Transformer secondary OCP?

[DeeKay]

Hey guys, having some trouble understanding the requirements for overcurrent protection on the secondary of a transformer. I read through 240.21 and 450.3 and some stuff makes it seem like I don't need OCP on the secondary and some stuff makes it sound like I do. I'm leaning towards not needing it but want to be sure.

Here's what I'm looking at. This will be supplying power to a connex box.
40A 480V breaker feeding a 15kVA transformer about 100' away.
Transformer feeds a 100 amp main breaker 240/120 load center. Do I even need a main breaker load center since the main breaker would only be used as a service disconnect or could I get away with main lug only?

Distance between the secondary and the load center is 3 feet at most. This is a secured industrial site so it's considered a "supervised location"

As always, thanks for the help.

 

[Bert_]

If you have protection on the secondary you can run a bigger breaker on the primary.

I've never had much trouble with inrush on transformers and often just do primary protection and end up splitting the secondary to multiple panels.

 

[DeeKay]

Thanks that was kind of my understanding too, I already have the 40 amp breaker so I'll just go that route then. Not too worried about inrush since this was sized pretty generously in the first place.

I don't think I need a main breaker load center right? I could get away with just a main lug load center and use the 480 breaker as the disconnect. Does that sound right?

 

[Terry D]

Your only good for 62.5 amps on the secondary of that 15 KVA single phase transformer. With just protecting the primary with a 40 amp breaker, You have the possibility to overload it. I would use a 30. Even at 125% your at 39 amps for primary protection.

 

[bjcouche]

OK, to answer your question, I believe you can get away with only primary protection because your transformer primary current at 125% is just under 40A, and you are able to use the next higher standard ampacity breaker.
Me personally, I like to always have both primary and secondary protection on transformers. That way if I have to increase the primary protection from 125% up to 250%, I already have the needed secondary protection. I've seen LOTS of instances, especially with control transformers (1-3KVA), where the primary was fused at 250% with slow blow fuses, and on power up of the transformer, with no load on the secondary, the primary fuses would sometimes blow due to transformer inrush. The solution can be to change the type of slow blow fuse to one that blows "slower". This is usually less of an issue with breakers though as they are sooooooo slooooooow to begin with, compared to fuses.

 

[DeeKay]

Your only good for 62.5 amps on the secondary of that 15 KVA single phase transformer. With just protecting the primary with a 40 amp breaker, You have the possibility to overload it. I would use a 30. Even at 125% your at 39 amps for primary protection.

I'm not too worried about overloading the this thing, company requires us to oversize transformers 25-30% of the expected load, so while it's possible it could be overloaded, it's really unlikely here.
I thought standard practice was to size breakers at 125% which is why I'm using the 40 amp, 125% of 31.25 is 39ish, next closest standard is 40. We have another 15kVA in my MCC with a 40 on the primary and an 80 on the secondary. Am I all wrong on my thinking about this?
I'm really trying not to buy another $800 breaker if I can avoid it haha

 

[Terry D]

I'm not too worried about overloading the this thing, company requires us to oversize transformers 25-30% of the expected load, so while it's possible it could be overloaded, it's really unlikely here.
I thought standard practice was to size breakers at 125% which is why I'm using the 40 amp, 125% of 31.25 is 39ish, next closest standard is 40. We have another 15kVA in my MCC with a 40 on the primary and an 80 on the secondary. Am I all wrong on my thinking about this?
I'm really trying not to buy another $800 breaker if I can avoid it haha

Its all about your connected load to it. If it is already oversize fot the loads, you will be fine. I guess i read it wrong, i was thinking you needed 100 amps or close to it

Sent from my SM-G960U using The Garage Journal mobile app

 

[DeeKay]

Its all about your connected load to it. If it is already oversize fot the loads, you will be fine. I guess i read it wrong, i was thinking you needed 100 amps or close to it

Sent from my SM-G960U using The Garage Journal mobile app

Oh ok cool, thanks Terry.
By the way is that picture from St. Louis? I grew up in south city, I always loved those old houses with the round turrets.

 

[Terry D]

Oh ok cool, thanks Terry.
By the way is that picture from St. Louis? I grew up in south city, I always loved those old houses with the round turrets.

Its just a picture off the internet. Not sure where it is from. I live in south city. Small world