Forums » Electrical Engineering

Sizing proper transformer

    • 1 posts
    January 7, 2020 6:22 PM PST

    good afternoon,

    I have recently renewed my facility and upgrading electrical system. Currently this building is fed a through a pad mounted transformer 630kVA 12kV/480V transformer. Another building is also fed by this pad mounted transformer and the total load of both buildings I came out with 554kVA. The question is, if I apply 1.25%, I will get around 692kVA. do I really need to apply 1.25%? without 1.25% factor the transformer will be loaded 88% which looks fine? please advise me the right decision.

    regards

    • 32 posts
    January 7, 2020 6:27 PM PST

    Hi Clauds, 

    if 554kVA load is a demand load, I would say, 630kVA transformer is fine and you do not need to apply 1.25% factor.

    • 67 posts
    January 8, 2020 12:55 PM PST
    Claud Sheldon said:

    good afternoon,

    I have recently renewed my facility and upgrading electrical system. Currently this building is fed a through a pad mounted transformer 630kVA 12kV/480V transformer. Another building is also fed by this pad mounted transformer and the total load of both buildings I came out with 554kVA. The question is, if I apply 1.25%, I will get around 692kVA. do I really need to apply 1.25%? without 1.25% factor the transformer will be loaded 88% which looks fine? please advise me the right decision.

    regards

     

    Claud -

    How did you come up with 554kVA?

    Is this a calculation based on an NEC article 220?

    Some other method?

    • 67 posts
    January 8, 2020 1:29 PM PST
    Edward Mccreary said:

    Hi Clauds, 

    if 554kVA load is a demand load, I would say, 630kVA transformer is fine and you do not need to apply 1.25% factor.

    Edward -

    I'm feeling kind of dumb (seriously).  Do you have a definition of "demand load".  I am seeing the term come up occasionally in colloquial conversation.  The term shows up in the NEC as a demand load for dryers and ranges (stoves) and that it.  So thislooks like a calculated term.

    It does show up in IEEE 100:

    demand (1) (electric power systems in commercial buildings) The electrical load at the receiving
    terminals averaged over a specified interval of time. Demand is expressed in kilowatts,
    kilovolt amperes,  kilovars,  amperes,  or  other  suitable  units. The interval of time is
    generally 15 minutes, 30 minutes, or 60 minutes. Note: If there are two 50 hp motors (which drive
    45 hp loads) connected to the electric power system, but only one load is operating at any time,
    the demand load is only 45 hp but the connected load is 100 hp. Synonym: demand load.

    (IA/PSE)  241-1990r, 141-1993r

    (2) The average value of power or a related quantity over a specified interval of time. Note: Demand
    is expressed in kilowatts, kilovolt-amperes, kilovars, or other suitable units.

    (ELM)  C12.1-1988

    (3) The rate at which electric energy is being used.

    (PE/PSE)  858-1993w

    (4) The rate of consumption, e.g., power, volume/hour.

    (AMR/SCC31)  1377-1997

     

    From IEEE, "Demand Load" is the measured load, averaged over a suitable time period.

    In your context, is "demand load" measured, or calculated?  If calculated, what is the calculation? (An example would be an NEC art 220 calculation)

     

     

    • 32 posts
    January 9, 2020 2:36 AM PST

    Hi Carl,

    for me it is clear. Please do not mind the following explanation, I know, you know all these factors, just for clarification.

    Demand Load: = Maximum load / connected load

    Inside building for distribution panel: It does not matter building type, commercial or residential. But demand factor applies for all equipment, if they are sockets, dryers, mechanical loads, lighting system and etc.

    If there are two buildings supplied from a transformer, there will be another demand factor which is called diversity factor. Here in UK, it is only called diversity factor. 

    The purpose of a diversity factor / demand factor is to size a proper transformer. If OP has already applied these factors in his calculation, and it is final demand load, I can say, it is not essential to apply 1.25% for transformer. You can load transformer even 100%

    best regards

     


    This post was edited by Edward Mccreary at January 9, 2020 2:39 AM PST
    • 67 posts
    January 10, 2020 7:48 AM PST

    Edward -

    My question is not about "diversity" or "demand factor".  The question is about the term "Demand Load".

    In specific, Is this "Demand Load" measured, as in using a clamp-on ammeter?

    Or, is "Demand Load" calculated?  An article 220 load calc would be an example of calculated.

    Hi Clauds, 

    if 554kVA load is a demand load, I would say, 630kVA transformer is fine and you do not need to apply 1.25% factor.

    I can't tell from the context which it is.

     

     

     

    • 32 posts
    January 10, 2020 12:44 PM PST
    Carl Coulter said:

    Edward -

    My question is not about "diversity" or "demand factor".  The question is about the term "Demand Load".

    In specific, Is this "Demand Load" measured, as in using a clamp-on ammeter?

    Or, is "Demand Load" calculated?  An article 220 load calc would be an example of calculated.

