Breaker vs Fuse sizing

[BURGH]

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?

 

[petersonra]

The code generally does not require you use a cb rated less than 15 Amps.

 

[BURGH]

The end user is requesting CB instead of fuses

 

[kwired]

For something that has a surge of current upon energizing it you may need to look into differences in trip curves of the two different devices just to ensure the startup current doesn't cause nuisance tripping.

 

[petersonra]

For something that has a surge of current upon energizing it you may need to look into differences in trip curves of the two different devices just to ensure the startup current doesn't cause nuisance tripping.

It seems unlikely to me that a xfmr that had its 1A fuse replaced with a 15A CB would ever have trouble with nuisance tripping.

 

[K8MHZ]

The code generally does not require you use a cb rated less than 15 Amps.

The code is not a design manual.

 

[K8MHZ]

The end user is requesting CB instead of fuses

Then it should be up to that person to spec you a circuit breaker.

But generally, the amp rating of a fuse and a circuit breaker indicate the same amount of protection, so a 1 amp breaker would be the replacement for a 1 amp fuse.

 

[K8MHZ]

It seems unlikely to me that a xfmr that had its 1A fuse replaced with a 15A CB would ever have trouble with nuisance tripping.

Why would you replace a 1 amp fuse with a 15 amp circuit breaker?

 

[templdl]

The code generally does not require you use a cb rated less than 15 Amps.

I think then you would consider a UL1077 supplimentary protective device which is addressed in the NEC.

 

[kwired]

It seems unlikely to me that a xfmr that had its 1A fuse replaced with a 15A CB would ever have trouble with nuisance tripping.

Correct the chance of nuisance tripping is pretty much non existent in that situation, but I was talking about replacing a 1 amp fuse with a 1 amp circuit breaker, or any amp fuse with same amp circuit breaker for that matter.

 

[petersonra]

Why would you replace a 1 amp fuse with a 15 amp circuit breaker?

I might not. But in the instance the poster mentioned (transformer protection), it would be code permissible to do so.

There might be other issues that would convince me to use a lesser rating on the CB, such as wire sizes, but for transformer protection it would be acceptable.

 

[templdl]

If you provide a sec OCPD is that item that protects the transfotmrer. The pri OCPD then protects the upstream distribution system should the transformer fail, short circuit protection. If you are allowed to provide only a pri OCPD it is then that it is sized to protect the transformer as a result of an overload on the secondary.
As a rule of thumb a sec. W/O a neutral may be allowed to have pri. protection only such as 1ph2w and 3ph3w secondaries.

 

[gadfly56]

It seems unlikely to me that a xfmr that had its 1A fuse replaced with a 15A CB would ever have trouble with nuisance tripping.

Except you might slag the occasional transformer. Which also might be considered a nuisance. hmy:

 

[petersonra]

Except you might slag the occasional transformer. Which also might be considered a nuisance. hmy:

Why would it hurt the transformer any? UL product standards have been harmonized with the NEC so since the NEC allows it, the product standard has to accommodate it.

 

[gadfly56]

Why would it hurt the transformer any? UL product standards have been harmonized with the NEC so since the NEC allows it, the product standard has to accommodate it.

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?

 

[kwired]

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?

Nothing until it is overloaded.

 

[winnie]

Guys, it appears that we are talking about a fuse in a piece of utilization equipment that has a transformer, not OCPD for a transformer installed to NEC requirements.

-Jon

 

[petersonra]

Guys, it appears that we are talking about a fuse in a piece of utilization equipment that has a transformer, not OCPD for a transformer installed to NEC requirements.

-Jon

I don't see that changes the answer any.

The reality is that if the piece of utilization equipment that has the xfmr in it is listed to UL standards, the same rules would likely apply as if it were a NEC piece of equipment being installed. UL508a would allow for a 15A CB just as the NEC does.

Personally I would not get real excited about the rating of the CB. I am a fan of using the highest rated CB I can most times to reduce the incidence of nuisance trips. I also size fuses that way. Small transformers are infamous for tripping OCPD on the primary side.

 

[gadfly56]

I don't see that changes the answer any.

The reality is that if the piece of utilization equipment that has the xfmr in it is listed to UL standards, the same rules would likely apply as if it were a NEC piece of equipment being installed. UL508a would allow for a 15A CB just as the NEC does.

Personally I would not get real excited about the rating of the CB. I am a fan of using the highest rated CB I can most times to reduce the incidence of nuisance trips. I also size fuses that way. Small transformers are infamous for tripping OCPD on the primary side.

Is that because the designer spec'd too small a primary side OCPD or an undersized transformer?

 

[gadfly56]

Nothing until it is overloaded.

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.