Service capacity calculation

[msf]

Article 220.87 requires when calculating feeder or service loads that serve existing loads the existing load be increased by 125%.

Article 215.2 requires that the minimum feeder conductor size shall not be less than 125% of the continuous plus 100% of the non-continuous.

The exception to 215.2 suggested that if the overcurrent device is rated for 100% of it's rating that the minimum feeder conductor size shall not be less than 100% of the continuous plus 100% of the non-continuous.

Question: If the overcurrent device is rated for 100% of it's rating, are you still required to increase the load for the existing loads by 125%?

 

[steve66]

Re: Service capacity calculation

Question: If the overcurrent device is rated for 100% of it's rating, are you still required to increase the load for the existing loads by 125%?

I think so. There is no exception to 220.87 that allows you to forget the extra 25%. Althought it may be tempting, you can't take the exception to 215.2 and apply it to 220.87.

Steve

 

[charlie b]

Re: Service capacity calculation

There are many places in the code that require adding 25% to something. They each have their own reason. In this case, you are taking measurements, and looking for the peak load during a time interval. I believe the 25% is added here in an effort to account for the possibility that the actual load might have been higher, if you had picked a different time period to take your measurements.

 

[rbalex]

Re: Service capacity calculation

The additional 25% is, as Charlie B states, a safety factor. (And a small one at that) It is always required.

I?ve always puzzled at the willingness we ?Electrical?s seem to show toward as marginal design as we can get. We don?t demand nearly the safety factors of 5, 10 or even 20 that are common in Structural or Mechanical standards. And there isn?t nearly as much safety factor in the Product Standards as we would like to believe. Simply look at the actual thermal/mechanical damage curves of conductors or transformers.

While I know the answer, I don?t think its clear from 220.87(2) the ?new? load does NOT require any ?continuous/non-continuous? adjustments either as long as the maximum ?existing? load from 220.87(1) or it's Exception is computed at 125%.