Transformer OCP

[augie47]

This is a quote from part of a paper prepared for a class:
See any errors ?

For example, a 75 kva 3 phase 480 to 208Y/120 transformer can be protected with a primary

overcurrent device rated at 125 amps (75000/480 X 1.732 = 90 X 1.25 = 112 next size = 125) with no specified secondary protection OR the transformer can have a primary overcurrent

device rated at 225 amps (75000/480 X 11.732 = 90 X 2.5 = 225) IF your secondary protection

does not exceed 300 amps (75000/208 X 1.732 = 208 X 1.25 = 260 (300 = next size up)

 

[charlie b]

1. You have a typo (11.732 should have been 1.732)
2. It fails to account for Note 3 on Table 450.3(B), the one that allows for higher primary side protection if you have built-in thermal overload protection.

 

[augie47]

1. You have a typo (11.732 should have been 1.732)
2. It fails to account for Note 3 on Table 450.3(B), the one that allows for higher primary side protection if you have built-in thermal overload protection.

Thanks. I've found at my age, anything longer than 5 words is likely to have a typo unless spellcheck catches it

I skipped Note 3 (and less than 9 amp) to keep it simple but, in hindsight, I best reference it.

 

[petersonra]

just because it is commonly a point of confusion you might want to add a note that this only protects the transformer itself and may not protect the transformer conductors.

 

[jim dungar]

2. It fails to account for Note 3 on Table 450.3(B), the one that allows for higher primary side protection if you have built-in thermal overload protection.

I would think that Note 3 applies to less than .1% of all transformers larger than 5kVA single phase.
For general purpose transformer training, I think it adds more confusion than it prevents.