Sizing Feeder and OCPD for Motors AND Other Loads

[Junior_EE]

Hi all,
I use this forum often and I can usually find the answers to my questions somewhere, but I cannot seem to find this. I know how to calculate conductors and OCPDs for motors and I know how to calculate conductors and OCPD for other loads, but I am getting tripped up when they are fed from the same OCPD and the same feeder. My question is a two-parter:

a.) I am sizing fused switches that supply power to panelboards that supply power to elevator motors and other miscellaneous elevator loads. How do I calculate the fused switch size and conductor size that supplies a panelboard that feeds a 30 HP motor and 14.8 FLC miscellaneous control and AC loads?

b.) I am sizing a feeder and OCPD that supplies a distribution panel that feeds a combination of motor and other loads. How do I perform this calculation? This may be the same as part (a).

 

[petersonra]

take a look at article 430 where it discusses motor feeders.

 

[charlie b]

I would say that it really starts with 220.40, under ?Feeder and Service Load Calculations?. But that will lead you to 430 eventually anyway. What you need to do is calculate the total load on each of the two panels. In general, that will involve taking 100% of all non-continuous loads, 125% of all continuous loads, and 125% of the largest motor. The phrase ?continuous load? is defined as something that runs at full capacity for more than 3 hours at a time. That is not likely for elevator-related equipment. You may also be able to take advantage of ?demand factors? for items that do not generally all run at their peak capacity at the same time. But that will depend on the nature of the loads on the two panels. One example is receptacle loads in non-dwelling unit applications. See table 220.44.

Welcome to the forum.

 

[Smart $]

I agree with Charlie's summation, in general. However, there may be some difference involved when it comes to sizing feeder OCPD.

As most are aware, depending on motor type, its duty cycle, and type of OCPD, a motor's maximum OCPD rating varies between 150% to 1,100%, allowing for start inrush. Where this value overshadows all other loads on the same feeder, you have to carry it up through the system (though not necessarily the same type OCPD and rating) until the other loads are no longer overshadowed, if at all.

For more details and how it may relate to your installation, see Article 430, Part V.