Transformer Conductors

[bubba207]

I am familar with the requirements of 450 and 240 when it comes to primary and secondary conductors of a transformer, but do the requirements of 215.2(A)1 apply as well? I.E. - 75kva transformer - primary side conductor - would require a mininum of a #2 copper (115a) or larger depending on the primary OCPD - but is it permitted to be smaller? (based on the primary OCPD size) and what about the secondary conductors? - do they have to be at least 260 amps - or just large enough to meet the secondary OCPD size. (I know they cannot be smaller than the OCPD - But is a 3/0 copper permitted with a 200 amp OCPD on the secondary of this 75kva transformer? (I have concerns for the conductor between the transformer and the OCPD on the secondary side..)Thanks for the help!!

 

[david luchini]

The conductors need to be sized per the OCPD and sized to carry the load per 215.2(A)(1), but they don't need to be to the transformer rating (ie, the 260A secondary that you list.

For instance, if your calculated secondary load was 162A, you could use a 175A c/b on the secondary.

 

[bubba207]

I guess I do not fully understand why article 215 does not apply for the conductor - it is a feeder - 450 does not cover the conductor - and 240 covers the OCPD requirement for the conductor - I would think you would have to go by a minimum "load" requirement for the conductor. If ther is no specific rule for a given application, doesent the general rule apply? (hence 215) Thanks again for your input.

 

[augie47]

Ignoring energy losses the transformer is not a load so there is no requirement to supply it at it's maximum capacity.
A 75 kva 480v transformer could be fed by a 15 amp breaker and there would be not violation (In real life, inrush would prevent such an install)
Your conductor and their corresponding OCP need to be addressed.
.

 

[LarryFine]

It sounds like the question is basically whether one can oversize a transformer without making any other changes.

It sounds like the answer is "yes." Yes?

 

[iceworm]

It sounds like the question is basically whether one can oversize a transformer without making any other changes.

It sounds like the answer is "yes." Yes?

Following on previous posts:
That depends on if one wants it to work reliabily or not. And that is a design issue not a code issue.

One can install a 75kva transformer for a 25kva load, size the primary feeder and OCP to the load. And if the primary CB trips every other time it is energized that is okay by the code.

As for the secondary one can easily size the conductors and OCP to the load with no adverse affects and the code thinks that is okay as well.

Now if one wants to size the primary and secondary, conductors and OCPs, to allow reliable energization and capability for future full load, that's also okay with the code - but it is a design issue.

ice

 

[iceworm]

A 75 kva 480v transformer could be fed by a 15 amp breaker and there would be not violation (In real life, inrush would prevent such an install).

(tongue in cheek humor - not picking on anyone)

Gus -
That is true. However, in design life, inrush would also prevent such an install.

But in EC value engineered life the install would be considered an excellent money saver.

ice

 

[bubba207]

I see a lot of answers regarding OCPD size of the transformer, and that is not what I am looking for, again I ask where in the NEC does it REQUIRE us to ignore article 215 for the conductors sizing into and out of the transformer. How is it not any different than ANY feeder when All loads are not on at the same time, yet possible to have them all on? Again, just my opinon...

 

[charlie b]

We are not required to ignore 215, and David gave you the answer you need in post #2. The design process always starts with a calculation of the load. Both 210 and 215 have the requirement to include 125% of the continuous load and 100% of the non-continuous load.

If there were no transformer to be considered, then the next two steps are to select a conductor that has enough ampacity to carry that load, and to select an OCPD capable of protecting that conductor.

When you have a transformer to consider, then once you now the load the transformer has to serve, you select a transformer that has at least that rating. You have to size the transformer?s primary and secondary feeders to handle the load, as before. So 215 comes into play at this point in the design process. You might want to size the primary and secondary feeders to handle the entire rated load of the transformer, and perhaps to handle another 25% as well. But you don?t have to.

 

[charlie b]

But is a 3/0 copper permitted with a 200 amp OCPD on the secondary of this 75kva transformer?

Yes it is, provided that the total calculated load (including the 125% factor for continuous loads) is not more than 200 amps.

(I have concerns for the conductor between the transformer and the OCPD on the secondary side..)

So do I, given that there are fault scenarios in which they can be loaded beyond their ampacity, and in which the OCPD could not protect them. But I recognize that the risk is small, given the requirements that the secondary conductors are limited in length, and that they must be protected against physical damage.

 

[bubba207]

Hypothitical situation - 75 kva transformer - primary protected at not more than 250% - 225 amp OCPD and 4/0 copper conductors - secondary protected at not more than 125% - 250 amp OCPD and 250 kcmil copper conductors - This all meets the NEC - Correct? Then what happens if the 250 kcmil conductors 'fault' between the transformer and the OCPD? will the primary OCPD open BEFORE the 250 kcmil conductors melt? Just my thoughts....

 

[charlie b]

This all meets the NEC - Correct?

Agreed.

Then what happens if the 250 kcmil conductors 'fault' between the transformer and the OCPD?

In my opinion, we don’t have to postulate that specific failure mechanism. We take extra steps to prevent such a fault, as I mention in my earlier post #10.

Will the primary OCPD open BEFORE the 250 kcmil conductors melt?

If you are talking about a direct phase to phase or phase to neutral short circuit, then there will be severe melting at the fault location. But some distance away from that point, perhaps a few feet in either direction, the cable will not be damaged.

If instead you are talking about a high impedance fault, one that cause a fault current higher than 250 amps (the ampacity of the secondary conductors) but lower than 519 amps (the primary OCPD setting, as reflected through the transformer), then no breaker will trip to terminate this event, and yes the 250 MCM secondary conductors could experience a melting of their insulation systems. Did I mention that we are not required to postulate this failure mechanism, and not required to be concerned over what might happen to the conductors if it did occur?

 

[bubba207]

Thanks Mr.Beck, the answer does, as always, lie in the actual 'load' - we cannot size anything properly without the actual load involved...and the transformer is merely a 'conductor' to the load.. A little more clearer, harder to teach...Really, Thank you for your insight. Bubba

 

[iceworm]

there are fault scenarios in which they can be loaded beyond their ampacity, and in which the OCPD could not protect them.

Then what happens if the 250 kcmil conductors 'fault' between the transformer and the OCPD? will the primary OCPD open BEFORE the 250 kcmil conductors melt?

bubba -
There is only about one fault scenerio that loads the conductors between the xfm and the ocpd.

Any current entering the xfm end of the conductors has to come out the ocpd end. The only exception is a backhoe or fork lift attack. And that is not an overload - that is a fault.

So, now that this has happened, there are no conductors left to protect - they are dead, faulted, finished. Now the issue is to protect the transformer from damage and get the fire put out. That is the job of the xfm primary ocpd

ice