Demand factor for process equipment

[Jennifer K.]

Hello - I am curious to see if anyone here has used NEC 430.26 to apply demand factor(s) to motors such as those that may be used in a process/manufacturing type facility? The code indicates "Where reduced heating of the conductors results from motors operating on duty-cycle, intermittently, or from all motors not operating at one time, the authority having jurisdiction may grant permission for feeder conductors to have an ampacity less than specified in 430.24...." In the handbook version there is a highlighted informational text box below this section that further indicates that AHJ may grant permission to allow a demand factor of less than 100%. Motors not running at the same time is completely clear to me - then the non-coincidental load section would apply and the smaller load could be dropped when sizing the feed however I am trying to figure out if anyone utilizes this section to apply other factors (for example 75%, 80%, etc.) to a motor load? It seems as though one would need detailed knowledge of the motor(s) in consideration to apply these demand factors and even in that case it would still be a complete estimation by the engineer (a cut sheet for the motor likely would not have a demand factor on it). Thanks in advance for any responses!

 

[petersonra]

what would be the point? the code allows the ahj to grant special permission for anything the ahj is willing to grant special permission for.

say you are able to save a few bucks on wire. the cost of getting the variance will likely exceed whatever tiny savings there is, and the chances of getting it is remote.

in any case, it would not be about the motor itself but about the load.

 

[gadfly56]

what would be the point? the code allows the ahj to grant special permission for anything the ahj is willing to grant special permission for.

say you are able to save a few bucks on wire. the cost of getting the variance will likely exceed whatever tiny savings there is, and the chances of getting it is remote.

in any case, it would not be about the motor itself but about the load.

I agree. You'd have to have a pretty significant diversity factor and a lot of kW in play to make it worth while. Any AHJ looking to cover his/her back side is going to want a thorough justification for granting that permission.

 

[tom baker]

In WA, special permission is applied for on a form. You end up paying for a no.

 

[Jennifer K.]

Thanks - I am just wanting to ensure our client that we are not oversizing the electrical system. NEC does not really allow for applying a demand factor for motor loads. It is difficult to explain this to clients as they may say that the motors in their facility do not necessarily run at full amperage all of the time (and I do not have a way to confirm this) - even if some do not run at full amperage all of the time my point is if they do sometimes (and all at once) then the feed, OCP, etc. needs to be sized appropriately.