Diversity Factor at Sub-main/ Final distribution Panel

[TMMB]

I have a School consists of 4 buildings, each building 3 floors GF, FF AND SF ,The demand loads for each Branch Panel as follow

BUILDING 1 , with main MDB1 and each floor with 3 Branch Panels
Demand load for LP1(GF)=62 KVA , LP2(FF)=65 KVA and LP3(SF)= 65KVA

BUILDING 2 , with main MDB2 and each floor with 3 Branch Panels
Demand load for LP1(GF)=68 KVA , LP2(FF)=80 KVA and LP3(SF)= 80KVA

BUILDING 3 , with main MDB3 and each floor with 3 Branch Panels
Demand load for LP1(GF)=55 KVA , LP2(FF)=46 KVA and LP3(SF)= 51KVA

BUILDING 4 , with main MDB4 and each floor with 3 Branch Panels
Demand load for LP1(GF)=68 KVA , LP2(FF)=62 KVA and LP3(SF)= 64KVA

My Q is : the demand load for MDB1 = 62+65+65=192 KVA is correct or can apply diversity factor to obtain the demand load for MDB1

and if the diversity factor is required , what equal? ( is it 0.9 or 0.8 or what according to NEC)


and we will calculate the demand load for MDB2, MDB3 and MDB4 BY THE SAME WAY


WHEN WE CALCULATE THE TOTAL DEMAND LOAD FOR THE WHOLE SCHOOL SHALL WE APPLY ANOTHER DIVERSITY FACTOR AT THE MAIN MDB WHICH WILL FEED MDB1, MDB2, MDB3 AND MDB4 OR NOT

PLEASE ADVICE

 

[Smart $]

In general, you have to do a load calculation for each distribution and subpanel. Demand factors based on number of units are not determined accurately by simply adding the calculated load for each subpanel. For example, kitchen equipment is based on the number of units. Say you had one kitchen unit on each of 3 subpanels. The demand factor for these units would be 100% for each subpanel feeder... but the demand factor for the MDB supply would be 90%.

 

[bob]

My Q is : the demand load for MDB1 = 62+65+65=192 KVA is correct or can apply diversity factor to obtain the demand load for MDB1

Rather than calculate, suggest you obtain a demand meter and clip it on MDB1 and measure the actual load over a period of 30 days.

and we will calculate the demand load for MDB2, MDB3 and MDB4 BY THE SAME WAY

Do the same for all panels.

For the main panel, you can get the max demand for the year from the utility and add 25 %. Or install the meter at the main. Something like this
http://www.microdaq.com/extech/power/voltage-current-data-logger.php

Or if the school is broke, maybe the utility will let you borrow one of theirs.

 

[steve66]

In most cases, you simply add the load together to get the total load. The NEC doesn't have any "blanket" demand factors that apply to all general loads.

There are specific demand factors that apply to some specific loads - The example Smart$ refered to is specific to kitchen loads.

For a school, I'd suggest you refer to the NEC. THere are some specific paragraphs that address schools based on if they have electric heat or cooling.

 

[charlie b]

Just a minor quibble with terms, with the intent of averting confusion. Along the way to obtaining a "calculated load," we are sometimes allowed to use "demand factors." The NEC does briefly use the phrase "load diversity," albeit in a totally useless way, but it does not give us any "diversity factors."

 

[Smart $]

Just a minor quibble with terms, with the intent of averting confusion. Along the way to obtaining a "calculated load," we are sometimes allowed to use "demand factors." The NEC does briefly use the phrase "load diversity," albeit in a totally useless way, but it does not give us any "diversity factors."

Just to expand on that, the word "diversity" is used 3 times in Chapters 1 though 9 of the 2008 NEC: FPN's of Table 310.15(B)(2)(a) and 440.5(B), and the footnote of Table 520.44.

 

[Smart $]

In most cases, you simply add the load together to get the total load. The NEC doesn't have any "blanket" demand factors that apply to all general loads.

There are specific demand factors that apply to some specific loads - The example Smart$ refered to is specific to kitchen loads.

For a school, I'd suggest you refer to the NEC. THere are some specific paragraphs that address schools based on if they have electric heat or cooling.

Kitchen loads are just one type of load which affects summing of sub-panel loads for the supply distribution panel.

Another is motor loads. Each panel must consider the largest motor at 125%. If each subpanel has one or more motor load, only the largest of would be 125% factored for the MDP.

The point is, summing of subpanel loads is not the prescribed method of calculating the load on the MDP supplying those subpanels, and will likely result in an error.