    Hi Clauds, 

    if 554kVA load is a demand load, I would say, 630kVA transformer is fine and you do not need to apply 1.25% factor.

    I can't tell from the context which it is.

     

     

     

    what do you mean by "demand load"?I think you are mixing between the demand load vs load profile. In small residential building you get the the demand load after applying diversity or demand factor. That is your demand load.

    load profile defines how an electricity customer uses its electricity over time,

    Using load profile, you can find the peak load over time, average load over time, base load over time. this is used for big power plant to find out which type of generation, when should be turned on.

    I asked OP, if he has already applied demand factor and this is the demand load, he does not need to apply 1.25%

    using a clamp-on ammeter you will get power consumption in a specific time, but in order you find the demand load, you need to measure in many different time intervals and finally you will get the average of that. That will be close to your demand load.

     

    using a clamp-on ammeter

     


    This post was edited by Edward Mccreary at January 10, 2020 12:45 PM PST
    • 27 posts
    January 10, 2020 1:23 PM PST

    The distinction that needs to be made is the difference between connected load and demand load.  The demand load is the maximum load that would be running at any one instance in time.  For example, you could have both an air conditioner and an electric furnace connected but you shouldn't be running both at the same time.  The Canadian Electrical Code (and I imagine the NEC as well) prescribes how to calculate the demand load for various types of occupancy.  Of course, the code books don't have every type of occupancy listed, and that is when you need an engineer to use his experience to calculate the final demand. The local utility usually has some rules about flicker that needs to be considered when sizing the transformer that they connect to.  The flicker needs to be calculated especially when there are large inductive loads that could cause a "brown out" when those inductive loads are started.

    • 154 posts
    January 10, 2020 1:49 PM PST

     

    Derek Nelson said:

    The distinction that needs to be made is the difference between connected load and demand load.  The demand load is the maximum load that would be running at any one instance in time.  For example, you could have both an air conditioner and an electric furnace connected but you shouldn't be running both at the same time. 

     

    In the NEC you would only calculate the load of the greater of the ac vs the electric heat which generally means the electric heat would be used.  In warmer climate we use heat pumps and in that case the units may be used at the same time so you have to figure it that way.

    The question the op is asking, as far as I can tell is whether the transformer needs to sized at 125% of the load. As stated earlier a transformer can be sized at 100% but it may not be the best design method.  

    Total Connected Load (TCL) is the mechanical and electrical load (in kW) that will be connected (or to consumed) for that particular area.The Maximum Demand (MD) is the total kW that actually contributes the total power used in one time after applying the diversity factor based on the Total Connected Load calculated

    Demand load is the calculated load.  There is a way to monitor a system but the monitor must be on during the time of year when most loads would be used and I believe it must be monitored for 30 days or so.

    So, IMO, demand load is calculated load and the TCL is all the loads available

     

     

     

    • 67 posts
    January 12, 2020 8:59 AM PST

    Thank you all for your replies.  Interestingly I see three somewhat different answers.  My mission is to understand how one derives the number associated with the term, "Demand Factor"

    Whoops sloppy me.  That should have been "Demand Load"

    edit 1/13/2020 - cc

    I will list out what I translated from the four replies - you guys can point out where I am goofed up.

    Edward:

    Demand Load: = Maximum load / connected load

    I am hoping this is a mis-type - because this would make "Demand Load" a percentage.

     

    Edward:

    In small residential building you get the the demand load after applying diversity or demand factor. That is your demand load.

    Okay, this is similar to an NEC 220, Branch-Circuit, Feeder, and Service Load Calculation.  This calculation applies Diversity, Demand Factors, Non-Coincident loads (Example: AC and Heat), and Continuous, Non-continuous Loads

    And I think this is similar to what Denis said.

    Dereck: 

    Of course, the code books don't have every type of occupancy listed, and that is when you need an engineer to use his experience to calculate the final demand.

    The NEC details just how crappy a job one can do and still have it not illegal.  So, yes, sometimes (often even) the customer specs require that the installation be upgraded to meet reliability and power quality standards.

     

    Dennis:

    So, IMO, demand load is calculated load and the TCL is all the loads available

    And I'm guessing the "calculated load" is an article 220 load calc?  That would make sense and consistent with Edward's and Derek's posts.

    Question for Edward and Derek:

    Edward:

    Is the term "Demand Load" used in the UK electrical code?  If so, is it defined?

     

    Derek:

    Is the term "Demand Load" used in the CEC?  If so, is it defined?

     

    It is okay if used and not defined - Especially if used consistently to where the context is clear.

     


    This post was edited by Carl Coulter at January 13, 2020 12:41 PM PST
    • 67 posts
    January 12, 2020 9:20 AM PST

    Guys:

    I am truly just trying to understand.  This is an international participation forum.  The NEC does not reign.  Terms get used that are common outside of North America.  Sometimes the terms are colloquial, sometimes defined by code.  And I don't always have a clear definition.  I'm guessing this is also true when some NEC/North American terms get used.  However, the Science/Physics is uniform across the earth.  And that unites us.

