Who is correct?

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95285

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I was speaking with an engineer on a change to a project. Here was his proposal: They are wanting to add 4 additional RTUs. Each RTU is 25 ton cooling only. The rating for the unit is MOC 90a
the unit draw 49.8 FLA. The engineer proposes to add one 200a panel with four 90a breakers. My question to him was the size of the panel, I proposed a 400a panel for the additional load. His response was since the load was non-continuos 200a panel for all four would be sufficient. Is he correct with the non-continuos reasoning? The project is in the midwest
 

hassaf

Member
Re: Who is correct?

The RTU's (Roof Top Units) can run 7 hours continuously on a hot day, this would include the compressor & the evap fan. In addition, all these units could be on at the same time.

You could ask any mechanical engineer and he would agree.

The evap fan could runs all the time, you could program it to recycle as an option.

So, I disagree with the engineer, I believe that this is a continuous load and should be calculated as such.
 

lady sparks lover

Senior Member
Re: Who is correct?

Originally posted by hassaf:
The RTU's (Roof Top Units) can run 7 hours continuously on a hot day, this would include the compressor & the evap fan. In addition, all these units could be on at the same time.

You could ask any mechanical engineer and he would agree.

The evap fan could runs all the time, you could program it to recycle as an option.

So, I disagree with the engineer, I believe that this is a continuous load and should be calculated as such.
I doubt that the cooling motors will run at the same time for all the units. Usually we used a diversity of 65% of the actual load, and that's pretty acurrate.

If he's not going to budge, so some of your own investigation. Try an Power panel with HVAC equipment on a hot day, and check all the legs and see what the actual is on the panel. I will even give it 75% tops.

Again, as a contractor you will see actual, but you should take a look at the utility bills, the loads are like half, and the service is always over-sized. :)

[ July 01, 2004, 06:20 PM: Message edited by: lady sparks lover ]
 

hassaf

Member
Re: Who is correct?

When you have 4 units only on a single panel, I doubt that a 65% would be appropriate; I would agree that a 65% might be ok on the total project service demand calculations.

I don't see any reason for these 4 units to be on at the same time. May be if they were 10 to 20 units, a greater demand factor (diversity) might apply and there a greater chance for the units for not running at the same time.

I'm sure you would agree that if there was a couple of units only, then there is a great chance for these unit to be on at the same time, and you would size your panel (especially if the panel is only feeding these two units) to carry the full loads of these units together.

I'm not sure where you draw the line, or how many units you need to start applying diversity, but in my opinion, 4 units could be running at the same time on a hot summer day, and they could be running for a long time.
 

iwire

Moderator
Staff member
Location
Massachusetts
Re: Who is correct?

We do not have all the facts to really answer this.

JMO after wiring many of these units, most units of this size have multiple compressors (or VFDs) and fans that come on as need in stages. The only time the unit will draw the name plate FLA is when all stages are running full.

IMO it is unlikely that all 4 units will have all stages active at the same time for more than 3 hours. If this does happen it sounds like the cooling is undersized for the building.

JMO, Bob
 

rbalex

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Location
Mission Viejo, CA
Occupation
Professional Electrical Engineer
Re: Who is correct?

I already mentioned in some other thread that Art 220 and a few of its internal cross references, such as in 220.14, say about all there is in the Code with regard to demand and diversity factors. For general industrial / commercial facilities, there is precious little of either that the Code recognizes.

In a limited since, the apparent oversize this causes is a safety factor. When you recognize some mechanical and structural codes require safety factors of 5, 10, 20 and occasionally more you realize we electrical folks actually get off fairly easy.

I co-authored a paper on reliability that will be published in a forthcoming IEEE Transactions. With regard to overall reliability, "oversizing" (underloading) is the simplest and most cost effective method to achieve reliability for simple systems. (We refer to it as "Simple-Robust.") It's a big enhancement for complex systems too. This is based on about a half-dozen FMEA (Failure Mode and Effects Analysis) studies we conducted over the last 6 years on several refineries including two of the largest in the world. We didn't deal directly with safety issues in the paper, but it enhances that too.
 

thunder15j

Member
Location
Cali
Re: Who is correct?

Most RTU's have listed on their nameplate a value which is the minimum circuit ampacity. The ampacity should already have continuous duty rating built into it (1.25 x actual combined load). Of course should of in not always what happens. If this is the case, continuous duty is already built in. One would have to check with mfg to be sure.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
Re: Who is correct?

I don't believe the code requires you to provide for the sum of the units MCA's on a feeder. I believe MCA mostly applies to branch circuits.

Personally, I would use the Minimum Circuit Ampacity (with its built-in safety factor) for the largest unit, and then add the Full Load Amps for the other units. If possible, I would also leave room for future units or expansion.

By the way, what happens if one unit goes bad, and is replaced with a 55 amp unit? Does that push you over the 200A limit?

Why not a 225A panel?

Steve
 

lady sparks lover

Senior Member
Re: Who is correct?

Originally posted by steve66:


By the way, what happens if one unit goes bad, and is replaced with a 55 amp unit? Does that push you over the 200A limit?

Why not a 225A panel?

Steve
I would even say a 250A panel with a 225 or 250A breaker. Sorry 95285, I would not go with a 200A per se, but for me 400A is way oversized.

Also Hassef, I would seriously doubt that all motors will be running, and after speaking to many manufacturers, they will tell you that almost 90-100% of the time that all the motors are not running. JMO!


