Sizing feeder by actual load instead of calculated

[Tbatson44]

I have a customer asking whether actual load can be used to size feeds. I am almost solely used to using FLC and calculated loads when determining feeds but I vaguely remember something about NEC allowing it if we have some mechanical interlock preventing loads from being run at the same time. Is anyone familiar with that or maybe some other options? They are saying the plant in Canada allows them to take into consideration duty cycle and actual loading.

Thank you for your help and informaton.

 

[drktmplr12]

code requires we size multi motor feeders according to connected load, not demand load. engineers still routinely apply blanket 80% demand factors on anything and everything that is a motor load at their own risk. this is one way to account for load diversity, although you would be hard pressed to prove where in the code book it is permitted (hint: it isn't). usually, the ratio of demand to connected load falls somewhere around 60%, that is for every 100 kVA of connected load, there is 60 kVA of demand load. This varies widely based on the process.

If you ever need to use actual load to size a feeder, refer to 220.87. Encourage you to review that paragraph and the exception.

it allows using maximum demand data (1-year minimum) from the power utility

or

if that data is not available, the maximum demand continuously recorded for a 30-day period using a recording ammeter or power meter connected to the highest loaded phase.

plus 25%

it's the plus 25% part that makes doing this for large feeders impractical.

 

[drktmplr12]

the interlock bit you are looking for is in 430.24, exception 3

 

[Tbatson44]

the interlock bit you are looking for is in 430.24, exception 3

I did see the 30-day rule as well. The interlocking was what I was thinking. I will definitely review those to see what we can do. Thank you for your help.