Disconnects

[patpappas]

What designates using a fused disconnect vs. a non-fused.

 

[480sparky]

Whether or not you need a fuse.

Some equipment have only fuses on their nameplate. If you feed it from the panel with a breaker, you'll need to install fuses to protect the equipment.

Other times, it's a job specification. Many times, EEs upsize wires feeding large loads to reduce voltage drop. You may see an RTU fed with #2 on a 100a breaker, but 80-amp fuses in the disco. The 100a breaker protects the wiring, the 80a fuses protects the RTU.

 

[weressl]

What designates using a fused disconnect vs. a non-fused.

If you have branch circuit overcurrent protection for an apparatus that is away from the location and out of sight, you may want to use a non-fused lockable disconnect to satisfy the OSHA Lockout requirements, and of course the Code.

 

[pete m.]

At times it may also be the necessity for current-limitation which some fuses provide to reduce the available fault current in a specific application.

Pete

 

[patpappas]

When using a disconnect on a dry 75kVA transformer primary, is it for over current or short circuit, I can not find anything for short circuit in 450. I would think a breaker would cover the over-current.

 

[haskindm]

The question is not, is it for over current or short circuit, but is it strictly a disconnect or is it there to provide protection of the equipment? If it is there for protection, it should be fused. If it is just there for disconnection of the load an unfused disconnect is fine.
For example some air conditioners are marked "Maximum Fuse Size". If the distribution panel uses circuit breakers this means that a fuse must be provided so fused disconnects are often installed. If the unit is marked "Maximum Fuse or HVACR Rated Circuit Breaker" the breaker in the distribution panel is sufficient and an unfused disconnect may be installed at the unit to disconnect it for servicing.

 

[ron]

Sometimes the available fault current at the switch will be higher than a non-fused can handle (withstand). Most non-fused are only rated to 5,000A. Fused switches get you up to 200,000A.