Transformer calculation

[geraldkinen]

I did this calculation, but I'm not shure that I got it righ? Would somebody check my answer? And If I got it wrong, let me know which formula I should use. Thanks!
The Minimum 75 degrees C. transformer secondary conductor not over 7.5 m long for a 480 volt primary to 208 volt secondary transformer suplied by a 100 ampere primary overcurrent device is: A. 6 awg xhhw al. B. 4 awg rhw al. C. 3 awg thw al. or D. 2 awg use al.
Answer I came up with is D. 2awg use al. using IS=EPXIP/ES

 

[charlie b]

It is not a matter of whether you got the answer right. The person who wrote the question got the question wrong. You don't size the secondary conductors on the basis of the primary OCPD. You base it on the transformer's KVA rating. That number was not given in the question.

To make the question seem even worse, there isn't a standard size transformer for which I would use a 100 amp primary OCPD. For a 50 KVA, I would use a 75 amp primary OCPD. For a 75 KVA, it would be 125 amps.

For the sake of discussion, since it comes closest to matching the given information, let's talk about a 75 KVA. Primary rated current is 90.2 amps. Multiply by 125% and you get 112.8 amps. You could choose to use a 100 amp primary OCPD, if you wished. The secondary current would be 208 amps. Multiply by 125% and you get 260 amps. The minimum 75C aluminum wire with that ampacity is 400 MCM. So I don't understand the available choices.

Bad question!

Edited to add the formula: Secondary rated current equals transformer KVA rating divided by 208 volts, then divided again by 1.732 (the square root of 3).

 

[charlie b]

. . . using IS=EPXIP/ES

OK, so 480 volts times 100 amps divided by 208 volts equals 231 amps. How is a #2 (or any of the other wire choices) going to handle 231 amps?

Are you certain you are presenting the question correctly?

 

[Jim Shorts]

240.21(C)(6)(1) (2005 NEC) reads that the secondary conductors must have an ampacity not less than the value of the primary to secondary voltage ratio (2.3) multiplied by 1/3 (33.3) of the rating of the overcurrent device protecting the primary of the transformer. Therefore:
2.3 x 33.3 = 76.92 minimum ampacity thus #2 aluminum conductors.

 

[charlie b]

OK, thanks Jim. Now I see their intent. But I will still call it a bad question. It spoke of secondary conductors, not of a transformer secondary tap. So I would look for conductors that can handle the rated load.

Gerald: Listen to Jim, and not to me, on this one at least.

p.s. The difference between your formula and Jim's is that he included the factor of 1/3. I don't know how that would have influenced your choice of cable size, but you appear to have gotten the right answer.

 

[geraldkinen]

xmfr

xmfr

Thanks! That's what i came up with.

 

[geraldkinen]

xmfr

xmfr

OK, thanks Jim. Now I see their intent. But I will still call it a bad question. It spoke of secondary conductors, not of a transformer secondary tap. So I would look for conductors that can handle the rated load.

Gerald: Listen to Jim, and not to me, on this one at least.

p.s. The difference between your formula and Jim's is that he included the factor of 1/3. I don't know how that would have influenced your choice of cable size, but you appear to have gotten the right answer.

Thanks! It is worded weird and I didn't multiply by 125% like you normally would. But that's because of wire not breaker, I think?

 

[charlie b]

. . . I didn't multiply by 125% like you normally would. But that's because of wire not breaker, I think?

Well, I normally figure out the wire size, then the breaker size. SoI suppose that if the wire has a 125% built in, the breaker will too.

But in this particular case, you don't use the 125% because you are not dealing with the entire capacity of the transformer. You are dealing with a single tap, and the formula to which Jim referred does not include a 125% factor.