Example D4(b)

[antenna2001]

Hi,
I am trying to solve example D4(b) in the 2008 NEC as preparation for the master test.

220.82(C)(5) allows a 40% factor applied to the 4 electric space heating units.

The factor is applied to the main feeder but not to the individual feeders going to each unit. It seems to me that the factor should apply to the individual feeders too.

Does anyone know why is the factor not used in the individual feeder calculation?

Thank you.

 

[Hameedulla-Ekhlas]

Hi,
I am trying to solve example D4(b) in the 2008 NEC as preparation for the master test.

220.82(C)(5) allows a 40% factor applied to the 4 electric space heating units.

The factor is applied to the main feeder but not to the individual feeders going to each unit. It seems to me that the factor should apply to the individual feeders too.

Does anyone know why is the factor not used in the individual feeder calculation?

Thank you.

I have to see the riser than tell you what to apply but you should consider the following factors usage.

We have two factors demand factors and diversity factors.
Demand factor applies for inside building and building feeder.
Diversity factor applies for distribution for more than different building sizing the main distribution feeder which is always greater than 1.

 

[antenna2001]

Thanks for the reply Hameedulla-Eckhlas,
Now you have me thinking. How do you know when a factor is of the demand or of the diversity type? I had not heard these distinctions before, although I see that most of the factors (if not all) in the NEC could be classified this way.

In the particular case I am looking at, 220.82(C)(6), the statement seems to be general in nature, applying to all feeders (seems to me), and this is why I don't see why it is only applied to the Main.

 

[kingpb]

There is a difference between demand and diversity. Basically, think of demand as what is the actual running load. Motors are the easiest to think of. Say you have a 5 hp motor 208V 3ph motor, the rated FLC is 16.7A. But the pump connected to the motor will never draw more than 87% of the motor power, and therefore the running load is 0.87 x 5Hp. So, when running it will never draw more than 14.6A. That is the demand (running) load.

Diversity is more easily determined, like your heating and cooling. It is either heating mode, or cooling mode. Say the heating load is 30A and the cooling mode is 22A. The total connected load is 55A, but the diversity would be to use the 30A since they can't run at the same time. Further, you determine that the rated load of 30A when operating is never going to be greater than 90% of the rated. So, the demand (running) is 27A.

For the above, diversity would be the motor FLC and rated heater load = 16.7 + 30 = 46.7A
But the demand (running) is 14.6 + 27 = 41.6A

A simple way to think of it is: If it's on or off, that's diversity, if it's running at a reduced output, that's demand.

As far as 220.82, you need to review the definition of dwelling unit, as it applies to each of the 40 units over 100A, not the overall service to the building.

 

[charlie b]

I think that what both Ekhlas and kingpb said is right, but off target. I will come back to this issue in my second post.

But first, let me answer the original question. I am not yet at work, and I don’t have an NEC with me, so I do not know how that particular example is set up. But I infer that it has four dwelling units, each with its own feeder, and there is an overall service that provides power to all of four. You can apply a demand factor to the overall service, based on the total number of items that are powered via that service. That would include all four space heaters. But when sizing a specific dwelling unit’s feeder, you have to note that each one will only power one of the space heaters. So you can’t use a demand factor that counts all four, when sizing a wire that powers only one.

 

[One-eyed Jack]

Hi,
I am trying to solve example D4(b) in the 2008 NEC as preparation for the master test.

220.82(C)(5) allows a 40% factor applied to the 4 electric space heating units.

The factor is applied to the main feeder but not to the individual feeders going to each unit. It seems to me that the factor should apply to the individual feeders too.

Does anyone know why is the factor not used in the individual feeder calculation?

Thank you.

When you have four or more electric heaters on individual thermostats it is likely that one or more will be off at any given point in time. You can apply that diversity to the service feeder. The branch circuit has no such diversity. When it is on it is at 100% so the branch circuit must be sized accordingly.

 

[charlie b]

Now back to my comment regarding the posts by Ekhlas and kingpb. What each of you said was correct, but in neither case was it related to the original question.

kingpb was correct in saying that there is a difference between demand and diversity. But there is also a difference between “demand,” a term that kingpb described well, and “demand factor,” which was the subject of the original question. A given motor will almost always operate at a power level that is lower than its full rating. But when we do a load calculation, we count it using its full load current, as given in the tables in article 430. We don’t take into account that it might be running at a level lower than that, and this fact is not related to any demand factor. The NEC recognizes that not everything that is connected will be operating at the same time, and that is the basis of demand factors. In the particular example cited in this thread, there is a demand factor that allows us to account for less than 100% of the total connected heating load.

