There is a problem in today’s design of power systems, and this problem is not being talked about or covered in any concerted way. Doing power and demand load calculations, is often not considered exciting. But, even though they are not sexy, simple power supply and demand load calculations are important and need to be done correctly.

Consider the following -

The IEEE Red Book defines “demand” as a time average load. (See ANSI/IEEE Std 141). This is a good definition for concept and ease of understanding. But, it does not help much during the actual design process.

The problem is that during an actual design, of say a low voltage power supply system for a commercial structure, the engineer/designer cannot determine the “time average” of most of the loads being connected into the system.

As an example, consider a typical HVAC load. The electrical engineer/designer does not know when the load is going to cycle on or off, or how long the load will stay on at any given time. So, the designer does not know enough about the load to do a time average calculation.

So what is an engineer actually doing during the design process? Usually, he or she is trying to determine what the actual load is during the worst case event. This is the load number which is then used to “design” the system. Note: This number is commonly referred to as the “Demand Load”, even though the usage does not agree with the IEEE definition of Demand.

Another consideration is the US National Electrical Code (ie: The NEC). The NEC does not define the terms Demand or Demand Load at all. It does define “Demand Factor” as the ratio of the maximum demand of a system to the connected load. Again this does not help except in those few places where the NEC gives a useful number. Places like Art. 220.44 where it gives 10kVA + 50% as the demand factors for determining the load of general use receptacles. But, this is not a demand load. It is a rule of thumb approximation method for determining the load during use. It is not a bad rule. It gives usable numbers. But, it is not a time average of the load, and it is not a calculation of the load under worst case conditions. And, the larger problem is that the NEC does not give useful demand factor numbers for most of the loads designers find in commercial structures. Note: Some think that the NEC actually prohibits the calculation of “demand loads”. This is not true. See below.

The point is that “demand” as defined by the IEEE does not agree with the load calculated by the demand factors given in the NEC, and neither of these help engineer/designers determine the actual loads that they need to design against.

This puts an engineer/designer trying to do a design, which is correct from a calculated worst case load point of view, in conflict with the IEEE definition of the term demand load, and the NEC usage of the term demand factor.

Again, during a design, what the engineer needs to do is determine what the actual load is during the worst case event. Then the equipment (eg: a panel) can be sized or designed based on that load. The worst case event is sometimes called the design event condition, because that is the condition or event on which the design is based.

To avoid the conflict in the usage of the terms demand or demand load, let’s define a new term. Let’s call the worst case event the Design Event Basis, or “DEB”. This name reflects what the designer is trying to do. As in, he or she is trying to define the event against which the design is based. And of course, the load that occurs during the DEB is the “DEB Load”.

Now, the design process looks like:

1.Determine what the Connected Loads are.
2.Determine what the DEB Load is.
3.Determine what the Design Load is.
4.Size the equipment to be equal to or greater than the Design Load.

In using this term, there is no conflict with the IEEE definitions, and the DEB Load can be determined in accordance with the NEC rules.

For most loads, the DEB Load can be determined using NEC Art. 220.60, for Noncoincident Loads. (And again for those pesky general use receptacles, we use NEC Art. 220.44.) Using knowledge gained during the design process, the designer determines which loads are on during the DEB, and those loads are counted toward the total DEB Load. While the loads which are off during the DEB are counted as zero. Art. 220.60 allows us to do all of this. Which means that determining the DEB Load in this way meets the NEC minimums.

The point here is that the industry needs to change it usage of the terms Demand and Demand Load to something which engineers can actually use during the design process. We propose the terms DEB and DEB Load.

For more detail on how to use and calculate DEB Loads, see the book “DEB Load Calculations”, from e-books.com or Amazon.com.

Is anyone out there having similiar problems? Would the term DEB Load help anyone else?

Ralph M. White, PE