Are "Demand Factors" in Load Flow Studies Permitted?

[Ekmast]

With NFPA 70B stipulating that Load Flow Studies be performed or reviewed every five years, I’ve been researching the requirements for conducting these studies. I’ve reviewed NEC Article 220, IEEE Std 3002.2-2018, IEEE 399-1997 (Chapter 6), and IEEE 141-1993 (Section 2.4).



One observation is there seems to be a difference between a demand load study and conducting a load flow study. SKM software treats them as two distinct types of studies. Demand load studies involve loading calculations to determine the load on a system, primarily used for sizing feeders and services. The NEC provides minimal allowance for incorporating “demand factors” in an industrial setting in these studies, with the biggest exception being a 1-year/30-day meter-recorded demand for existing systems (NEC 220.87).



Load flow studies, on the other hand, are more comprehensive, focusing on how load impacts the entire system, including factors such as power flow in branch circuits, source loading, voltage magnitude, voltage drop, phase angles, and more. One area of uncertainty for me is whether "demand factors" are permissible in load flow studies. Since maximum demand data is often readily available from the utility, a demand factor could potentially be derived from this for use in the load flow study. However, I’m unsure if there is an allowance for this practice.



Some IEEE documents suggest that analyzing different loading levels in an electrical system is indeed permitted. For example, one document notes: “System loading demand should be considered. Both maximum and minimum loading conditions can be useful to determine the system operating limits (i.e., under- and overvoltage conditions).”

From our experience, using the connected load in load flow studies almost always results in the entire system appearing to fail. However, when checked or trended with a meter, this is usually not the case.

 

[petersonra]

The NEC provides minimal allowance for incorporating “demand factors” in an industrial setting in these studies, with the biggest exception being a 1-year/30-day meter-recorded demand for existing systems (NEC 220.87).

The NEC only allows this method if you are adding loads.

 

[Elect117]

Demand data from the utility is a reliable source for sizing services and feeders. The purpose of demand data is to size conductors and OCPD so that they do not fail yet meet your load criteria. You could always have a higher current than the rating on a given circuit as long as it is short enough not to trip an element or damage the conductor. I don't think it would be as useful in a load flow study since you are concerned with more than just the steady state health of conductors and their overcurrent device.

1) Utility metering can provide you with kW but not include kvar which is an obvious consideration in more industrial style plants. Especially when conducting load flow.

2) The demand information is a average demand over a given time period (in seconds typically) and it keeps the highest recorded for 30 days (metering cycle). That may or may not be precise enough for your load flow study. More than likely you care more for inrush and things of those nature to which demand data averages for that window.

3) Utility metering may or may not include bi-directional demand if there is more than one source on a single meter. So their might not be a way to isolate the maximum demands of differing systems without a way to compare their time stamps or having a special SEL relay.

 

[ron]

With NFPA 70B stipulating that Load Flow Studies be performed or reviewed every five years, I’ve been researching the requirements for conducting these studies. I’ve reviewed NEC Article 220, IEEE Std 3002.2-2018, IEEE 399-1997 (Chapter 6), and IEEE 141-1993 (Section 2.4).

NFPA 70B is not an adopted code in any jurisdiction that I've heard in the USA. With that in mind, NFPA 70, Article 220 is part of an adopted code (NEC) and only allows certain reductions in the load calculation, and those are the only ones that matter. Nobody will use 70B to ask for load studies, unless you are using it to only provide voltage drop calcs for the adopted energy code.

 

[Ekmast]

NFPA 70B is not an adopted code in any jurisdiction that I've heard in the USA. With that in mind, NFPA 70, Article 220 is part of an adopted code (NEC) and only allows certain reductions in the load calculation, and those are the only ones that matter. Nobody will use 70B to ask for load studies, unless you are using it to only provide voltage drop calcs for the adopted energy code.

NFPA 70E and 70B currently hold the same level of enforceability. Some industrial facilities we work with do care about 70E and are starting to take a look at 70B because of their safety culture goals and/or because their insurance providers require a certain level of compliance. With 70B recently being promoted to a standard, we've started seeing more inquiries from sites about it. If insurance providers begin incorporating elements of 70B into their requirements, I believe it will gradually become more prevalent across the industry.

 

[Ekmast]

As a side note to my original post, I’ve since learned that coordination studies and load-flow studies aren’t explicitly required by 70B. The wording in the standard can be a bit tricky...

NFPA 70B 6.5.1 "If a load-flow study is needed to allow maintenance to be performed, the load-flow study shall be updated when changes occur that could affect the results of the study." The If makes it optional.

NFPA 70B 6.4.1 A coordination study shall be created as necessary to support risk assessments and the intended system design goals.. The as necessary makes it optional.

I hope NFPA makes this less ambiguous in their next revision.

 

[petersonra]

As a side note to my original post, I’ve since learned that coordination studies and load-flow studies aren’t explicitly required by 70B. The wording in the standard can be a bit tricky...

NFPA 70B 6.5.1 "If a load-flow study is needed to allow maintenance to be performed, the load-flow study shall be updated when changes occur that could affect the results of the study." The If makes it optional.

NFPA 70B 6.4.1 A coordination study shall be created as necessary to support risk assessments and the intended system design goals.. The as necessary makes it optional.

I hope NFPA makes this less ambiguous in their next revision.

Coordination studies are not really optional. It just requires you to make a determination if it is necessary or not. So if you do a risk assessment and come to the conclusion that a coordination study is required, it is required. You cannot just decide not to do it to save money.

 

[Ekmast]

Coordination studies are not really optional. It just requires you to make a determination if it is necessary or not. So if you do a risk assessment and come to the conclusion that a coordination study is required, it is required. You cannot just decide not to do it to save money.

I probably should have worded it "Optional, if risk assessments have determined coordination studies to be of no benefit"