Fuses 100% or 80% rated ?

[Davebones]

We have a oven that has been retrofitted with some new controls . It has a Wattlow SCR power controller controlling the heater element . The fuses that feed the SCR are ( Ferraz-Shawmut amp-trap HSJ60 amp fuses ) 60 amp . Are these type fuses rated for 100% of the load or should we only feed them at 80% of their rating ?

 

[__dan]

Don't know the answer to exactly what you asked.

What I do know is that with the SCR control, it may no longer act like a clean resistive load. With the SCR constantly chopping the sinewave, the load will be very noisy, and without substantial filtering to reduce harmonics, the branch circuit, conductors, breakers, will just run a lot warmer than you expect. Seen it before, surprisingly so, with SCR controlled heating elements. Oversized conduits and panels running much warmer.

So, if your question was if you should expect nuisance tripping, the fuses will not like constantly running warm.

Not quoting the code, I would have to look it up, but heating type appliances I would expect are continuous loads, so the source would be at 125%. Not sure if you are trying to use the exception for 100% rated equipment.

 

[templdl]

We have a oven that has been retrofitted with some new controls . It has a Wattlow SCR power controller controlling the heater element . The fuses that feed the SCR are ( Ferraz-Shawmut amp-trap HSJ60 amp fuses ) 60 amp . Are these type fuses rated for 100% of the load or should we only feed them at 80% of their rating ?

Isn't the wire sized for 125% of the continuous load + 100% of the none continuous load? The wire must be sized to carry that load. There, isn't the OCPD (fuse) sized to protect the wire? If the wire rating it is not a std. ampacity for a fuse size are you not allowed to round up to the next std. fuse sizer?
As such, wouldn't the fuse by default be sized at the very least at 125%?

 

[ron]

The fuses can accommodate the 100% rating. It is the place they are mounted and the wire terminations that need to be special tested for 100% operation

http://www.cooperindustries.com/con...se_Operation_at_100_Percent_Rated_Current.pdf

 

[don_resqcapt19]

The fuses can accommodate the 100% rating. It is the place they are mounted and the wire terminations that need to be special tested for 100% operation

http://www.cooperindustries.com/con...se_Operation_at_100_Percent_Rated_Current.pdf

And that is exactly the same for breakers, they are tested at 100%, but not in an enclosure and not with other breakers. The 80% is an allowance for the additional heat build up with an enclosure and other breakers that may also be producing heat.