30kva primary side over current device

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mrblue1482

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I have a 30kva 480v primary 208v secondary transformer I?m wiring. My question is what size ocd should I use for the primary side? I've calculated a 45 amp breaker is needed but can I round up or should I always round down?

30,000va\ (480x1.732) =36.08A
36.08A x 1.25=45.1

I?m using a 50amp breaker on the primary side.

I did browse other posts but found nothing with this particular example.

Any help will be appreciated.
 
Should be fine on the 45 amp OCP, 50 will be ok too. Very unlikely that the entire 30kva is continuous, generally. You can drop the 0.1 amp anyway 220.5(B).

Welcome to the forum!
 
The 125% "rule" is for transformers without secondary overcurrent protection (addressing Art 450 only, not Art 240).
Although there may be some debate on the 50 vs 45 since the calculations result in 45.1 amps, and although I would probably go with 45, my opinion is that what you describe is Code compliant.
 
augie47 said:
The 125% "rule" is for transformers without secondary overcurrent protection (addressing Art 450 only, not Art 240).
Although there may be some debate on the 50 vs 45 since the calculations result in 45.1 amps, and although I would probably go with 45, my opinion is that what you describe is Code compliant.
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What part of the code would allow you to increase to 50 when the amps are 45 and they make a 45. Im not disagreeing just learning:)
 
well..it's not exactly 45 it's 45.1
the notes to Table 450.3b allow you to go to the enxt higher (in some circumstances)
 
augie47 said:
well..it's not exactly 45 it's 45.1
the notes to Table 450.3b allow you to go to the enxt higher (in some circumstances)
Click to expand...

But you are supposed to drop the .1 right?
I see 450.3b mentions 2 cases it allows secondary conductors to be protected by an overcurrent device right? Is that only 2 cases you can protect secondary conductors on a transformer?
 
I don't know of a Code section that prescribes that you drop fractions.
Art 220 discusses it, but that's for calculations and "allows" you to, doesn't require you to.
Be very careful discussing conductors when you reference Art 450.
Arty 450 has NOTHING to do with conductors. It only covers transformers and transformer protection.
Once you determine your OCP for your transformer, then you must go to Art 240 to see about OCP for conductors, more specifically 240.4(F) and 240.21(C)
 
augie47 said:
I don't know of a Code section that prescribes that you drop fractions.
Art 220 discusses it, but that's for calculations and "allows" you to, doesn't require you to.
Be very careful discussing conductors when you reference Art 450.
Arty 450 has NOTHING to do with conductors. It only covers transformers and transformer protection.
Once you determine your OCP for your transformer, then you must go to Art 240 to see about OCP for conductors, more specifically 240.4(F) and 240.21(C)[/QUOTE

I confused myself i was looking at the handbook and it mentioned 240.4(f) and 450.
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mrblue1482 said:
I have a 30kva 480v primary 208v secondary transformer I’m wiring. My question is what size ocd should I use for the primary side? I've calculated a 45 amp breaker is needed but can I round up or should I always round down?

30,000va\ (480x1.732) =36.08A
36.08A x 1.25=45.1

I’m using a 50amp breaker on the primary side.

I did browse other posts but found nothing with this particular example.

Any help will be appreciated.
Click to expand...

You most likely will be required to provide both priamary and secondary protection.

Primary side

30*1000/480/1.73*2.5=90.317 Amps.

So the NEC would allow a 0 to 90 amp primary overcurrent protective device.


Secondary side

30*1000/208/1.73*1.25=104.212 Amps

In this case note 1 of Table 450.3(B) allows us to roll up to the next standard size.

So the NEC would allow a 0 to 125 amp secondary side overcurrent protective device.
 
Thanks for the replies guys. I?d like to add that I do have a 3 pole 100 amp breaker on the secondary side that is feeding a panel less than 10 ft away. So I believe I should use 125% and not 250%. An inspector is telling me I need to use a 45 amp breaker. Oh did I mention this application will be done 12 times in this building? I have the 50 amp breakers already and it is going to cost big bucks to replace so I need to get this right. Thanks
 
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