Lady :)
 

rbalex

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Location
Mission Viejo, CA
Occupation
Professional Electrical Engineer
Re: Who is correct?

ARTICLE 220 Branch-Circuit, Feeder, and Service Calculations .
This article provides requirements for computing branch-circuit, feeder, and service loads.
Exception: Branch-circuit and feeder calculations for electrolytic cells as covered in 668.3(C)(1) and (4).
220.2 Computations.
(A) Voltages. Unless other voltages are specified, for purposes of computing branch-circuit and feeder loads, nominal system voltages of 120, 120/240, 208Y/120, 240, 347, 480Y/277, 480, 600Y/347, and 600 volts shall be used.
(B) Fractions of an Ampere. Where computations result in a fraction of an ampere that is less than 0.5, such fractions shall be permitted to be dropped.
220.3 Computation of Branch Circuit Loads.
Branch-circuit loads shall be computed as shown in 220.3(A) through (C).
(A) Lighting Load for Specified Occupancies.
?
(B) Other Loads - All Occupancies. In all occupancies, the minimum load for each outlet for general-use receptacles and outlets not used for general illumination shall not be less than that computed in 220.3(B)(1) through (11)
?
(C) Loads for Additions to Existing Installations.
?
(2) Other Than Dwelling Units. Loads for new circuits or extended circuits in other than dwelling units shall be computed in accordance with either 220.3(A) or (B), as applicable.
220.4 Maximum Loads.
The total load shall not exceed the rating of the branch circuit, and it shall not exceed the maximum loads specified in 220.4(A) through (C) under the conditions specified therein.
(A) Motor-Operated and Combination Loads. Where a circuit supplies only motor-operated loads, Article 430 shall apply. Where a circuit supplies only air-conditioning equipment, refrigerating equipment, or both, Article 440 shall apply. For circuits supplying loads consisting of motor-operated utilization equipment that is fastened in place and has a motor larger than hp in combination with other loads, the total computed load shall be based on 125 percent of the largest motor load plus the sum of the other loads.
(B) Inductive Lighting Loads. For circuits supplying lighting units that have ballasts, transformers, or autotransformers, the computed load shall be based on the total ampere ratings of such units and not on the total watts of the lamps.
...
II. Feeders and Services
220.10 General.
The computed load of a feeder or service shall not be less than the sum of the loads on the branch circuits supplied, as determined by Part I of this article, after any applicable demand factors permitted by Parts II, III, or IV have been applied.
FPN: See Examples D1(A) through D10 in Annex D. See 220.4(B) for the maximum load in amperes permitted for lighting units operating at less than 100 percent power factor.
220.11 General Lighting.
The demand factors specified in Table 220.11 shall apply to that portion of the total branch-circuit load computed for general illumination. They shall not be applied in determining the number of branch circuits for general illumination.
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220.12 Show-Window and Track Lighting.
?

220.14 Motors.
Motor loads shall be computed in accordance with 430.24, 430.25, and 430.26 and with 440.6 for hermetic refrigerant motor compressors.
220.15 Fixed Electric Space Heating.
?

220.21 Noncoincident Loads.
Where it is unlikely that two or more noncoincident loads will be in use simultaneously, it shall be permissible to use only the largest load(s) that will be used at one time, in computing the total load of a feeder or service.
220.22 Feeder or Service Neutral Load.
?
III. Optional Calculations for Computing Feeder and Service Loads
...

220.34 Optional Method - Schools.
...
220.35 Optional Calculations for Determining Existing Loads.
The calculation of a feeder or service load for existing installations shall be permitted to use actual maximum demand to determine the existing load under the following conditions:
(1) The maximum demand data is available for a 1-year period.
Exception: If the maximum demand data for a 1-year period is not available, the calculated load shall be permitted to be based on the maximum demand (measure of average power demand over a 15-minute period) continuously recorded over a minimum 30-day period using a recording ammeter or power meter connected to the highest loaded phase of the feeder or service, based on the initial loading at the start of the recording. The recording shall reflect the maximum demand of the feeder or service by being taken when the building or space is occupied and shall include by measurement or calculation the larger of the heating or cooling equipment load, and other loads that may be periodic in nature due to seasonal or similar conditions.
(2) The maximum demand at 125 percent plus the new load does not exceed the ampacity of the feeder or rating of the service.
(3) The feeder has overcurrent protection in accordance with 240.4, and the service has overload protection in accordance with 230.90.
220.36 Optional Calculation - New Restaurants.
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IV. Method for Computing Farm Loads
...
I don?t do residential.

I?ve taken Article 220 and deleted all ?dwelling? sections - as indicated by an ellipsis (?). In some of the remaining subsections I retained only the title and left the review of the content as an exercise for the student. I retained the critical cross-references to external Sections that effect industrial / commercial design and the key ?basic? rules within the Article.

I draw particular attention to 220.10. Since I have also left the references to ?? applicable demand factors permitted by Parts II, III, or IV?? (220.14 and 220.21) how would you size the panel under question? Cite your Code references.

Remember: "noncoincident loads" are generally mutually exclusive such as electric space heating and air conditioning or "running" and "spares." They are not "probabilities."
 

dahualin

Senior Member
If I am going to size the OCPD, I will count the biggest MOCP and other three FLAs. That is 90A + 49.8 x 3 = 239.4A. Either next size up (250A) or next size down (225A) shall work for the OCPD.
 
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