Terminology regarding "Loads"

[RonEl]

I'm not sure if this is the correct topic heading for this question .... it's more a question of Basic Education as opposed to "Continuing Education". Please be kind, this is my first time posting a question.

As an Interim Plans Examiner I'm trying to get versed in use of, and understanding, the Electric Code. I will need to eventually take, and pass, the exam. I've ordered Mike's "Understnding the NEC" but haven't yet gotten very far into it. But in the course of doing my job, I'm trying to learn and familiarize myself with the code as best I can.
I can tell that my first problem is with terminology ... I have 2 questions (1 general, 1 more specific):

1. What is the best way to familiarize myself with the vast and very specific terminolgy used throughout the NEC?

2. While reviewing plans, I've noticed that many times the engineer/architect will list "Connected Load" and "Demand Load". So, when sizing conductors (I think I've learned they are not 'wires'), what is the relationship (if any) between 'connected/demand loads' vs. 'non-continuous/continuous loads'? The definition of "Continuous Load' in Chapter 1 does not seem to help.

If there is a better category for questions like this, please advise.
Thank you for sharing the vast amount of knowledge contained by the members and moderators using this forum!

 

[al hildenbrand]

Welcome to the Forum !

Whew! For a beginner, that is an advanced question.

It's like starting to study Spanish for the first time so that one can write a poem (in spanish) to win the heart of another.

I know, after 30 years I'm still learning the lingo.
With respect to "load", load is comprised of the things that draw current when they are "on". You've read the definitions of continuous and non-continuous loads. The "connected load" or "demand load" is the result of the electrical designer's summing up the various things that are known to need electrical power, how they will use it, and will also include estimates of the electrical power needs of apparatus that is never named, along with a fudge factor or two that may be things like a safety factor or a future expansion factor.

 

[charlie b]

In the ?olden days,? the word ?load? was used to describe the flow of current, in units of amps. Nowadays, we used the word ?load? to describe the power that is supplied by a source, or the power that is drawn by an item that is connected to the source. We measure load in units of volt-amps (VA). The word ?load? is also used in connection with the current-carrying device itself.

Here is an example of the usage, in which there may be two correct answers to the same question:
Question: What is the load supplied by this circuit breaker?
Answer: It is a resistive heater.
Question: What is the load supplied by this circuit breaker?
Answer: 2500 VA

There may be a plethora of devices being supplied by any given panelboard. The same panelboard might supply lights, receptacles, heaters, motors, fans, and a host of other stuff. The complete collection of ?stuff? that can receive its power from the same panelboard is referred to as that panelboard?s ?total connected load.? But it is commonly recognized that not every single item will be running at the same time, and that not all items that are running will be running at their individual full capacity. When we calculate the service load for a facility, or calculate the load on any single feeder within a facility, the NEC gives us some leeway to count only a portion of the total connected load. We are given ?demand factors? that we can apply under certain specific circumstances. For example, if an apartment building has a total of 15 units, and each unit has its own clothes dryer, we don?t have to size the service to accommodate all 15 dryers running at the same time. We are allowed (by Table 220.54) to take the total load of 15 dryers (15 times 5,000 VA equals 75,000 VA), and multiply that value by a demand factor of 43% (a value I obtained from the instructions in the table), and assign a dryer load of only 32,250 VA. When we do all the related math for the other loads in the building, the result we get is the ?demand load? for the service or for the feeder.

Welcome to the forum. I hope this helps you understand the difference between connected load and demand load.

 

[PEDRO ESCOVILLA]

WELCOME, I'm newe here also. If you have the opprtunity to monitor apprenticeship classes in your area, that may be one way to master the NEC. It is farily easy, not simple, and for me, requires that I stay in it.If apprenticeship classes aren't available to you, there is an inspector training outfit in colorado (i don't have info at hand, they are on the net) could be the best spent $ you'll use in your career, especially if you need to test for certification at some point. Good luck, hang around, enjoy the learning process!

 

[quogueelectric]

I'm not sure if this is the correct topic heading for this question .... it's more a question of Basic Education as opposed to "Continuing Education". Please be kind, this is my first time posting a question.

As an Interim Plans Examiner I'm trying to get versed in use of, and understanding, the Electric Code. I will need to eventually take, and pass, the exam. I've ordered Mike's "Understnding the NEC" but haven't yet gotten very far into it. But in the course of doing my job, I'm trying to learn and familiarize myself with the code as best I can.
I can tell that my first problem is with terminology ... I have 2 questions (1 general, 1 more specific):

1. What is the best way to familiarize myself with the vast and very specific terminolgy used throughout the NEC?

2. While reviewing plans, I've noticed that many times the engineer/architect will list "Connected Load" and "Demand Load". So, when sizing conductors (I think I've learned they are not 'wires'), what is the relationship (if any) between 'connected/demand loads' vs. 'non-continuous/continuous loads'? The definition of "Continuous Load' in Chapter 1 does not seem to help.

If there is a better category for questions like this, please advise.
Thank you for sharing the vast amount of knowledge contained by the members and moderators using this forum!

