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Discussion Starter · #1 ·
Dear Sirs,

I have a question regarding the calculation of the service conductors that are connected to a raceway where 2 dwellings are feed.

which ones should be the design criteria to use?

Let's say that the feeder of each dwelling has a 22.5 KVA load demand, which factor must i apply to the load in order to calculate the main service cables?

Should I sum both loads and calculate the main load for the main conductors, this without applying any factor?

Or should i apply a factor to the total of load?
I know that exists a table for more than 3 dwellings, but i am not sure how is in this particular case, 2 dwellings.

An example please,

Thanks in advance for any help,
 

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The table is for 3 or more so if you only have 2 units then I assume it must be done at 100%. I am assuming the 22.5kva is calculated according to art. 220
 

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Discussion Starter · #3 ·
The table is for 3 or more so if you only have 2 units then I assume it must be done at 100%. I am assuming the 22.5kva is calculated according to art. 220
Hello,


Thanks for your reply,


I have found in a web site the following information:


A word about two-family dwellings. The feeder for a two-family dwelling unit is calculated using the standard method in Part III of Art. 220. When that calculated load exceeds the calculation for three identical units using the optional method of 220.84, the lesser of the two calculations is permitted to be used [220.85].


But i am not clear about this data that have been extracted from the NEC, i was trying to find an example about it, but i could not.

Maybe somebody can give me a better explanation with an example, i would appreciate it so much,

Thanks in advance for your responses,
 

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I am not sure what data you are looking for. Did you calculate the load for the dwelling using Part III. If so then multiply that by 3 and use the 45% demand factor. If that is lower than the sum of just 2 units then you can use that.

22.5kva + 22.5kva= 45000/240=187.5

22.5kva * 3= 67500/240= 281.25 *.45= 126.56

As I see this you can use the 126.56 or a 150 amp service. Around here it would cost more for a 150 amp service than a 200 amp service simply because no one stocks 150 amp panels. However it may be different where you are
 

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Discussion Starter · #5 ·
Thanks

Hi Dennis,

Thanks for your reply, I was a little bit confused, but your answer has been very clear.

And yeah in CR, would be better for cost to get a 150 amperes service instead of one of 200 amperes

Pura Vida!

Costa Rica, third city illuminated by electricity around the world!, after New York and Paris,


I am not sure what data you are looking for. Did you calculate the load for the dwelling using Part III. If so then multiply that by 3 and use the 45% demand factor. If that is lower than the sum of just 2 units then you can use that.

22.5kva + 22.5kva= 45000/240=187.5

22.5kva * 3= 67500/240= 281.25 *.45= 126.56

As I see this you can use the 126.56 or a 150 amp service. Around here it would cost more for a 150 amp service than a 200 amp service simply because no one stocks 150 amp panels. However it may be different where you are
 
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