Transformer size vs secondary output

[cppoly]

If you have a 225 kVA transformer, maximum output on the secondary is 625 amps @ 208/120V.

How would you size the secondary OCPD? 650 amp fuses or 800A fuses if you wanted to assume a continuous load at 125% (781 amps).

The part that I'm questioning / confused about is, if someone down the road sees an 800 amp fused disconnect - is it not wrong for them to assume they can load that up to 800 amps of non continuous load ? But they'll only get 625 amps max out of it.

 

[charlie b]

We don't design or build with the expectation that some future person might misinterpret our installation. That person is responsible for assuring that what they change is code-compliant.

 

[ron]

Although Table 450.3(A) Maximum Rating or Setting of Overcurrent Protection for Transformers Over 1000 Volts (as a Percentage of Transformer-Rated Current), shows the maximum, Footnote 1 allows you to round up.
If someone wants to load it to 800A without looking at the feeder, that is their mistake.

 

[cppoly]

A follow up. Looking at Table 450.3(B) with primary protection at 125% and secondary protection not required, this would mean the OCPD upstream would never trip unless the primary load exceed 125% of the transformer's FLA?? Transferring that onto the secondary, does that mean a the transformer would never trip with 125% of it's secondary FLA? So a 225 kVA transformer could run 625A x 125% = 781 without damage or just without tripping?

 

[jim dungar]

So a 225 kVA transformer could run 625A x 125% = 781 without damage or just without tripping?

Probably.

The NEC is only concerned with conductor sizing and conductor protection. Transformer, and motor, loading is a design issue to be taken up with the manufacturer.

 

[don_resqcapt19]

Note that where you size the secondary overcurrent protection to 800 amps the conductors on the load side of that OCPD must have an ampacity of 800 amps. Also note that between the secondary terminals and that OCPD, you need conductors with an actual ampacity of 800 amps because 240.21(C) says "Section 240.4(B) shall not be permitted for transformer secondary conductors.".

 

[ron]

Note that where you size the secondary overcurrent protection to 800 amps the conductors on the load side of that OCPD must have an ampacity of 800 amps. Also note that between the secondary terminals and that OCPD, you need conductors with an actual ampacity of 800 amps because 240.21(C) says "Section 240.4(B) shall not be permitted for transformer secondary conductors.".

Also recall that 240.21(C)(1) says that a delta wye transformer isn't protected by primary only protection.

 

[Joethemechanic]

I think just the nature of your typical 208/120 WYE service is that even if they are the type to max it out without checking, they probably aren't really. The loads are mostly not continuous.

And really like said above, it's their bad

 

[jim dungar]

Also recall that 240.21(C)(1) says that a delta wye transformer isn't protected by primary only protection.

Actually, it says the secondary conductors of a delta-wye transformer aren't protected.
450.3 allows the transformer, itself, to be protected by a primary device sized at not more 125%.