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#42997 - 10/01/04 12:36 PM DISCONNECT AND WIRE SIZE?  
rad74ss  Offline
Joined: Apr 2004
Posts: 156
Pryor, OK USA
I have a large HVAC unit. The unit RLA is 631, MCA is 665, and Max Fuse is 800A. Max breaker is 1000A. I figure using the 75 degree column of 310-16 the customer would probably use 2 runs of 500 MCM.

I was sent a revision stating the customer was bringing in 4 four inch conduits with 4 350 MCM wires and a 3/0 ground in each conduit. They are putting in a 1200A disconnect. It did not state what type of disconnect.

The ground makes me think it is AL or CU/AL wire. (Our prints state copper wire only.) I cannot figure out how the customer came up with the wire and disco. size. I am trying to see how far away the disco. is. Besides the fact they are bringing in 4W with ground and our unit is 3W 3PH w/grd, why would they bring in four sets of oversized wire?

I build end user equipment and I don't have alot of knowledge on the service entrance side, so I would appreciate any advice or explanations on this one.



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#42998 - 10/04/04 07:05 PM Re: DISCONNECT AND WIRE SIZE?  
Ray97502  Offline
Joined: Oct 2004
Posts: 55
Central Point,OR
I am not sure what the abreviations RLA and MCA mean. If you have the 2002 NEC, you may want to look at Article 440.22. If they used values you have for RLA or MCA to size the protection they may be sizing the wire to that also. If the feed is directly to your equipment, it definately sounds like whoever wrote the revision is confused about the supply requirments. Does your equipment have circuit protection or a disconnect means on board? Are they planning to install any other equipment ahead of your unit? ie a panel 277 lighting. If they have to set a disconnect they maybe oversizing and bringing a grounded conductor for future loads.
NEC Article 250.122 (F)(1) "Conductors in Parallel" requires a grounding conductor in each run sized to the rating of the over current device. With a 1200A disconnect 3/0 copper would be required in each raceway.

#42999 - 10/04/04 09:21 PM Re: DISCONNECT AND WIRE SIZE?  
Scott35  Offline

Broom Pusher and
Joined: Oct 2000
Posts: 2,707
Anaheim, CA. USA
The Abbreviations are as follows:


[*] RLA = Running Load Amperes - same as Full Load Amperes, but more direct per HVAC and Motors,

[*] MCA = Minimum Circuit Ampacity - RLA with adjustments made to address the Circuit's Design Value ... If that makes any sense...

[*] LRA = Locked Rotor Amperes - Relates to the "Inrush" during Starting of Motors, normally used to classify Fuse types, Transformer and Circuit Breaker Design types, Circuitry specs, and lastly to add with Fault Calcs.

As to the Client's install Specs - with the Four 4" Ducts:


[*] The Four 350 MCMs per Duct is correct, figuring there will be 4 to 6 Current Carrying Conductors per Raceway.
350 THHN cu from the 90 degree column = 350 Amperes. 80 Percent of 350 equals 280 Amperes, so it's a little shy of 300 Amperes for each Raceway.

[*] 350 MCM under the 75 Degree column equals out to 310 Amperes, so even if the Parallel Feeders are just using the 3 Ungrounded Conductors and no Common Grounded Conductor "AKA Neutral" is used, the Feeders are still compliant.

[*] Minimum Equipment Grounding Conductor size for Circuits with an OCPD not exceeding 1200 Amperes is 3/0 cu, so the 3/0 will comply - as long as it's Copper, or "cu".

[*] Each Raceway will need a full size EGC, so its one 3/0 per Parallel Feeder Raceway, or Conduit.

Not a bad idea to include the Grounded Conductor to an Aux. or Remote Gear Section.

If this installation is for Service Equipment from a Utility Owned Transformer, or such, then the System's Grounded Conductor definitely needs to be brought to the Service Equipment and Bonded to the Enclosure(s) plus the Grounding Electrode System at the Building. This is how the Fault Path is provided back to the System's Origin (Transformers) when a Ground Fault occurs.

Also if this will be for Service Equipment, the PoCo may not even use the EGCs, as they are not of use.

So from all this, it's My opinion that the Client's Proposed installations will be to NEC Minimums.

A little more information would be helpful.


Edited tu fikx baad sphellieng [Linked Image]

[This message has been edited by Scott35 (edited 10-04-2004).]

