Breaker vs Fuse sizing

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gadfly56 said:
Well, perhaps I am displaying my ignorance of transformer design, but I did assume that if the original protection was 1 amp, at 10 amps the transformer would definitely be overloaded.
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Other then the current it takes to keep the field magnetized, the rest of current is determined by connected load, not by the overcurrent device.

Put a 100 amp fuse or breaker on it and if the connected load is only 1/2 amp it still only draws 1/2 amp. When too much load gets connected or there is a short circuit or ground fault is when the setting of overcurrent device actually becomes important.
 
kwired said:
Other then the current it takes to keep the field magnetized, the rest of current is determined by connected load, not by the overcurrent device.

Put a 100 amp fuse or breaker on it and if the connected load is only 1/2 amp it still only draws 1/2 amp. When too much load gets connected or there is a short circuit or ground fault is when the setting of overcurrent device actually becomes important.
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Which was exactly the situation I outlined:

"If the transformer requires a 1 amp fuse for protection and you replace it with a 15 amp fuse, what do you suppose will happen when you put a 10 amp load on the transformer?"
 
gadfly56 said:
Which was exactly the situation I outlined:

"If the transformer requires a 1 amp fuse for protection and you replace it with a 15 amp fuse, what do you suppose will happen when you put a 10 amp load on the transformer?"
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who says anyone is going to put a ten amp load on it?

in any case, if that was the purpose of primary OCPD on a xfmr, why does the code allow 250% OCPD on the primary.

In any case, what transformer ever requires a 1 amp rating on its primary OCPD.
 
We can play word games all day, bottom line is the connected load determines the normal actual operating current, not the overcurrent protection device.

The OPD limits how much current will flow and or how long it will flow.
 
petersonra said:
who says anyone is going to put a ten amp load on it?

in any case, if that was the purpose of primary OCPD on a xfmr, why does the code allow 250% OCPD on the primary.

In any case, what transformer ever requires a 1 amp rating on its primary OCPD.
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Um, the OP's?

If you have a 1 amp fuse protecting a device like a transformer. And you want to replace it with a miniature breaker. Do you size the breaker at 1 amp also and if so how do you determine the trip curve?
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petersonra said:
gadfly. what code would require a CB rated less than 15A?
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How did this become a question about code? The OP has a transformer, or actually, a device "like a transformer" that's protected by a 1 amp fuse. We don't know if it's hanging in a ceiling or inside a piece of equipment. We can only assume that someone had a good reason for limiting the current to 1 amp. We have no way of knowing what the loads on the transformer/device are, or what might happen if the load side were to wind up in a bolted-fault condition. How does it make sense to say you can replace a 1 amp fuse with a 15 amp circuit breaker, given all the unknowns?
 
Many small capacity transformers have either internal thermal protection or are impedance protected. Those could be on a 15 amp breaker and they still have overload protection.
 
The NEC is not a design manual.

If the NEC says it's 'OK' to use a 15 amp breaker in place of a 1 amp fuse, that just means is legal in jurisdictions that have adopted the NEC as their electrical rule book.

That doesn't make it a good practice or a good design.

One of my favorite code instructors had a saying that the electrician that followed the NEC to the letter was basically the worst electrician allowed by law. I think the saying is funny and to a certain extent, very true. Others, I am sure, will disagree.

Let's look again at the OP's question:

If you have a 1 amp fuse protecting a device like a transformer. And you want to replace it with a miniature breaker. Do you size the breaker at 1 amp also and if so how do you determine the trip curve?
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The OP didn't ask what the legal minimum was.

I would use the same amp breaker as the replaced fuse and try to get the trip curve from the manufacturer if possible. If a trip curve is a requirement, then I would look for a breaker that I could get that had that info.

Even before that, I would want to know why the client wants to do away with the fuse before I modified it for a breaker. There may be no good reason to do so, after a bit of discussion. If the reason is excessive tripping, then there is more to look at than putting in a breaker.
 
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