Transformer OCPD Incorrect?

[Sahib]

That was my mistake, assuming a 3-phase transformer.

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One interesting thing in the formula you used (post7)is application of .9 efficiency factor. Does it mean output KVA is generally used instead of input KVA?

 

[ramsy]

One interesting thing in the formula you used (post7)is application of .9 efficiency factor. Does it mean output KVA is generally used instead of input KVA?

Good point, xfmr efficiency ratings are OEM specified on Nameplate, and modern hi-efficiency transformers are far below traditional eff. tables based on KVA, so inrush currents are higher, blowing traditional overload selections. NEC Annex D - Example D10 does not explain where 0.9eff came from, but using xfmr Nameplate values has become more critical.

Further confusion, is NEC sections 310 Tables, 430, 440, & Annex D - Examples, which show conductor ampacity, motor, & xfmr calcs under load, rather than xfmr output (line voltage). Actual load voltage in the wild is unknown, but the NEC Tables & examples assume 460vac for 480v services, 230vac for 240v services, and 110vac for 120v services, which piles more assumptions on actual values.

 

[Jraef]

Wouldn't that be 120 amps?

100,000/480/1.73=120

It's a single phase transformer.

 

[Cow]

I work as a maintenance electrician in metal foundry, started in April, and I have a question concerning an existing transformer that I believe the OCPD is not properly sized.

It is a 100kva single phase transformer with primary voltage of 480vac and secondary voltage of 240vac. The secondary is feeding a panel, less than 10ft away. The panel has 200amp main with 17 of the 20amp breakers being used out of the 40 available slots.

The transformer primary is being protected by a 100amp breaker in a 400amp protected power distribution cabinet. My calculations for primary only protection show that this transformer should be protected by a 300amp OCPD.

Am I missing something to where the 100amp breaker is adequate protection?

It sounds like someone installed a 100kva, where a 50kva would of sufficed. The 100 amp breaker is not an issue, unless it's tripping.