Question transformer undersized but dont see anything in NEC 2017

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hhsting

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I have 30kva transformer delta to wye. Primary voltage 380v three phase and secondary voltage 208/120v. Primary breaker is sized 40A and secondary breaker is sized 100A. Wverything is upto NEC 2017 this far.

The secondary side total demand amps comes to 99A. It wont trip the secondary 100A breaker but the total demand kvA comes to 99*.208.1.732= 35kva.

So question 99A demand wont trip the secondary breaker 100A but wont 30kva transformer be undersized due to 35kva demand load?
 
hhsting said:
I have 30kva transformer delta to wye. Primary voltage 380v three phase and secondary voltage 208/120v. Primary breaker is sized 40A and secondary breaker is sized 100A. Wverything is upto NEC 2017 this far.

The secondary side total demand amps comes to 99A. It wont trip the secondary 100A breaker but the total demand kvA comes to 99*.208.1.732= 35kva.

So question 99A demand wont trip the secondary breaker 100A but wont 30kva transformer be undersized due to 35kva demand load?
Click to expand...

Thats 480V not 380V in the above post.


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The code would permit the secondary breaker to be 110 amps, I don't see a real world issue.
I don't thing there are any rules about sizing that transformer in the NEC
 
The NEC does not provide direct help for sizing transformers, it's focus is on OCPD selection.
 
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Could anything in 110.3 be used to make using a transformer in excess of its rated load a violation of the code?

I'm thinking of things like listing, labeling, or manufacturers installation instructions....
 
steve66 said:
Could anything in 110.3 be used to make using a transformer in excess of its rated load a violation of the code?

I'm thinking of things like listing, labeling, or manufacturers installation instructions....
Click to expand...
You would have to show me such an instruction. I have never seen one that would prohibit the installation described in post 1
 
steve66 said:
Could anything in 110.3 be used to make using a transformer in excess of its rated load a violation of the code?

I'm thinking of things like listing, labeling, or manufacturers installation instructions....
Click to expand...
Transformer capacity is based on not exceeding the temperature of the transformer insulation. If the ambient air is cooler than 30°C the transformer can handle more load than it's nameplate. If there is sufficient cooling time, a transfomer can also handle overloading.
Both of these conditions would allow an application such as in the OP.
 
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