There is also a difference between “diversity,” a term that kingpb also described well, and “diversity factor,” a phrase that Eklhas correctly placed within the realm of larger distribution systems. The one thing that most of us need to know about “diversity factor” is that we should disregard it disdainfully. It never comes into an article 220 load calculation. I don’t think it even appears in the NEC. There is an occasional use of the phrase “load diversity,” but that is not the same thing. Here again, I am not at work, and my textbook is at my desk, so I am going from memory by saying “diversity factor” is a ratio obtained by dividing the maximum load seen by an overall system by the average load over a period of time. That makes it a value that must necessarily be 1.0 or higher, as Ekhlas pointed out. It is used in the realm of distribution system planning, not in the everyday design of dwelling units.

 

[kingpb]

I will agree that for dwelling units these terms probably do not apply. The NEC does a reasonable job at providing guidance for determining the minimum size service for resi- and commerical loads.

I don't use the NEC for the work I do, it is all engineered. In that case, we have motors ranging from 25,000Hp down to fractional Hp. We have 2 x 50%, 3 x 33%, 3 x 50%, 4 x 33%, etc. Needless to say we look very closely at load diversity, and running versus connected.

 

[antenna2001]

Thanks!

Thanks!

Well, I am glad I posted this question. Reading through the replies has been a great learning experience and I found that I had several misconceptions so I will adjust my thinking accordingly. To better understand the subject, can anyone recommend a text where I can study these concepts in depth?
Thank you all.

 

[Hameedulla-Ekhlas]

Well, I am glad I posted this question. Reading through the replies has been a great learning experience and I found that I had several misconceptions so I will adjust my thinking accordingly. To better understand the subject, can anyone recommend a text where I can study these concepts in depth?
Thank you all.

Ok click the below books

power_distribution_planning_referen

 

[LarryFine]

The factor is applied to the main feeder but not to the individual feeders going to each unit. It seems to me that the factor should apply to the individual feeders too.

Very basic example: Thanksgiving.

If someone is cooking T-giving dinner at home, they may well be using the oven and several burners, so the range circuit will be well-loaded, and its load will be on the individual unit's feeder.

However, the odds are that several neighbors will be having dinner with relatives, so it's unlikely that all of the units' feeders will be carrying their range load, nor will the building's service.

Since HVAC loads, as well as most other heavy-appliance loads, are cyclic, the odds are against them all running simultaneously, so we can consider that. You're certainly allowed to go bigger.

 

[Hameedulla-Ekhlas]

Now back to my comment regarding the posts by Ekhlas and kingpb. What each of you said was correct, but in neither case was it related to the original question.
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charlie for one building contains four heaters you are completely right. But the question is not clear whether one building contains four heaters or each four buildings contain one heater. That is why I asked him the riser before giving the explaination.

Your explain is very good charlie, if there would be four building each contain one heater and all of them are served from one main feeder than what would have been the diversity factory there?

 

[charlie b]

. . . if there would be four building each contain one heater and all of them are served from one main feeder than what would have been the diversity factory there?

When calculating the main feeder, the feeder that carries current to all four dwelling units, it may be possible to count only 40% of the total heater load. When calculating the feeder to a single dwelling unit, we count 100% of the heating load in that unit.

Two things worth noting:

1. You can only use the 40% factor if you are using the optional method, and if the four heaters are controlled separately. I infer that we are talking about one heater per household, so they would certainly be controlled separately. See 220.82(C)(5).
2. It is called a "demand factor." The phrase "diversity factor" does not come into play, when performing an article 220 load calculation.

 

[Hameedulla-Ekhlas]

When calculating the main feeder, the feeder that carries current to all four dwelling units, it may be possible to count only 40% of the total heater load. When calculating the feeder to a single dwelling unit, we count 100% of the heating load in that unit.

Two things worth noting:

1. You can only use the 40% factor if you are using the optional method, and if the four heaters are controlled separately. I infer that we are talking about one heater per household, so they would certainly be controlled separately. See 220.82(C)(5).
2. It is called a "demand factor." The phrase "diversity factor" does not come into play, when performing an article 220 load calculation.

Ok good thanks for explaination one more thing. If I use 2.5 diversity factor would it be any problem.

 

[charlie b]

If I use 2.5 diversity factor would it be any problem.

Use it for what? If you are designing a transmission system feeding half a continent, and if you are performing an "optimal power flow" calculation, then perhaps "diversity factor" might come into play. But it has no place in a discussion of NEC service load calculations.

 

[charlie b]

Now that I have my old textbook available, let me correct an earlier statement:

Here again, I am not at work, and my textbook is at my desk, so I am going from memory by saying ?diversity factor? is a ratio obtained by dividing the maximum load seen by an overall system by the average load over a period of time.