    All I am saying is, Nothing wrong with being prepared to define a term when asked.  I try to be - especially when it is a North American specific.

     

    • 70 posts
    January 12, 2020 2:55 PM PST

    Concept is same worldwide.

     

    BS7671 Part 3 Chapter 31 Purpose, Supplies and Structure

    311 Maximum Demand and Diversity

    Regulation 311.1 …….. In determining the maximum demand of an installation diversity may be taken into account.

    What this means is that not everything will be switched on at the same time and we can make allowances for that.

    Domestic installations:

    Applying diversity to the maximum load (Amps)

    Lighting: 66%
    Heating: 100% of total up to 10A and 50% of all above
    Cooking: 10A + 30% of cooker full load above 10A and + 5A if there is a cooker socket
    Water Heaters: (Instant – eg. showers) 100% of largest + 100% second largest + 25% of remaining
    Water Heaters: (Immersion cylinder) 100%
    Floor: 100%
    Storage Heaters: 100%
    Sockets: 100% of largest appliance + 40% at every other point

    more for BS7671 .....

    http://www.napitonline.com/downloads/CP%204%2007%20P%2010-11%2016th%20Diversity.pdf

    • 32 posts
    January 14, 2020 8:54 AM PST
    Carl Coulter said:

     

    Question for Edward and Derek:

    Edward:

    Is the term "Demand Load" used in the UK electrical code?  If so, is it defined?

     

     

     

    I think Reynaldo Danilo has already replied. Is it clear now for you Carl?

    • 27 posts
    January 14, 2020 9:28 AM PST

    FYI all, the CEC does not have a formal definition of "demand load".

    • 67 posts
    January 14, 2020 1:39 PM PST
    Edward Mccreary said:

    I think Reynaldo Danilo has already replied. Is it clear now for you Carl?

     

    Thanks Edward

    I am fine

     

    carl

    • 67 posts
    January 14, 2020 1:50 PM PST

     

    Derek Nelson

    FYI all, the CEC does not have a formal definition of "demand load".

    I suspected that was the case and the term was being used colloquially.  And sometimes the definition was not always the same.

    Which is okay.  I just did not know what it meant.  However, I m now understanding it appears to be a calculation similar to NEC 220.  Which takes into account:

    Diversity

    Demand Factors (which is different from "Demand Load")

    continuous loads

    non-continuous loads

    non-coincident loads

    Does that sound about right?

    • 67 posts
    January 14, 2020 2:08 PM PST

     

    Reynaldo Danilo said:

    Concept is same worldwide.

    BS7671 Part 3 Chapter 31 Purpose, Supplies and Structure

    311 Maximum Demand and Diversity

    Regulation 311.1 …….. In determining the maximum demand of an installation diversity may be taken into account.

    What this means is that not everything will be switched on at the same time and we can make allowances for that.

    Domestic installations:

    Applying diversity to the maximum load (Amps)

    Lighting: 66%
    Heating: 100% of total up to 10A and 50% of all above
    Cooking: 10A + 30% of cooker full load above 10A and + 5A if there is a cooker socket
    Water Heaters: (Instant – eg. showers) 100% of largest + 100% second largest + 25% of remaining
    Water Heaters: (Immersion cylinder) 100%
    Floor: 100%
    Storage Heaters: 100%
    Sockets: 100% of largest appliance + 40% at every other point

    more for BS7671 .....

    http://www.napitonline.com/downloads/CP%204%2007%20P%2010-11%2016th%20Diversity.pdf

    Reynaldo - Thank you for your response.

    I believe I understand the concepts of Diversity, continuous/non-continuous, non-coincident.  However, at 554KW, this likely is not a Domestic Installation.

    How would you calculate the following industrial loads to arrive at the "Demand Load"?

    3 manufacturing units each - 100KW continuous process heating

    3 manufacturing units each - 100KW motor, continuous run, full load

    100KW process area lighting, always on, 24/7

    100kw Environmental heat

    100Kw Environmental Air Conditioning

    consider the previous two, heat and air conditioning, to be non-coincident loads

    After going through all the previous posts, my answer would be:

    3 X 100KW continuous process heat = 1.25 X 100Kw + 100kw + 100kw = 325 KW

    3 X 100KW motor, continuous run, full load = 1.25 X 100KW + 100kw + 100kw = 325 KW

    100KW process area lighting, always on, 24/7 = 1.25 X 100KW = 125KW

    100kw Environmental heat or

    100Kw Environmental Air Conditioning (non-coincident) =100KW

    If I understand the Demand Load = 875KW

    Does that sound about right?


    This post was edited by Carl Coulter at January 14, 2020 2:10 PM PST