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

#43000 - 10/04/04 10:23 PM Re: DISCONNECT AND WIRE SIZE?  
e57  Offline
Joined: May 2003
Posts: 2,876
"I have a large HVAC unit." My WORD it's a BEAST! Using a 1200A disco because they couldn't find anything available for continuous ratings. Yet to see a 850A panel... These guys must love you, love them back, make it bigger. The power company must like it too, the thing is a transformer to itself, or it dims the neighborhood. Unless it gets walked into operation.

OK I have to ask, as I can not imagine a building with an HVAC unit this large under 20 stories, what is this unit for? What is it's LRA?

Mark Heller
"Well - I oughta....." -Jackie Gleason

#43001 - 10/04/04 10:45 PM Re: DISCONNECT AND WIRE SIZE?  
Ray97502  Offline
Joined: Oct 2004
Posts: 55
Central Point,OR
Scott35, thanks for such a clear reply. Great job. I didn't think about the need to derate for the 4th conductor. thanks again.

#43002 - 10/05/04 12:15 AM Re: DISCONNECT AND WIRE SIZE?  
Bjarney  Offline
Joined: Apr 2002
Posts: 2,527
West-Southern Inner-Northeast ...
Unit votlage is not mentioned. What is it?

208V 3ø » 2.8A/kVA So, 631A is 225kVA

480V 3ø » 1.2A/kVA So, 631A is 526kVA

#43003 - 10/05/04 09:30 AM Re: DISCONNECT AND WIRE SIZE?  
rad74ss  Offline
Joined: Apr 2004
Posts: 156
Pryor, OK USA
The unit is 208V 3PH 3-Wire. We do not have a disconnecting means on the unit. It is an option for us to mount a disconnect. We have a terminal block for the customers termination and a ground lug.

LRA is 691 amps. These are Italian made Screw compressors. They are Temporary Part Wind Start. You should see the amps for Direct Over Line start!

If they are running additional load off of their disconnect and my unit is rated for 800A max fuse, or 1000A max HACR breaker, then should they not have a second disconnect between the 1200A and my unit?

I would hope they are not intending to tap another load off of my panel since it is close to its maximum load as is.

As to the fourth current carrying conductor, is the neutral (grounded) conductor a current carrying conductor. I'm sure there are quite a few threads on that subject in the forums. I am working on a government job right now (cannon plugs and teflon wire) and don't have enough time to set down and do a thorough search.

Here is the disclaimer at the top of every power page we make.

To protected power source per N.E.C. For MCA value of 100 amps or less, use type 60 degree C. If greater than 100 amps use type 75 degree C. copper conductors only.

I have been told that that was to cover the licensing of our equipment. When pressed they told me to close my eyes and say "I believe."

Thanks for the information so far. Still no word from the customer as to what they had in mind.


#43004 - 10/05/04 09:47 AM Re: DISCONNECT AND WIRE SIZE?  
Trumpy  Offline

Joined: Jul 2002
Posts: 8,211
SI,New Zealand
As to the fourth current carrying conductor, is the neutral (grounded) conductor a current carrying conductor.

In a Star (wye) system it is, it carries any out of balance current and also the current from any single phase loads in the equipment.

Let's face it, these days if you're not young, you're old - Red Green grin

#43005 - 10/05/04 10:48 AM Re: DISCONNECT AND WIRE SIZE?  
resqcapt19  Offline
Joined: Nov 2000
Posts: 2,148
The unit is 208V 3PH 3-Wire.

If this is the case, the grounded conductor is not a current carrying conductor and there isn't even a reason to install it.


#43006 - 10/05/04 01:35 PM Re: DISCONNECT AND WIRE SIZE?  
KennyFrank  Offline
Joined: Jul 2004
Posts: 29
Chicago, IL USA
Looks to me like there would be no need to size for larger than 800 amp feed: your LRA is at 691 amps for one thing. RLA is at 631 amps, normally I would figure 125% over that for my feeder which is 788.75 amps. Note that the max fusing is 800 amps, I'd size for that. Sizing to a 1200 feeder seems like a waste of money, unless there is a DEFINITE upgrade sometime in the near future. And I'd agree that only 3 conductors plus the equipment ground is the required cables, neutral would be a waste of money.

Even a blind hog can find an acorn every now and then

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