240.21(b)(3)

[ken987]

Need some help Understanding :
240.21(b)(3)2

The conductors supplied by the secondary of the transformer shall have an ampacity that, when multiplied by the ratio of the secondary to primary voltage, is at least one third the rating of the OCPD protecting the feeder

Now I understand the intent, of this, it's the calculation part of it that has me a little confused. Mike holts Understanding the nec gives no example of the calculation, however a stallcup's book I have does. His calculation:

480V/208V *1/3*250A= 192A
This is for a 480-208 step down transformer with a feeder OCPD of 250A

isn't this calculation showing a primary to secondary ratio, when the code is asking for a secondary to primary ratio?

Any help Appreciated

Edited in an attempt to break the text lines. Charlie

[ November 06, 2004, 09:19 PM: Message edited by: charlie ]

 

[charlie]

Re: 240.21(b)(3)

Ken, if a normal 25 ft tap were being used, you would size the tap conductors for at leas 1/3 of the feeder OCPD. This is no different except you are now going through a transformer to accomplish the objective.

 

[plt]

Re: 240.21(b)(3)

That is a good question, the language used is not all that clear to me. but the primary current is 1/3 that of the secondary.

2.5 amp primary = 7.5 amps available on the secondary. I wouldn't size it at 1/3 of the 2.5 amps, it won't hold. I would not size it 1/3 of the 7.5 amps either because a worst case senario here says fire or injury or at least damage to system. I could use some help with this one too.

 

[ken987]

Re: 240.21(b)(3)

That's how I feel I'm studying for the NJ exam(been working through the 3000 questions in Mike's exam prep book gonna do every last question) given by experior, this probably won't be on there but since I'm studying I feel I might as well take the time and understand why and where the code applies. I'd like to know and understand than just know random codes.

 

[resistance]

Re: 240.21(b)(3)

I would write MIke Holt in reference to the concern of "WHY" it's not explained.

 

[paul32]

Re: 240.21(b)(3)

There is no contradiction. Look closer at the wording:

The conductors supplied by the secondary of the transformer shall have an ampacity that, when multiplied by the ratio of the secondary to primary voltage, is at least one third the rating of the OCPD protecting the feeder

In math terms, this says:

(ampacity of sec cond) * (sec volt/prim volt) >= 1/3 * OCPD

Rearrange by multiplying both sides by (prim volt/sec volt):

(ampacity of sec cond) >= (prim volt/sec volt) * (1/3) * OPCD

This is then the form the stallcup book has.

 

[john m. caloggero]

Re: 240.21(b)(3)

See Section 240.21(C)(6). The transformer is a step down from 480V to 208V. The states a secondary to primary ratio: this is 208 divided by 480 = .433 ratio. When you multiply .433 times the ampacity of the secondary conductors, the resultiing value must be equal to or greater than 1/3 of the ampere rating of the primary overcurrent device.

 

[ken987]

Re: 240.21(b)(3)

Thank you
I was a little confused at first, but with everyones help it's pretty clear.