I remembered it wrong. Diversity factor is defined as "the ratio of the sum of the individual maximum demands of the various subsystems of a system to the maximum demand of the whole system."

Here is the process:
Consider a set of 10 buildings. Monitor their power use. Take the highest demand of each building, regardless of what time any given building reaches its peak. Add those numbers together, and call that "X." Now take the highest demand on the service conductors that provide power to the entire set of buildings as a whole. Call that "Y." We will observe that not all buildings will reach their individual peak values at the same time, so X will be a larger number than Y. Diversity factor is defined as X/Y, and therefore it will be larger than 1.0.

Question:
Does this process sound like it has anything to do with a service load calculation? I don't think so either.

Conclusion:
We should forget that we ever heard the phrase "diversity factor."

 

[Hameedulla-Ekhlas]

[FONT=Times. Diversity factor is defined as X/Y, and therefore it will be larger than 1.0.[/SIZE][/FONT]

Question:
Does this process sound like it has anything to do with a service load calculation? I don't think so either.

Conclusion:
We should forget that we ever heard the phrase "diversity factor."

Now suppose you have two buildings one is Barracks and the other is Admin building. You know better when is the peak load at Barrack and when is the peak load for Admin Building. Now if you want to design main feeder which supplies both panel boards. Now can we not apply diversity factor for this? Eventhough it is not mentioned in NEC at all for such a case.

 

[charlie b]

It's a good question, and it shows that you understand the concepts. My answer is that you cannot. My reason is simple: I am constrained to follow the rules in the NEC, and I don't see anything in the NEC that would let me.

But let's take a step back and look at what we are trying to do. The intent is to provide enough power to any given buiding to allow safe operation of the equipment in that building. The NEC is only concerned with the minimums required for safety; it does not care of the owner wants more. We calculate the load using the processes described in article 220, and that is the basis for the minimum power we should provide to the building.

If the engineer, or the owner, or another interested party wants to use a smaller value than the calculated load, or wants to use smaller wires to feed the building, or wants to use a smaller transformer outside the building, and is willing to take responsibility for the decision, then they are operating outside the NEC. An electric utility, as one example, is not constrained to follow the NEC. They will typically provide a power source that is 40-50% of what the calculated load said was needed. That is based on their experience with similar buildings over decades of operation. They know the NEC values are conservative, that the real load will never be as high as the calculated load, and they can save money by providing smaller transformers. In our area of the world, the laws give them the authority to make that decision.

But if you, as an engineer, wanted to reduce the calculated load by a technique that is not permitted by the NEC, then you would have to deal with the local authorities, to see if they will allow you to take that action. For my part, I would not want to go there.

 

[Hameedulla-Ekhlas]

It's a good question, and it shows that you understand the concepts. My answer is that you cannot. My reason is simple: I am constrained to follow the rules in the NEC, and I don't see anything in the NEC that would let me.

But let's take a step back and look at what we are trying to do. The intent is to provide enough power to any given buiding to allow safe operation of the equipment in that building. The NEC is only concerned with the minimums required for safety; it does not care of the owner wants more. We calculate the load using the processes described in article 220, and that is the basis for the minimum power we should provide to the building.

If the engineer, or the owner, or another interested party wants to use a smaller value than the calculated load, or wants to use smaller wires to feed the building, or wants to use a smaller transformer outside the building, and is willing to take responsibility for the decision, then they are operating outside the NEC. An electric utility, as one example, is not constrained to follow the NEC. They will typically provide a power source that is 40-50% of what the calculated load said was needed. That is based on their experience with similar buildings over decades of operation. They know the NEC values are conservative, that the real load will never be as high as the calculated load, and they can save money by providing smaller transformers. In our area of the world, the laws give them the authority to make that decision.

But if you, as an engineer, wanted to reduce the calculated load by a technique that is not permitted by the NEC, then you would have to deal with the local authorities, to see if they will allow you to take that action. For my part, I would not want to go there.

Yes, charli thanks for information. I understand both, we all know that NEC is excellent book for safety. Indeed NEC just for residential buildings in that case if we have more than 100 buildings, the diversity factor will be 1 and wont affect.

But when we are talking about different buildings not typic than diversity factor plays great rule in power generation and feeder and transformer sizing.

Demand factor is only for inside building and equipment. If we have distribution for different building than in this case diversity factor plays a crucial rule which one of the most important part of enginering and design.

I am wondering why NEC book has not considered that but the reason could be only one reason. NEC is only talking about residential buildings in that case diversity is 1.

 

[charlie b]

I am wondering why NEC book has not considered that but the reason could be only one reason. NEC is only talking about residential buildings in that case diversity is 1.

No, the NEC also addresses non-dwelling units. It applies to hospitals and laboratories, factories, office buildings, retail outlets, and any other type of building you can name.