Elevator / Motor Feeder

[mjs]

Say I have a feeder supplying 7 elevator motors.
620.14 allows me to apply a 0.77 demand
430.24 wants me to add 25% to the largest motor (assume for this case they are all the same size)
Do I use these in combination? I.e., add 25% to the largest motor?
Do I add the 25% before or after the 0.77 factor?
TIA-
MS

 

[Volta]

Welcome to the forum .

Reading 620.14, I would expect to add 25% of the largest motor first, then apply any demand factor allowed to the entire feeder.

Oh, I see you have been here, so welcome to the discussions!

 

[benaround]

mjs,

If you add the 25% first then x .77 it will be a lower number than if done the other way.

 

[Volta]

mjs,

If you add the 25% first then x .77 it will be a lower number than if done the other way.

But, a * b * c = a * c * b . . .

The 25% need only be applied to the largest motor, not the sum of the motors.

 

[benaround]

The question was, 'do I add the 25% before or after the .77 factor.

I have 7 motors at 10a. 1.25 x 10 = 12.5a

12.5 + 60 = 72.5

.77 x 72.5 = 55.85

#2

.77 x 70 = 53.9

53.9 + 2.5 = 56.4

Now I've done it twice, thank you!

 

[Volta]

Well you sure are right about that , your welcome .

But if we choose to use it, we apply the Demand Factor to the summation of the ampere ratings of the motors as determined by the listed Sections from Article 430. Those must already include 125 or 140% of the nameplate depending, as passenger elevators are classified as intermittent duty. This modified rating is to be used in the summation.

To that total we can apply the Demand Factor.

 

[benaround]

And that will result in a lower number than if done the other way

 

[erickench]

Look at problem D9 of Annex D in the back of the NEC. It shows the method for calculating the feeder for a group of elevators. It appears that the 125% rule is only used to size the controller not the feeder. Since these motors are used for intermittent duty you must multiply the nameplate current by 140% and then add them up and apply the demand factor .77 and this will give you the feeder
diverse ampacity.

Now the total feeder ampacity is the sum of the diverse ampacity as determined above and all the controller continuous current if the controller is separate.