Guidelines on Load Diversity 3

[Isonergy]

I am trying to find guidelines the electrical community has concerning load diversity. The aim is to try and determine the appropriate service capacity size for an expansion at a clients facility, the expansion will have a certain level of connected load. We feel the site electrical engineer is being overly conservative by using 100% diversity, and we'd like a basis for a counterargument.

 

[dpc]

Diversity will vary drastically depending on the type of facility. I don't think you will find much published data giving you specific diversity that may be used to support your case.

The IEEE Grey Book and perhaps the Red Book talk about diversity, but only provide general guidelines.

Electric utilties have learned through experience diversity factors that can be applied to residential and commercial loads.

But industrial loads are difficult to generalize. There are facilities where pretty much everything runs balls out all the time. In others, there is a lot of spare equipment and significant diversity.

The best approach is to get utility billing data and check out the peak demand for the past year. Compare this with the connected load and you'll get an idea on the diversity, after you allow for appropriate demand factors.

 

[rbalex]

I already mentioned in some other thread that Art 220 of the NEC 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.

From the 2002 National Electrical Code

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.
…

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.
…

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 equipment 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."

 

[Isonergy]

Thank you both (dpc & rbalex) for your input. Most helpful.

 

[dpc]

The plant engineer is being conservative and it's hard to fault that. No one has ever been fired for oversizing the electrical service. If it turns out to be too small, being able to say that it "meets the code" is small comfort.

It's pretty common to size distribution equipment based on connected load - loads tend to grow with time and the excess capacity often saves a lot of time and effort (and downtime) later on.

You will often see the facility's service conductors meeting utility incoming conductors about half their size.

 

[rempman]

If you can get access to Pacific Gas & Electric's design manual there is a great section on diversity of loads. Puget Sound Energy also has a good discusion on the subject.