I will give you a real life scenario of the situation you describe Lets say you have a conveyor belt in a factory which moves equipment and or Elephants from the first floor to the second floor when you turn on the belt no load amperage may be 10 amps per feeder conductor to the motor. The FLA amps may be 15 amps. When you put an Elephant on the belt to send it to the second floor it may very well go as high as 14.99 amps to move the Elephant yet when it is idling at no load it may coast at 10 amps per leg.

 

[ActionDave]

You have done work in a factory that conveyed elephants between floors?!!!

 

[John120/240]

Welcome to the Forum. Continous load are those that are on for 3 hours or more. For

example the lights in shopping center/mall would be continous. On/Off control can be

various methods, sometimes you can just use the branch circuit breaker at the panel. If you

go this route the breaker must be switch rated. Take a look at Cutler Hammer classic breaker

the one with the tan handle. You will find 15 or 20 with SWD on the handle.

 

[RonEl]

Thank you for your responses so far. I'm confident I have found a good spot to begin understanding this stuff.

From what I've discovered so far, when addresing conductor and circuit protection, the NEC uses the terminology continuous and non-continuous loads, and those terms are almost self descriptive (hurray for the NEC on this one). I guess my problem is that some plans I've looked at list their load summary as "connected" and "demand"; and that terminology seems to be what was presently confusing me when I posted my original question.

So, I think what I'm hearing from these posts: the term connected is the same as non-continuous (total load of all devices that could require power from the source) and demand is equivalent to continuous (a reduced power requirement allowed for some devices). Is my understanding to this point correct?

 

[charlie b]

Is my understanding to this point correct?

Sorry, but no. The notion of "connected versus demand" is not at all related to the notion of "continuous versus non-continuous." Take look at the definitions, in article 100, of the terms "continuous load" and "demand factor." See if that helps you understand.

 

[gndrod]

Sorry, but no. The notion of "connected versus demand" is not at all related to the notion of "continuous versus non-continuous." Take look at the definitions, in article 100, of the terms "continuous load" and "demand factor." See if that helps you understand.

I agree. Referring to a DEMAND Factor (DF) refers to load averaging calculations from Art. 220. This can be easier to comprehend when looking at [220] for determining a system service 'load' capacity where a continuous load is considered 100% and other circuit loads permit lower DF percentages when calculating an entire service panel sizing.

 

[weressl]

In the ?olden days,? the word ?load? was used to describe the flow of current, in units of amps. Nowadays, we used the word ?load? to describe the power that is supplied by a source, or the power that is drawn by an item that is connected to the source. We measure load in units of volt-amps (VA). The word ?load? is also used in connection with the current-carrying device itself.

Here is an example of the usage, in which there may be two correct answers to the same question:
Question: What is the load supplied by this circuit breaker?
Answer: It is a resistive heater.
Question: What is the load supplied by this circuit breaker?
Answer: 2500 VA

There may be a plethora of devices being supplied by any given panelboard. The same panelboard might supply lights, receptacles, heaters, motors, fans, and a host of other stuff. The complete collection of ?stuff? that can receive its power from the same panelboard is referred to as that panelboard?s ?total connected load.? But it is commonly recognized that not every single item will be running at the same time, and that not all items that are running will be running at their individual full capacity. When we calculate the service load for a facility, or calculate the load on any single feeder within a facility, the NEC gives us some leeway to count only a portion of the total connected load. We are given ?demand factors? that we can apply under certain specific circumstances. For example, if an apartment building has a total of 15 units, and each unit has its own clothes dryer, we don?t have to size the service to accommodate all 15 dryers running at the same time. We are allowed (by Table 220.54) to take the total load of 15 dryers (15 times 5,000 VA equals 75,000 VA), and multiply that value by a demand factor of 43% (a value I obtained from the instructions in the table), and assign a dryer load of only 32,250 VA. When we do all the related math for the other loads in the building, the result we get is the ?demand load? for the service or for the feeder.

Welcome to the forum. I hope this helps you understand the difference between connected load and demand load.

Perhaps it is easier to understand when you examine how an electric dryer or water heater works. When you first turn it on it is continuously on that could be anywhere between 5 to 20 minutes and then the thermostat begins to cycle the load. when you look at CB or fuse curves you can see that it takes literally hours for either of those to trip at 125% of the FLA. All this is still adequately protecting the cable when sized per the NEC.

 

[Smart $]

...

So, I think what I'm hearing from these posts: the term connected is the same as non-continuous (total load of all devices that could require power from the source) and demand is equivalent to continuous (a reduced power requirement allowed for some devices). Is my understanding to this point correct?

Ditto on the others "no".

Connected load is as if all were on at the same time. For example, 300 100W luminaires connected to the system is a 30kW connected lighting load.

Demand load is a lower value that results from NEC allowances for loads almost never being on at the same time. Continuing the example, there will likely never be any more than 100 luminaires on at any one time. The demand lighting load is 10kW.

(Note the example is for demonstrative purposes only. The NEC method does not calculate lighting [demand] load in this manner.)