Can you load a fused switch past 80% of its rating?

[samblv]

On a recent job walk, I noticed the nameplates on these fused switches at the main switchboard said, "Do not load past 80% of rating".

I know you figure LCL at 125% + non-LCL loads at 100% = less than 80% of the rating of the fuse? Or can you load the fuse past 80% as long as you have taken in consideration for LCL, Largest motor, etc.?

I was told by a manufacturer recently that the fuses are tested in free air and it is not recommended to load them past 80%.

Is this true?

 

[Smart $]

On a recent job walk, I noticed the nameplates on these fused switches at the main switchboard said, "Do not load past 80% of rating".

I know you figure LCL at 125% + non-LCL loads at 100% =? Or can you load the fuse past 80% as long as you have taken in consideration for LCL, Largest motor, etc.?

I was told by a manufacturer recently that the fuses are tested in free air and it is not recommended to load them past 80%.

Is this true?

TTBOMK, you can load a fused switch to over 80% of the fuse rating with non-continuous loads. This, however, does not assure the fuse will not blow under extreme condiitons of safe operation.

BTW, that would be 125% LCL + 100% LNCL ≤ 100% of fuse rating... not "= less than 80% of the rating of the fuse"

Do you have a manufacturer and model number for the referenced switches?

 

[jim dungar]

There is no UL Listed "enclosed" overcurrent protective devices (breaker or fuse) that is rated above 80% continuous unless specifically labled.

 

[haskindm]

As others have said the 80% requirement is only for continuous loads. Non-continuous loads may be up to 100% of the rating of the fuse or circuit breaker. Of course motors are a whole different subject and you would not want a motor load anywhere near 80% of the capacity of an overcurrent device. This is a fallacy that has permeated our industry and it is widely accepted that fuses cannot be loaded to more than 80%, but nowhere is this stated in the NEC, except for continuous loads. Also see Table 210.24 for multi-outlet branch circuits. As electricians we tend to try to simplify things. It is easier to say that an overcurrent device cannot be loaded passed 80% of its rating than to explain what a continuous load is and how it affects ampacity. This rates right up there with the statement "the white wire can't hurt you". It is often quoted, often believed, but not true!