How to properly size a transformer

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BigMike

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I have a 480/3P panelboard. I need to feed 66kVA of 208V/120 connected load. This translates to 86kVA of demand when you calculate it based on the NEC demand factors listed in Art.220. (Its mostly the track lighting length that drives this demand factor). I know that the demand doesn't factor into the branch circuit for each track light calculation, but it does for the feeder. So does this mean I need to size the transformer based on the demand for the feeder or the connected? I know...a very basic question, but its driving me crazy.


Thanks.
 
The simple answer is that you size everything (panels, feeders, branch circuit conductors, transformers, you name it) on the basis of the calculated load. So if you calculated 86 KVA, you need a transformer with at least that high a rating. You also need a panel with a rating of at least the corresponding current (239 amps).

But I don't see how you went from 66 to 86, a 30% increase. I see that 220.43(B) has us adding some extra VA for certain track lighting applications. Is that what you used? Also, how did you arrive at the "connected" load of 66 KVA? Was that just a sum of the light bulbs, or did that come from a "VA per linear foot" calculation?

By the way, I am accustomed to using the phrase "demand factor" only in the context of something multiplied (i.e., not added in, as 220.43(B) would have us do), and only for values lower than 1.0. For example, when you add 25% for continuous loads (that is, you multiply by 1.25), I would not call the 1.25 a "demand factor," if for no better reason than that it is higher than 1.0. But others might see that phrase used differently.
 
I have 21kVA of track lighting and 25kVA of general lighting and 20kVA of general receptacle load.

So....

(21kVA + 25kVA) * 1.25 = 57.5kVA

and

20kVA (10kVA * 1 + 10kVA * .5) = 15kVA

and

180' of track lighting

(150W per 2ft) = 180/2 * 150 = 13.5kVA

So that adds up to 66kVA connected and 88kVA demand.

Am I missing something?

Thanks!
 
Utilities and Authorites Having Jurisdiction around here make a very clear distinction between "demand" and "connected" or "calculated" load. Connected is what your calculations tell you that you have. Demand is what your meter on the back of the house says you have.

But we always size for connected load, which IMO, is as it should be.
 
Let's not confuse "demand," a term used by the utility to help them send you a bill, and "demand factor," a term used by the NEC to allow us to size things smaller than would otherwise be needed to accommodate every connected load. "Demand factor" takes into account the likelihood that not every load will be running at the same time.
 
BigMike said:
So that adds up to 66kVA connected and 88kVA demand.
Click to expand...
No, it adds up to 86 kVA calculated load. That is what you should use to size the transformer and the panel, as well as the feeder between the two.
 
It looks like you counted your track lighting twice.

You would normally use the larger of the connected load *1.25 which looks like it is 21 *1.25 = 26.25 KVA or the calculated load which is 150VA/ 2 ft or 13.5KVA.

The thing that is confusing me is that your connected load of 26.25 KVA is larger than the 13.5KVA. That is possible, but it would be about a 100 watt lamp every foot for the entire 180'. Is that right? If so you can forget about the 150W per 2 feet and use the connected load * 1.25.

Steve
 
big Mike
When selecting the size of TR, consider your average load, and select base that load, but not less than NEC load,
That is if your not see more additional loads on the future.
The actual or average load that youre going to put in your TR should not be less than 70% of the TR capacity youve selected.
Lets say youve finalized your calculation, to 86 kva@66kva ave.load
then find 90kva(not commercially available), 100 is the nearest, its ok but 125 is not ok ( 66/125 = 52.8% )
This will make you (consume)pay more than what youre using, since it eats up more power to magnetize 125 kva core, than a 100 or a 90.


davidv
es
 
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