cheaper. The MCA is 93.2A, and the allowable ampacities of #4AWG(CU) at 60 degree C is 105A that is larger than MCA of the condensing unit. I did the voltage drop calculation that is Ok. So we don't have to upsize the wiring.
 
dahualin said:
They all meet NEC requirement. The #2 method is cheapest among them. But I would change the wiring to 3#4AWG(CU)+#6G(CU) to make it more cheaper. The MCA is 93.2A, and the allowable ampacities of #4AWG(CU) at 60 degree C is 105A that is larger than MCA of the condensing unit. I did the voltage drop calculation that is Ok. So we don't have to upsize the wiring.
Click to expand...

dahualin, The only place I see #4 rated @ 105a is in free air !!!
 
Do the rules in Article 440 apply here or would this installation be under the rules in 430? It is my opinion that 430 applies and there are no provisions to use the minimum circuit ampacity and maximum OCPD information found on the nameplate in Article 430.
 
Reply to Don (don_resqcapt19)

Reply to Don (don_resqcapt19)

Don,

I am applying Article 430 to these scenarios, as it appears to be the more applicable Article.

Benaround & others;

I went with #2 because it is more available than #3 - out of habit!!!

All;

I posted these scenarios to check up on my Designs - per as many factors possible, to keep myself in check.

The examples introduced here are actual, real-world examples - just posted with several methods for de-bunking purposes.

The CU is from an older project, and is a small unit compared to the typical Racks I deal with normally.

In addition to the CU(s) at a given Rack, there will be an Evaporative Condenser, and several Air Handler Units (Evaporators), all fed from the same Rack.

Feeders to the Rack typically are Two (2) 600 Amp, 460V 3 Phase 3 Wire Circuits.

I will post some additional details regarding these systems later.

Thanks to all for the participation.

Scott
 
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