Protecting Conductors versus Protecting Transformers

[charlie b]

Per 240.4 (C), if the required ampacity of a conductor is over 800 amps, and if it does not match a standard rating for an overcurrent protection device, you must select the next lower rated device, not the next higher rated device.

But it appears to me that there is no similar rule for protecting the secondary of a transformer. For example, a 300 KVA transformer with a secondary voltage of 120/208V would have a rated secondary current of 833 amps. The maximum rating for the secondary protection device is 125% of this value, or 1042 amps. Despite the fact that this is more than 800, I am allowed to select the next higher rated device, and use a 1200 amp secondary breaker.

Am I reading this right? Is there something special about transformers that makes this a safe and acceptable practice?

 

[steve66]

Re: Protecting Conductors versus Protecting Transformers

I think you are right about the OCP size.

I think the "special thing" might be 220 deg. C insulation on a transformer rated for 150 deg. C rise. Add 25 deg ambient, and that gives an extra 45 deg of play.

Also, large transformers are usually enclosed in a NEMA 1 (or better) enclosure that would help prevent fires even if it did "melt down".

Steve

 

[bob]

Re: Protecting Conductors versus Protecting Transformers

The maximum rating for the secondary protection device is 125% of this value, or 1042 amps. Despite the fact that this is more than 800, I am allowed to select the next higher rated device, and use a 1200 amp secondary breaker.

Charlie
You already know this but the 125% from table 450.3B is for the protection of the transformer not the conductor.
240.21.C.2.1b requires the secondary conductor to match the OC device. If the 125% rating does not match a std device you are alloweed to go up but
the size of the sec conductor must also be increased.
The overcurrent device located at the termination serves as overload protection not as overcurrent protection. That is why the conductors must be sized to handle the load.

 

[charlie b]

Re: Protecting Conductors versus Protecting Transformers

Thanks for the replies. I suppose it does make sense that the insulation system for a transformer would be stronger than that of a conductor. I came across this while doing a review of another engineer?s design. The numbers I cite in my question are essentially what the design shows. I initially balked at the notion of going to a higher ODCP setting, since this was over 800 amps. Then I read the rules again. I just wanted a ?sanity check? on my interpretation of the rules.

 

[finhead]

Re: Protecting Conductors versus Protecting Transformers

Also, if the secondary of the transformer is rated for 833 amps, that is the maximum load permissible. The 125% OCPD on the secondary not only reflects higher insulation ratings on the transformer windings, but also that fact that - like motors - transformers have inrush current that could cause nusance tripping if the OCPD were sized at 100%

 

[beanland]

Re: Protecting Conductors versus Protecting Transformers

Many transformers are designed to exceed their nominal rating for various periods of time. ANSI standards use a "loss of life" standard for large transformers. For example, in the "utility" world, operating transformers at 150% of their nominal rating for 4 hours is considered acceptable practice. But, then, utilities do not follow the NEC.

 

[mc5w]

Re: Protecting Conductors versus Protecting Transformers

Even if you are below 800 amps some devices are NOT listed for conductors that are slightly smaller than the overcurrent device. An example is a panelboard with a 200% rated neutral.

Also, using a conductor that is less than the rating of an overcurrent device will affect the calibration of a thermal magnetic circuit breaker. The performance and calibration warranty for SquareD's circuit breakers if the conductor is rated less than the overcurrent device.

 

[john m. caloggero]

Re: Protecting Conductors versus Protecting Transformers

Hi charlie, remember, the 125 % value in the secondary of the xformer is used in concert with the 250 % value of OCP in the primary and is associated ONLY with the xformer, therefore, the conductors on the load-side of the 250 % OCD would have to have an ampacity suitable for the calculated value. Then you can apply 240.4 (B) or (C). We protect the source first, then the output conductors applying two different rules, 450.3(B) and 240.4.