Demand Factor in Distribution

[jimgineer]

I'm *fairly familiar with demand factors and service calculations, but I was recently having a discussion with a colleague on calculating demand factors and was stumped. In the past I have always used excel sheets for the load center level (and all other distribution equipment). When it came to demand factors, they would be placed in the excel sheet based on NEC to pop out the adjusted kVA and adjusted amps. When it comes to an individual load center, this is not a big deal.

The question involves the distribution as a whole, though. Motors for example the demand factor is based on the largest motor load, then the rest are sized differently. If you have a distribution board feeding two separate load centers that fed motors, if I am now thinking about the demand factor correctly, the dist. board would have no way of knowing which panel had the largest motor within it, and therefore no way to appropriately apply the demand factor to the largest motor. Or does the demand factor only apply to the load-level of distribution - in other words, you apply the demand factor to one panel, and then the adjusted kVA and amps cascade their way up the system... (I'm hoping this is the 'right' way to do distribution and interpret the NEC as this is how I have thought it was done.)

And hopefully this made sense enough for someone to comment. Thanks.

 

[davidbeach]

What I think you are doing won't get you into trouble with the code, but it does give a higher demand than the code requires. The Code demand factor rules apply at any and every level of the system you care to look at. So at the service you only need that extra 25% of the largest motor connected to the service, but for every branch panel you need to include 25% for the largest motor on that panel.

Use a real system modeling program, such as SKM Power Tools, Easy Power, or many others, and the program will do all that book keeping for you. If you are going to do it in excel you will need to track largest motor as a separate category and then at each level use MAX() of all the largest motors at the next level down.

 

[waross]

Calculate based on the motor loads and then, to each panel, add 25% of the largest motor fed from that panel.
For the main panel use the largest motor in the plant and add the allowance to the main panel. For each area power distribution panel, use the largest motor in that area for that panel. For each panel use the largest motor on that panel.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[burnt2x]

Demand factor is always less than 1. It is the ratio of the maximum demand of a system or part to the total connected load of the system or part under consideration. Although a feeder/circuit should have an ampacity sufficient to carry the load, the ampacity of the feeder/circuit may not always be equal to the total of all loads on all branches/circuits connected to it.

Remember that in some cases, a "demand factor" may be applied to the total load, and it is allowed to have a feeder conductor with an ampacity less than 100% of the sum of all branch circuit loads connected to the feeder. This is so because the actual demand will always be lesser than the rated load/capacity of the units.

To derive the maximum demand load, the sum of the maximum demand loads for two or more feeders is divided by the "Diversity factor" (ratio of the sum of the individual maximum demands of the various parts of the system to the maximum demand of the whole system). Please take a look at NEC 220.3 to 220.35 and section 430.26 for motors.

Your local AHJ may allow less than 100% demand factor if operation procedures, production demands or the nature of the work is such that not all motors run at one time.

Hope this helps.