Disconnects

[Alwayslearningelec]

Is there a "rule of thumb" as to when a disconnect switch needs to be fused or unfused?

A side note. I'm tapping an existing utility service in the building at the service end box and running feeders to a disconnect switch which feeds a main switchboard.

 

[LarryFine]

A disconnect can be just a switch. Tap rules may apply.

 

[PD1972]

I am not too familiar with what a service end box is supposed to be, but a considering that you are using the term "service" I am going to assume that this location on your electrical distribution system is pretty far upstream. Non-fused switches typically have a short circuit current rating of only 10kA. You may not be able to use a non-fused switch at your proposed location depending on what the maximum available fault current is at your tap location.

If there is upstream OCPD, you may potentially be able to series rate the non-fused switch for a higher SCCR. In my opinion, if your available fault current exceeds 10kA, just use a fusible switch to avoid the hassle (but this is coming from a non-cost-conscious-maximum-risk-mitigation consultant perspective).

 

[augie47]

From your post it appears you are "tapping" service conductors ahead of your existing service disconnect(s).
In that case the 240.22 "tap" rules do not apply as the applicable Article is Art230.
I can't think of a scenario where you would incorporate a non-fuse disconnect in the situation.
Most NF disconnects, other than"meter disconnects" are supplied from an overcurrent device and are commonly used only as a means of disconnect for equipment..

 

[infinity]

Is there a "rule of thumb" as to when a disconnect switch needs to be fused or unfused?

A side note. I'm tapping an existing utility service in the building at the service end box and running feeders to a disconnect switch which feeds a main switchboard.

That would be a service disconnect and it would require an OCPD.

 

[David Castor]

In the US, NEMA-rated unfused disconnect switches have a maximum short circuit withstand rating of 10,000 A. If the available fault current exceeds 10 kA, an unfused disconnect switch can't really be used.