Motor Disconnects

[Alwayslearningelec]

I thought most motors got NON FUSED disconnects. What determines whether the disconnect needs to be fused or non fused? Thanks.

 

[raberding]

If the MOCP is less than the ckt amps - like maybe 10 or 15 on this 20A ckt
- then a fused disc could be used.

 

[Alwayslearningelec]

If the MOCP is less than the ckt amps - like maybe 10 or 15 on this 20A ckt
- then a fused disc could be used.

MOCP??? Motor overcurrent protective device?

 

[Alwayslearningelec]

On a side but related note how do you guys attach/mount a loose disconnect to a unit as shown? Just curious what hardware etc. is required .

 

[mayanees]

It's the fault current environment that determines when a disconnect must be fused. Unfused disconnects have a short-circuit rating of 10kA, unless rated with the breaker or fuse that feeds it.
I was involved in a design 15 years ago where the drywall manufacturing facility had around 100 disconnects for motors. Value engineering said replace any of the fused disconnects with unfused, and for that facility I think we ended up only replacing around 1/3 or 33 units, which is to say that the need for fused disconnects (>10kA) is more likely.

 

[ptonsparky]

The fused disconnects could be tapping from a feeder.

 

[raberding]

MOCP??? Motor overcurrent protective device?

Maximum Overcurrent Protective Device. Info on the AC equipment nameplate.

 

[augie47]

In addition to the above:
Some equipment calls for "fuse protection only" in which case there must be fuses in the supply.
We often used fusible as the units turned out to have different values that expected it was easier to find odd size fuses than breakers.

 

[Strathead]

It's the fault current environment that determines when a disconnect must be fused. Unfused disconnects have a short-circuit rating of 10kA, unless rated with the breaker or fuse that feeds it.
I was involved in a design 15 years ago where the drywall manufacturing facility had around 100 disconnects for motors. Value engineering said replace any of the fused disconnects with unfused, and for that facility I think we ended up only replacing around 1/3 or 33 units, which is to say that the need for fused disconnects (>10kA) is more likely.

This is a disputed concept, and quite in depth. I bet someone here can point you to some of the discussion here. But OP, he means when the SCCR (short circuit current rating) of the equipment is below the fault current calculated to be available at the unit, then you use fuses to reduce the available fault current. Again, some people come down that it is acceptable, others state only when the entire assemble is tested and listed for that reduction.

 

[Strathead]

In addition to the above:
Some equipment calls for "fuse protection only" in which case there must be fuses in the supply.
We often used fusible as the units turned out to have different values that expected it was easier to find odd size fuses than breakers.

Yeah, more specifically when the nameplates states, MOCP then you can use a breaker. Other nomenclature is often, Max fuse, or Max fuse or breaker, and I have seen unites that actually list different values for max fuse vs. max breaker.

 

[kwired]

Thread title suggests motors - which is art 430, but your attachments in OP suggest you are really asking about art 440 applications, which can sometimes be more specific at calling for fuses as overcurrent devices than art 430 applications are.