Transformer OCPD

[304sparky]

Hey guys,

I’m trying to completely wrap my head around transformer OCPD. I was watching a video on the calculations for sizing over current protection, and here is the example presented.

480V single phase primary, 12kva load
120V single phase secondary, 12kva load

With that said the FLA for the secondary is 100 amps, and the primary FLA being 25 amps.

In the explanation, the FLA on the primary was multiplied by 250% the get the maximum OCPD. He then explained that you don’t HAVE to choose maximum OCPD but you can’t go too low because of the influx at the start up of the transformer.

My question is, why would I choose to not use maximum OCPD? Also, how low can I go without causing overcurrent at start up?

 

[winnie]

The primary circuit conductors get sized to match the primary OCPD. So using a larger OCPD than necessary will cost more.

The minimum OCPD that you can use without problems of tripping on inrush depends on lots of factors: the supply available short circuit current, the circuit impedance, the breaker trip curve, the transformer inrush characteristics, ....

We are just discussing this in another thread. A 125% primary OCPD will probably work for most installations.

Jon

 

[Elect117]

I agree with Winnie. I would also like to add, OCPD is a protective device. Going straight to the maximum can lead to some long term issues. For example, overloading the transformer intentionally reduces the useful life.

You might also run into issues with 240.21(C) on the sizing of the secondary conductors. Your primary OCPD*XFMR Ratio*TAP% (based on tap rule) less than Ampacity of Secondary conductors. So if you are using 250% on the primary side, you can see how the size of the secondary conductors also increases.

 

[topgone]

I agree with Winnie. I would also like to add, OCPD is a protective device. Going straight to the maximum can lead to some long term issues. For example, overloading the transformer intentionally reduces the useful life.

You might also run into issues with 240.21(C) on the sizing of the secondary conductors. Your primary OCPD*XFMR Ratio*TAP% (based on tap rule) less than Ampacity of Secondary conductors. So if you are using 250% on the primary side, you can see how the size of the secondary conductors also increases.

Not in my experience. Overloading can only occur a few hours of the day in the utilities sector. Maybe you are thinking about fixed 24/7 loads of a manufacturing plant. Utilities intentionally allow their transformers to take in overloads for certain hours of the day. It would be more cost-effective to run their transformers hot for a period, the succeeding light loading periods will surely return the transformer temperature to normal operating temp without sacrificing loss of useful life. You cannot achieve those if your overcurrent protection device settings are just a tad over the rated full-load amps.

 

[Tip DS]

Hey guys,

I’m trying to completely wrap my head around transformer OCPD. I was watching a video on the calculations for sizing over current protection, and here is the example presented.

480V single phase primary, 12kva load
120V single phase secondary, 12kva load

With that said the FLA for the secondary is 100 amps, and the primary FLA being 25 amps.

In the explanation, the FLA on the primary was multiplied by 250% the get the maximum OCPD. He then explained that you don’t HAVE to choose maximum OCPD but you can’t go too low because of the influx at the start up of the transformer.

My question is, why would I choose to not use maximum OCPD? Also, how low can I go without causing overcurrent at start up?

The 250% rule assumes you are also installing secondary winding transformer breakers/fuses. Actually, it isn't assuming, it's mandating. There is another rule that allows you to only install a breaker on the primary side - it requires a max breaker size of 125% rated primary current.

In some scenarios, you can let the transformer protection double as the conductor protection as well. So, if you put a gigantic breaker on the primary of the transformer, then you also have to provide secondary transformer breakers, and probably another set of breakers to protect the wiring. - If you are installing the transformer for a specific purpose (say you just need a transformer to operate a 480VAC skid in a 600VAC environment.) Let's further say you only NEED 15kVA of capacity, but the only transformer you can get on-site within your time budget is a 30kVA. Do you really need (or even want) to put big 70A breakers on the primary, 45A secondary, then have to put smaller 20A breakers in to protect your wiring to the skid? OR would you be happier putting the 125% breaker on the primary (35A), not having to install a secondary breaker, then use your 20A breaker for the wiring (which you would have had on the skid anyway, as that's the rated value of the skid.)?