MV transformer requirements

[Elect117]

I will break down for you my thoughts on it.

The short answer is yes. There is no code requirement that says you can not overload a transformer. Instead, you must not overload the conductor. So increasing the conductor can make you code compliant.

There are some design considerations to make as an engineer though.

1) Is this a facility that runs 24/7 at full load or is there down time over night? Most transformers that have time to cool overnight can be effectively overloaded with minimal or no loss of life expectancy. That is based on the temperature rise and insulating oil of the transformer and cooling method.

2) What will be the load based on 220.87? Take a years worth of metering info (even if it isn't a perfect match for the code) and calculate what you believe the demand will be. Be conservative a little and reach out to others if you are unsure.

3) Cost. Are you going to double the price of the project by suggesting increasing the size of the transformer? If they are prepared to upsize the transformer, it can be worth it if they see more expansions in the future. The cost today for the transformer and labor will be less than the cost 5 years from now.

I believe your transformer will be just fine to fulfill the 3000A board. I would up size the secondary runs for the new loads or add additional runs.

 

[electrofelon]

Sorry for the confusion. The NEC demand load is based on calculated load, not measured load.

If the demand load is 2753.7A, and a 2000kVA TX can only supply 2405A on the secondary, is that code compliant? Wouldn't they have to upsize to a 2500kVA?

And as has been mentioned, an NEC calculated load is often double the actual load. I don't really deal with NEC calcs anymore because of this.

 

[PD1972]

You'll find most utility companies undersize their MV transformers based on their own "factors" applied to the connected loads that you provide them. They essentially ignore the NEC 220 load calcs and allowable demand factors as they don't need to adhere to those requirements.

You state that the transformer is well past its end of life date. If the transformer were truly overloaded, I doubt it would be able to make it this long without insulation failure due to the constant overloading. If metering/usage data was obtained to determine the peak demand as defined by NEC 220, you will probably find that the transformer is appropriately sized for its load. This is essentially what a lot of other posters are alluding to.

 

[jim dungar]

Make sure you are keeping up with the 'oil' testing. This is often one of the best indicators of transformer life. But, testing needs to be done regularly, so you can develop a trend curve.

 

[Mr. Chippers]

I got the metered demand data over the past year, which has a peak demand of 1529kW, which at 0.9 pf comes out to 2043.4A. Well below what the transformer is rated for. So looks like a simple 1for1 replacement will be fine.

It's also a dry type TX FYI.

 

[petersonra]

Sorry for the confusion. The NEC demand load is based on calculated load, not measured load.

If the demand load is 2753.7A, and a 2000kVA TX can only supply 2405A on the secondary, is that code compliant? Wouldn't they have to upsize to a 2500kVA?

Where in the code is there any sizing requirements for transformers.

 

[jim dungar]

Where in the code is there any sizing requirements for transformers.

I think it is near sizing motors.