Service Calculations

[tuna]

I have a multi family building with 13 residential units, common laundry room and house. The engineer has calculated the service size. After taking the diversities allowed, he has come up with a total diversified load of 691 amps @ 120/240volts. The main circuit breaker size is 700amps. I have questioned him about the main size and he insists that the size is correct because the load is not considered a continuous load. I have looked through the code and cannot find and verbage as to what this diversified load is.

Anybody have any ideas.

Tuna

 

[roger]

Re: Service Calculations

Diversity in this sense is the allowed reductions per article 220, and his figures are probably correct.

To just open the book to article 220 and try to understand it is not an easy task.

Roger

 

[charlie b]

Re: Service Calculations

You are looking for a word that does not appear anywhere in the NEC. I did a word search for ?diversified,? and found no occurrences. I then looked for ?diversity.? I found 6 occurrences, but none were in the context of your question. You are caught in the middle of a miscommunication that happens whenever conversational terms are mixed with precise code language.

I think that the engineer should have said that all applicable ?demand factors? were taken into account, and should have described the 691 amps as the ?calculated service load.? If the calculation was performed correctly (i.e., per section 220), then the answer might well be acceptable. I might have recommended a larger main (and upsizing the boxes and wires accordingly), just to allow some room for load growth. But that would be a design choice, not a code issue.

Having said that, now let me add that the following statement is utter nonsense, and leads me to question the engineer?s understanding of the calculation process:

. . . he insists that the size is correct because the load is not considered a continuous load.

At any residential facility, some of the loads are going to be considered ?continuous.? The 125% factor will show up in one or more places in the calculation process. It is true that you don?t take the final answer, and then decide to multiply the 691 amps by 125%, depending on whether the overall load is continuous. But that is so obvious that I can?t believe that that was the engineer?s intended meaning.

 

[tuna]

Re: Service Calculations

Charlie B.

I agree with you 100%. The other 7 building services have similiar design. For instance, one building has a diversified load of 186 amps and the engineer has specified a 200amp service. I believe we are headed for some problems. I also explained to the engineer in my 25 years as a licensed electrical/contractor that I have never encountered such service sizes.

Its his professional stamp on the drawings and his design. I cannot question his professional status further.

Thanks: TUNA

 

[charlie b]

Re: Service Calculations

Originally posted by tuna:Its his professional stamp on the drawings and his design. I cannot question his professional status further.

You are not questioning his professional status. You are looking after the client?s interests. His Number One duty is to safeguard the health and safety of the public and the occupants of the building. His Number Two duty is to look after the interests of the client. If it were my PE stamp on the drawings, I would want you to ask me such questions.

 

[lady sparks lover]

Re: Service Calculations

It's not just diversity, it's also demand factors. For example, the applicances can range from 100%-35% per aritcle 220. So there might be several reason. Plus a previous utility bill might suggest that the demand is only at 60%.

I've had this discussion with contractors in the past, and personally I think that the services are oversized in most cases. If they added the 125% to get 700, but also consider the demand factor, then I say that might be the reason. The connected load is almost never the demand load, IMHO, from what I've seen even in the worst case.