xfmr: 150C vs 115 C rise

[malachi constant]

Hi all,

I am a consulting engineer. One individual in my distributor community has recently been pushing for me to approve 150C rise transformers instead of our specified 115C rise transformers. His request is based on two arguments:
A) The 150C used to be less efficient than the 115C, but now they are of similar efficiencies.
B) The 150C can be utilized to better achieve OCPD coordination where it is required (such as on Article 700 emergency systems).

The efficiencies appear to be the same, so that checks out. We haven't had a major / unsolvable issue with OCPD coordination yet.

Is the 150C generally more robust? Does it cost more? Assuming roughly equal costs (or a credit to go to 150C), are there reasons NOT to go to 150C? Are there other pros/cons I'm not thinking of?

Thank you!

 

[jim dungar]

The 115C unit should produce less heat at full load, therefore your building system would have less heat to get rid of.

Do not look at efficiency ratings of transformers. Instead look at the losses of your transformer based on its intended loading profile.
The published DOE energies standards are based on 35% loading.

Typically a 115C transformer is a 'derated' 150C unit, so it is likely more robus. MEP consulting engineers are about the only people that specify 115C units. In my experience, most transformers sold over a distributor counter or into industrial plants are 150C units.

 

[ron]

We have found that 150 deg rise transformers are ok if you plan for the system to almost never be fully loaded.
The 115 deg rise offers some headroom for equipment performance and reduced heat release.
the 2016 efficiency requirements are the same for all temp rise ratings, so that is BS.
That manufacture may have different inrush with their 150 deg rise versus the 115 deg rise transformers, but I've not seen anything advertised. The difference is often one being a derated version of the other. similar to a k-rated transformer.

 

[retirede]

The 115C unit should produce less heat at full load, therefore your building system would have less heat to get rid of.

Do not look at efficiency ratings of transformers. Instead look at the losses of your transformer based on its intended loading profile.
The published DOE energies standards are based on 35% loading.

Typically a 115C transformer is a 'derated' 150C unit, so it is likely more robus. MEP consulting engineers are about the only people that specify 115C units. In my experience, most transformers sold over a distributor counter or into industrial plants are 150C units.

If they are the same efficiency, how can one produce more heat than the other?

 

[winnie]

There are several ways you can have a higher temperature rise.

1) A less efficient transformer, so that more heat is produced.

2) A more compact transformer, so that the same heat production is packed into a smaller space

3) A more poorly cooled transformer, so that the same heat production results in a higher internal temperature.

Things to consider are losses (and heat production) at _expected_ loading, head room for overload, and relationship between transformer temperature and its insulation temperature rating.

-Jon

 

[jim dungar]

If they are the same efficiency, how can one produce more heat than the other?

Efficiency varies with the loading of the transformer. A transformer with high core losses and low conductor losses will have different efficiencies at 0%, 30% and 100% loading than a similarly rated unit with low core losses and higher conductor losses.

And for those of you that want to nit pick, I am using the term 'efficiency' in a general sense.