Transformer Secondary conductor sizing

[petersonra]

I have a headache.

 

[jrohe]

Well if thats the case, then we would use 312.5 amps to size wire? WOW Im sorry if Im not seeing it guys. can someone size this transformer for me so I can see. Secondary nameplate reads 75 kva 120/240v single phase. Size the OCPD and Wire for this secondary size?

What I would do:

Transformer secondary FLA = 75000 VA /240 volts = 312.5 amps

Transformer Secondary OCPD = 312.5 amps x 125% (NEC T450.3(B)) = 390.6 amps. Round up to 400 amps per T450.3(B) Note 1.

Secondary Conductors = #600kcmil (copper) rated at 420 amps because NEC 240.21(C)(2) requires transformer secondary conductors to have an ampacity not less than the rating of the OCPD they terminate on. I am assuming the conductor ampacity does not need to be derated due to ambient temperature or number of current carrying conductors in a raceway.

 

[omega83]

What I would do:

Transformer secondary FLA = 75000 VA /240 volts = 312.5 amps

Transformer Secondary OCPD = 312.5 amps x 125% (NEC T450.3(B)) = 390.6 amps. Round up to 400 amps per T450.3(B) Note 1.

Secondary Conductors = #600kcmil (copper) rated at 420 amps because NEC 240.21(C)(2) requires transformer secondary conductors to have an ampacity not less than the rating of the OCPD they terminate on. I am assuming the conductor ampacity does not need to be derated due to ambient temperature or number of current carrying conductors in a raceway.

Thanks I think I got it now. Sorry guys, thank you guys for being patience.

 

[jumper]

Thanks I think I got it now. Sorry guys, thank you guys for being patience.

What if the calculated load is 200 amps for the secondary load? What then for the secondary sizing of OCPD and conductors?

 

[jrohe]

What if the calculated load is 200 amps for the secondary load? What then for the secondary sizing of OCPD and conductors?

You are permitted to use a smaller OCPD, but that just wastes transformer capacity. In your example, if the calculated load is 200 amps and you want to make the remaining 27 kVA of the 75 kVA transformer unusable, then I would go with a 200 amp secondary OCPD and #3/0 copper conductors (again assuming derating is not an issue).

Or drop the transformer to a 50 kVA transformer.

 

[jumper]

You are permitted to use a smaller OCPD, but that just wastes transformer capacity. In your example, if the calculated load is 200 amps and you want to make the remaining 27 kVA of the 75 kVA transformer unusable, then I would go with a 200 amp secondary OCPD and #3/0 copper conductors (again assuming derating is not an issue).

Or drop the transformer to a 50 kVA transformer.

Jason, my question was for the OP to understand this point.

 

[jumper]

works for me :slaphead:
(my head hurts)

You know, I hear that a lot after people talk to me.:angel: Especially my wife.

 

[omega83]

What if the calculated load is 200 amps for the secondary load? What then for the secondary sizing of OCPD and conductors?

If the calculated load was 200 amps, then you would use a 200 amp breaker with 3/0 ( 75 c)

 

[jumper]

If the calculated load was 200 amps, then you would use a 200 amp breaker with 3/0 ( 75 c)

And if it is a 312.5 amp load?

 

[don_resqcapt19]

And if it is a 312.5 amp load?

500 kcmil with 350 amp OCPD.

 

[kwired]

500 kcmil with 350 amp OCPD.

Good answer. People often forget there is standard sizes other than 200, 400, 600, 800, Below 200 they remember most except for maybe 25,35,45, 80,90, 110 and 175.

 

[david luchini]

500 kcmil with 350 amp OCPD.

Good answer. People often forget there is standard sizes other than 200, 400, 600, 800...

That may be true, but 600 mcm with a 400A OCPD is also an acceptable answer to the question.

 

[jumper]

And if it is a 312.5 amp load?

500 kcmil with 350 amp OCPD.

Good answer. People often forget there is standard sizes other than 200, 400, 600, 800, Below 200 they remember most except for maybe 25,35,45, 80,90, 110 and 175.

That may be true, but 600 mcm with a 400A OCPD is also an acceptable answer to the question.

I am pretty sure I was hoping the OP would answer.:slaphead:

There was a point to my question to him.