Transformer Loading Guideline? 3

[fms77]

I looking for a loading guide for distribution transformers. At what point is a transformer overloaded? At nameplate KVA, at 20% over nameplate? How does the seasonal peak factor in? Mfgs tend to recommend not loading beyond the nameplate KVA. What about "degrees C rise"? -- Thanks.

 

[GOTWW]

Concerning the three residential units, with an estimated NEC KVA of 10 to 12Kva, now extrapolate this to table 230.32 for 3-5 dwelling units gives a demand factor of 45%. So this gives a range of 3*(10 to 12)*0.45= 13.2 KVA to 16.2 KVA, at which nearest transformer would be 15 KVA, that by the way has 62.5 FLA. So this is my explanation.

It is funny though having to do a full blown NEC load study for a commercial building upgrade to justify keeping the 400A main service, whereas the utility company leaves an existing 10KVA unit of the pole, no questions asked.

And No I am not a utility engineer, I have to wear allot of hats with varying degrees of incompetence

 

[jghrist]

Utilities would use a variety of methods for sizing the transformers, based on their local experience. You do need to take into account major loads such as electric heating and A/C. As an example, an old guide from Public Service New Mexico gives 2.7 W/ft² for 1600-2000 ft² houses with gas heat and evaporative A/C, 6.9 W/ft² for houses with gas heat and refrigerant A/C, and 7.9 W/ft² for total electric houses. They used a coincidence factor of 0.66 for 3 houses with gas heat and evaporative A/C, 0.88 for 3 houses with gas heat and refrigerant A/C, and 0.69 for 3 total electric houses.

 

[GOTWW]

Thanks jghrist, for the valuable insight into the thinking of the "evil" empire.

 

[Bung]

Following on from jghrist, utilities also often have a set of standard size transformers they use. This gives economies through discounts for quantitites, allows standard padmount or pole-top designs, and reduces reuirements for spares. So you can easily see a much larger than expected transformer in a given situation just because it is the nearest standard size, or was the only one available in time to meet the customer's required date for supply.

We also have to bear in mind the differences in "time constants" at play. A transformer takes hours, at worst tens of minutes, to be damaged by overload. An overcurrent relay (or fuse) takes fractions of seconds, at worst seconds, to operate. No point in "throttling" the transformer for short term overloads (in the order fo seconds) just because that's how the overcurrent curve is set. How would you deal with (for example) large air-con motor starting?


Bung
Life is non-linear...

 

[rempman]

Your question is a good one. My response makes the assumption that your firm falls under the ANSI C2 - NESC instead of NFPA 70 - NEC. In distribution utility applications within a residential setting you can plan to overload transformers up to 200% or more of nameplate. This is depending of course on the coincidental loading of the services that are being served, the length of time the peak will be inplace, and the ambient air temperature.

However you may be overlooking a more important issue if you choose to follow this route. I strongly recommend that you look at your voltage drop calculations to the service that is most at risk of being served at a voltage below your mandated requirements (Typically 114V at the meter) You will also need to review your flicker criteria in the case of heat pumps and air conditioning.

In the end I would wager you will be limited to around 120% of nameplate as a result of the voltage issues you may incur.