Supervised Location Overcurrent Protection of Transformer Secondary Conductors

[jstpie5]

There is an existing site that has a Delta/WYE (w/ HRG) 3500kVA 4160V / 480V xfmr feeding a 4000A 480V switchgear lineup.

The transformer & primary conductors are protected on the primary by a 5kV breaker / relay combo.

The secondary conductors terminate at a single switch (non-fused) and the switchgear feeds 1-4000A breaker and 3-1600A breakers.

Are the secondary conductors permitted to be protected by the primary overcurrent protection device as stated in 240.92(E) - Protection by Primary Overcurrent Device?

or

Are they required to be protected in accordance with the methods stated in 240.92(C) - Transformer Secondary Conductors of Separately Derived Systems?

 

[don_resqcapt19]

If this is a "supervised" location, the transformer secondary conductors must be protected by the rules in 240.92(C).

What are you using for the transformer secondary conductors? It appears that this installation requires 8,800 amps of secondary conductor.

Not sure the language in 240.92(C)(2)(1) permits the switch on the line side of the switchgear. As I read that language, I see it requiring the secondary conductors to land directly on the line side bus of the switchgear.

 

[jstpie5]

It is indeed a supervised location. The existing secondary is a 4000A bus duct and has been in service for 40 years (not that it makes things any better).

Is this installation NEC compliant, if the transformer primary OCPD is set to protect the 4000A bus between the transformer secondary and the SWGR?

 

[don_resqcapt19]

It is indeed a supervised location. The existing secondary is a 4000A bus duct and has been in service for 40 years (not that it makes things any better).

Is this installation NEC compliant, if the transformer primary OCPD is set to protect the 4000A bus between the transformer secondary and the SWGR?

NO. The bus is the secondary conductor and must be protected by either 240.21(C) or 240.92(C).

 

[jim dungar]

NO. The bus is the secondary conductor and must be protected by either 240.21(C) or 240.92(C).

But some of these rules have changed over the past 40 years. We used to treat industrial MV transformers like they were utility services rather than per the NEC.

 

[don_resqcapt19]

But some of these rules have changed over the past 40 years. We used to treat industrial MV transformers like they were utility services rather than per the NEC.

Jim,
Was that permitted by the code, or just a common industrial practice?

 

[jim dungar]

Jim,
Was that permitted by the code, or just a common industrial practice?

I dont remember, at that time it was commonly allowed by AHJs for industrial. By the 80's the NEC clarified it language, to about what we have now, and the old practices were finished.

I remember doing a site survey, as a recent hire, at a paper mill in 2005 where my senior engineer was going to cite one of these installations as being non-NEC compliant. I pointed out it was compliant when I designed it back in the 80's.

 

[don_resqcapt19]

I dont remember, at that time it was commonly allowed by AHJs for industrial. By the 80's the NEC clarified it language, to about what we have now, and the old practices were finished.

I remember doing a site survey, as a recent hire, at a paper mill in 2005 where my senior engineer was going to cite one of these installations as being non-NEC compliant. I pointed out it was compliant when I designed it back in the 80's.

I know I worked on a lot of smaller transformers in industrial applications where there was no secondary protection ...from the transformer terminals into a MLO panel. Never knew if it was just an accepted industrial practice, or permitted by the code.
I know that over the years a number of code changes were made for industrial installations where the substantiation was, "we have been doing that successfully for many years in the industrial world."

 

[ron]

As a side note of your question, the short circuit rating of a non-fused switch is generally very low, and mostly too low to be fed by a 3500kVA transformer, even if it has high relative impedance, the downstream 3 phase fault current will be higher than a typical non-fused switch.

 

[jstpie5]

As a side note of your question, the short circuit rating of a non-fused switch is generally very low, and mostly too low to be fed by a 3500kVA transformer, even if it has high relative impedance, the downstream 3 phase fault current will be higher than a typical non-fused switch.

Good looking out! These (bolt-loc) switches appear to be rated for 85kA+. W/ xfmr Z=5.83%, they are good. Close, but good.