Sizing Transformer protection

[sellakc@gmail.com]

In article 450, sizing of primary and secondary protection for a transformer is based upon voltage and the transformer voltages and rated impedance. If I have a 13.8kV primary/480V secondary transformer that has dual ratings, as in 1500 KVA/1950KVA, depending on whether or not fans are operational and the oil cooler is functional, which transformer-rated current do I use to calculate primary protection? The table is based upon maximum current of the transformer. This transformer has two primary currents listed. Others have multiple ratings based upon cooling classes. Any guidance here? Is it appropriate to just use the maximum current rating as the table in 450 says (in other words, the highest listed on the transformer)?

 

[electrofelon]

In article 450, sizing of primary and secondary protection for a transformer is based upon voltage and the transformer voltages and rated impedance. If I have a 13.8kV primary/480V secondary transformer that has dual ratings, as in 1500 KVA/1950KVA, depending on whether or not fans are operational and the oil cooler is functional, which transformer-rated current do I use to calculate primary protection? The table is based upon maximum current of the transformer. This transformer has two primary currents listed. Others have multiple ratings based upon cooling classes. Any guidance here? Is it appropriate to just use the maximum current rating as the table in 450 says (in other words, the highest listed on the transformer)?

I dont have any experience with auxiliary transformer cooling, but I would think the applicable FLC would just be whatever the transformer is set up/provided with.

 

[sellakc@gmail.com]

I have been told by several people to always size at the maximum rating of a transformer that has dual ratings when auxiliary cooling is provided. The concept makes sense to me on cable sizes, or even larger cables for efficiency and lower impedance, but protection would seem to widely vary by application. The NEC doesn't seem to address protection of these types of transformers directly. Perhaps it is more of an engineering decision.

 

[jim dungar]

I have been told by several people to always size at the maximum rating of a transformer that has dual ratings when auxiliary cooling is provided.

Is this a new design or is the transformer already purchased?

You should size based on the expected load. If load is expected to use the full kVA, then you need conductors able to handle that load. But, there are situations where the extra capacity was never intended to be used, rather the cooling system was installed to address environmental issues versus transformer life. There are also times when the extra capacity is actually more than will ever be required (i.e. the secondary gear is only 2500A).

 

[sellakc@gmail.com]

This was a case of new design. Normally I would agree about expected load. This is a unique design for a site using fuel cells as the main power supply to the site (off-grid). The owner is requesting maximum usage of transformer KVA to reduce install costs. It seems to not matter about the reduced life span of the transformer with such loading, but the main issue is primary/secondary protection. The conversation has gone back and forth about good engineering practices, and now is on "well, what is code-legal?" I cannot find an NEC statement saying you can or cannot size protection based upon max rating of such transformers. The site has another transformer that is 3000 kVa but 4200 kVa with oil radiators and fans running. Both transformers have multiple protection schemes - temp shutoffs, pressure shutoffs and more.

So, with no specific code statements fire regarding the sizing of fuses/breakers based on the upper rating, we have contacted the transformer manufacturer, who currently is also hem-hawing on a specific answer. Is this just commonly assumed to be based upon "good engineering best practice?" It sure seems so.

I fully agree about the situations and extra capacity limitations. This company seems to not be concerned, as these factors have all been mentioned. Very unique installation and risky company, in my opinion. Long term, it will be very costly, especially based on the sizes of these transformers.

 

[jim dungar]

Don't over complicate it.
The NEC says to protect transformers based on their output. The NEC does not say how to determine the transformer output. So if you, and the manufacturer, say the max output is 1950kVA then that is what it is. If the expected load is 1950KVA then that is the amount of conductors you need to have.