I have a 75kva transformer, 600v to 208v. The primary rating is 72.1 amps.

26-256(1): The primary overcurrent is to be 125%, or 90.125 amps; and, that will protect the secondary conductors rated at 125% of the rated secondary.

The rated secondary current is calculated to be 208.18 amps, so the secondary conductors must be good for 260.2 amps. I select 250 MCM copper with an ampacity of 265 amps.

26-256(3) provides that where 125% of the primary current does not correspond to the standard rating of an overcurrent device, the next highter rating shall be permitted. 90.125 amps is greater than the standard size of 90 amps, so the next standard size is 100 amps.

There is no stipulation that when the larger overcurrent is used, the size of the secondary conductor must be increased.

The inspector suggests that rule 14-606 must be applied to the conductors. I disagree that the rule about panels applies to conductors.

It is my position that the code allows for overcurrent to be greater than the ampacity of the conductors in certain circumstances. Motor feeders are an example, as is 14-104(a) and the associated Table 13. 26-256 is just another example.

Are we missing something? What do you think?

Good question, I would tend to think the construction of the 75kva xformer should have a current limiting effect so conductors should not have to be increased. In your installation, why not just go down to 90 amp overcurrent and keep all your feeder sizes as you have calculated them?

Because I shouldn't need to.

I agree that this rule does not apply to conductors; it applies to the protection of panelboards.
If you are feeding a panel board and the inspector's concern is for the protection of the panelboard, then I could understand how rule 14-606 would apply and either a panelboard with a higher ampacity or a smaller fuse at the primary of the transformer would satisfy rule 14-606.
If you are feeding some other type of load other than a panelboard then I would think rule 14-606 has absolutely no bearing on your situation.

TWH What is connected to the Transformer? is there secondary O/C protection? If you were feeding a 225 amp panel the largest O/C protection on the primary is 78 amps as the ratio of 2.88 from 225 amps to the primary of 78.125 amps. A 70 or 75 amp primary O/C protection is ok to protect the panel. A 200 amp panel would limit the primary o/c protection to 60 amps and you might blow that fuse just turning it on. If you are protecting a load then the next available fuse or breaker might be ok. You are required to protect conductors but Panel boards have over current rules too. Unless the panel has more than 90% motor loads then you cannot overfuse a panel.
Now if you come off the secondary to an overcurrent device in say a 225 amp panel then the 90 or even 100 amp is OK. So why would you argue about the 100 amp fuse over a 90 anyway? Or is this just for the sake of discussion?

mikesh: The primary fusing protects the secondary conductors. The panel is 400 amp and the load on the panel is not mainly motors.

At this point, it's for the sake of discussion. I'm upset because the inspector threatened to withhold approval on another part of the job, because of this. I am now aware that he stands in the minority with his view, at least in this part of the country.

TWH while I appreciate the inspectors point of view I cannot see a violation with the added info re the panel size of 400 amps. I think the safer installation is the 90 amp O/C device.

Whoa Stop the presses. I changed my mind a little. TWH you need to reread 26-256(1) and so do I. It says no more than 125% except that you may go to the next size in subrule 3. You would have to prove to me that the 90 amp OC protection is incapable of protecting the transformer because it is tripping on inrush.
That means You can use a 90.125 amp fuse as a maximim value. You cannot just automatically go up in size unless you can show the transformer protection won't hold at 90 amps. then the .125 amps gives you the right to go to the next size.

TWH,

What province are you in?


[This message has been edited by Cosimo Diano (edited 06-28-2006).]

mikesh: 26-258(3) says that if 125% does not correspond to the standard rating of the overcurrent device, the next higher standard rating shall be permitted.

"Shall be permitted" means that the inspector shall permit the use of the next higher standard size. It does not mean that the inspector can add extra requirements before it is permitted.

Show me where it says that I must prove anything beyond that 90.1 is greater than 90. I insist that if the rules apply to electricians, they apply to inspectors, too.

Cosimo Diano: I'm in Saskatchewan.

[This message has been edited by twh (edited 06-22-2006).]

TWH..
Well said in my opinion..100% agreed

TWH
I see you are correct in applying the rule and you win the argument but I still think you are arguing this so you can avoid changing to a 90 amp breaker. I know I am splitting hairs but for 1/8th of an amp you are going to up 9 7/8 amps because you can? A fuse or circuit breaker needs around 6 times it's current rating to operate in its instantaneous range so we now need between 500 and 600 amps to trip in the instantaneous range instead of about 60 amps less with a 90 amp o/c device. never mind a high impedance fault. The first subrule in 26-256 says maximim of 125% so it is obvious that the intent is to limit the overcurrent device to some level below 125%. Subrule 3 gives you the ability to match OC devices to the products available at the wholesaler and to be able to ensure tha transformer O/C won't open on excitation. For that 1/8th of an amp the inspector might have to allow the installation as you have done it but he is still correct in asking you to use a 90 amp O/C device.

Imagine for a moment you were installing a big transformer and the breaker was 1000.125 amps. Next size larger is 1200. Of course you wouldn't do that if it also meant spending another 2500 for the next frame size.

So I concede you have met the code but have made a less safe installation on principle. Who wins? Maybe you actually installed the 90 amp breaker and am just looking for a moral victory.
I think I would have told you to install the 90 amp O/C and let you appeal it. I don't intend for this to become a pi**ing match and it is a valuable argument for discussion purposes. It also made me think about the question from a different perspective. I can see from your other postings you do know how to read a code book.

[This message has been edited by Cosimo Diano (edited 06-28-2006).]