Transformer sizing

[mehdi1351]

I would like to know whether I should size my Transformer based on "Connected loads" or "Operating loads".
I know the first one is a bit more conservative but I want to know what is the the standard practise

Thanks

 

[jim dungar]

I would like to know whether I should size my Transformer based on "Connected loads" or "Operating loads".
I know the first one is a bit more conservative but I want to know what is the the standard practise

Thanks

The NEC provides no guidance as to how to size a transformer.

I have always used 'operating loads'.

 

[mehdi1351]

Yap, I am thinking like you. It is more sense to size the transformer based on operating loads rather than connected loads but i was not sure about american standards, NPFA70 etc.

 

[Jpflex]

I would like to know whether I should size my Transformer based on "Connected loads" or "Operating loads".
I know the first one is a bit more conservative but I want to know what is the the standard practise

Thanks

My boss used to order a transformer but did not know how to do load calculations and would constantly add new loads after purchase so he would often h ave to buy another larger transformer

 

[kingpb]

I would like to know whether I should size my Transformer based on "Connected loads" or "Operating loads".
I know the first one is a bit more conservative but I want to know what is the the standard practise

Thanks

How are you sizing your panel that has the loads connected to it? Simply pick the closest (rounding up) size to the panel. Some will size the transformer for the full capacity of the panel or board, it is connected to if there is a very likely chance that additional loads will need to be added later.

 

[KevinKos]

The standard practice for transformer sizing generally involves using "Connected Loads" rather than "Operating Loads." The connected load represents the total possible load that could be connected to the transformer, providing a more conservative and safer approach, especially for sizing the transformer to handle peak loads and future expansion.

Key Points:​

1. Conservative Approach:
   • Connected Loads include all the possible loads that might be connected to the system. This ensures that the transformer can handle the maximum demand without being overloaded, providing a margin of safety and allowing for future expansion without the need for immediate upgrades.
2. Practical Considerations:
   • Operating Loads reflect the actual load expected under normal operating conditions. While this might be closer to real-world usage, it doesn't account for peak loads or potential increases in demand over time.
3. NEC Guidelines:
   • According to NEC guidelines, transformer sizing often considers the maximum possible load to ensure safety and reliability. For example, NEC Article 450.3 specifies overcurrent protection based on the maximum current expected under peak load conditions .
4. Efficiency and Cost:
   • Using connected loads for transformer sizing might be more expensive initially due to the higher capacity required. However, it avoids potential costs and downtime associated with upgrading undersized transformers later.
5. Standard Practices:
   • Electrical utilities and industry standards typically recommend using the connected load to size transformers to avoid overloading and ensure the transformer can handle unexpected surges and future load increases.

Conclusion:​

Sizing transformers based on connected loads is a standard and recommended practice due to its conservative nature, ensuring safety, reliability, and accommodation for future load growth. While it may seem more conservative and potentially costlier initially, it provides a robust solution to avoid the risks and costs associated with transformer overloads and upgrades.

For more detailed guidelines and specific calculations, refer to NEC Article 450 and other relevant sections that discuss transformer sizing and protection .

 

[jim dungar]

For the load to operate it must be connected.
But, if it is connected it may not be operating. Not all connected loads are coincident.

The NEC contains no language about the sizing of transformers.

Consulting engineers routinely oversize transformers in an effort to cover themselves under the guise of unexpected future growth. Isn't their it job to anticipate the future?
Utilities like to undersize transformers selected using NEC load estimates in an effort to reduce transformer losses.

Some of the early DOE transformer requirements were to base efficiencies at 35% loading as their surveys, in the 80's, showed this was a typical average loading in offices and schools.

 

[mehdi1351]

@KevinKos and @jim dungar thanks for the response. I guess there should not be a prescription on this matter. I think it is thoroughly depends on the type of the loads, i.e some connected loads are duty/standby and it goes without saying that both duty and standby won't come into the line regardless of whether you are going to increase the plant capacity or not.
In Middle East, we size the transformer based on IEC60076, which is based on "operating loads" plus 35% over specially due to temp effect on the transformer de-rating and then we do the adequacy checks to see the selected KVA would meet the requirement.
BUT as I want to manufacture a new plant in Texas, I don't really know what is the US regulations, i.e NEC, NFPA70 etc tells about the Transformer sizing.
If you really sure about that those codes & Standards telling about transformer sizing criteria, please share it with me
Thanks heaps
Mehdi