Transformer Breaker Question

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david luchini said:
Looking at your jpeg, I think you're generalizing a little bit. You don't need to have an adjustable magnetic trip capability on the primary c/b, though it might be required in the example you show. The coordination depends on the specific transfomer and circuit breaker that you are using.

I agree that the NEC rules for transformers don't necessarily protect the transformer, but, if you do proper coordination, you probably could find a transformer/breaker combination that would protect the transformer from through-faults.
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Your correct, you wouldn't have to have an "adjustable breaker", but the breaker would need to have a fixed instantaneous trip that was at least 10X the thermal, otherwise it puts you below the inrush.

As far as generalizing, isn't that exactly what the NEC does, is give general guidelines that many people use as the design basis? That's exactly why I showed it as general as possible.

Of course, I can pick breakers that more closely match what I need and get it to coordinate better; perhaps fuses would be better on the HV side. I simply picked standard catalog equipment and plotted it, which is what many are going to do in the field without understanding the consequences. Which, is the whole reason of my point - AWARENESS, i.e. don't just blindly follow the NEC.
 
mull982 said:
I believe this would only be the case if the transformer primary OCPD was the same the feeder OCPD (primary breaker located remotely from transmormer)

But what if the primary OCPD was located right at the transformer. Then techincally this could not protect the feeders because this would fall under the tap rule. So then you would have to protect the feeder cables upstream where they origanate. This was the breaker sizing that I was referring to. Since this breaker would simply be protecting the cables. Although I would imagine it should be at least the same size if not bigger than the transformer primary OCPD.
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If you have a feeder supplying other loads, from which you tap off for a transformer with its own primary OCPD, then the feeder OCPD must be sized for the load on both the transformer and the other loads. The OCPD would be sized for not less than 100% of non-continuous load plus 125% of continuous load.
 
kingpb said:
Your correct, you wouldn't have to have an "adjustable breaker", but the breaker would need to have a fixed instantaneous trip that was at least 10X the thermal, otherwise it puts you below the inrush.
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No, it depends on the specific transformer. If you had a 75kVA transformer with an inrush of 5.3xprimary rated current (square d #EE75T3HF) = 478A inrush. A 225A c/b with a magnetic of 2.5x the thermal would handle the inrush.
 
david luchini said:
If you have a feeder supplying other loads, from which you tap off for a transformer with its own primary OCPD, then the feeder OCPD must be sized for the load on both the transformer and the other loads. The OCPD would be sized for not less than 100% of non-continuous load plus 125% of continuous load.
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I was referring to a case where there is a breaker in an MCC protecing feeders out to the primary of a transformer. (transformer is only device on circuit) And then at the transformer primary you had a breaker for the primary of the transformer (redundant I know).

So for this case, I was referring to the sizing of the breaker in the MCC.
 
mull982 said:
I was referring to a case where there is a breaker in an MCC protecing feeders out to the primary of a transformer. (transformer is only device on circuit) And then at the transformer primary you had a breaker for the primary of the transformer (redundant I know).

So for this case, I was referring to the sizing of the breaker in the MCC.
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Now I see. Yes, that would be redundant - not sure why you would do that. I think you could make that breaker any size you wanted, and match the feeder size to the breaker.

If the breaker at the MCC was smaller than the breaker at the transformer, I'd think the MCC breaker would be considered as protecting the transformer, assuming it coordinated with the downstream c/b so that it would trip first. But it could be the same size or it could be larger.

But why would you want to install it this way. The breaker in the MCC is sufficient to protect the transformer (sized per 430.3.) Why add another breaker into the circuit?
 
david luchini said:
No, it depends on the specific transformer. If you had a 75kVA transformer with an inrush of 5.3xprimary rated current (square d #EE75T3HF) = 478A inrush. A 225A c/b with a magnetic of 2.5x the thermal would handle the inrush.
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Correct again, it does depend on the transformer. So, you have shown twice that blindly following the NEC guidelines does not guarantee a good installation. The proper coordination of transformer protection is based on the specific equipment selected, not what you can apply from a general Table in the NEC.

Thanks for helping prove the point.
 
david luchini said:
Now I see. Yes, that would be redundant - not sure why you would do that. I think you could make that breaker any size you wanted, and match the feeder size to the breaker.

If the breaker at the MCC was smaller than the breaker at the transformer, I'd think the MCC breaker would be considered as protecting the transformer, assuming it coordinated with the downstream c/b so that it would trip first. But it could be the same size or it could be larger.

But why would you want to install it this way. The breaker in the MCC is sufficient to protect the transformer (sized per 430.3.) Why add another breaker into the circuit?
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I agree they are redundant I'm just asking "what if."

I guess if only one breaker is used at MCC and used as primary transformer breaker then you would size it at 125%, 250% or whatever to handle inrush and the feeder cables would have to be sized to be protected by the breaker.

If using 2 breaker as mentioned then MCC breaker technically only has to protect feeder cables which only need to be sized for transformer primary full load current. However as mentioned it would be best to increase the MCC breaker and therefore cable size to match or exceed the primary transformer breaker so that they are coordinated.
 
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