Protecting a Fused Disconnect

charlie b

Moderator
Staff member
Location
Seattle, WA
Occupation
Electrical Engineer
Disconnect switches are not included in Table 240.3. They do have industry standard ratings (i.e., 30, 60, 100, 200, 400, etc. amps), and we select one that is higher than the calculated load. But do we need to protect the switch itself against overcurrent, using an upstream OCPD that is no higher than the disconnect switch's rating? Here are the specifics:
  • Existing installation: A 400 amp breaker and 500 MCM conductors provide power to an old rooftop HVAC unit that is about to be replaced.
  • The new HVAC unit has an MCA of 152 amps and an MOCP of 175 amps. The manufacturer recommends dual element 175 amp fuses.
  • New installation: We will retain the 400 amp breaker and 500 MCM conductors, install a new disconnect switch with the recommended fuses.
  • Question: Does the disconnect switch have to be rated at 400 amps, given the upstream OCPD is 400 amps, or can it be rated at 200 amps, given the downstream load is lower than 200 amps?

I could not find the answer in article 404 or 240, and I am allergic to article 430.
 

jap

Senior Member
Keep in mind the lug size of a 200 amp disconnect if that's what you opt to go with.


JAP>
 

jap

Senior Member
I'd go with a 400 and fuse reduce it just for the wire bending space and keeping the full amperage to the roof.
No need to choke it.

But that's just me.


JAP>
 

charlie b

Moderator
Staff member
Location
Seattle, WA
Occupation
Electrical Engineer
I'd go with a 400 and fuse reduce it just for the wire bending space and keeping the full amperage to the roof.
The EC is telling me that the difference between a 200 amp and a 400 amp fused disconnect is about $1,500. At the moment, I am planning on telling him that a 200 amp rated disconnect is acceptable. Many thanks to all for your responses.

 

jap

Senior Member
The EC is telling me that the difference between a 200 amp and a 400 amp fused disconnect is about $1,500. At the moment, I am planning on telling him that a 200 amp rated disconnect is acceptable. Many thanks to all for your responses.

Then when the wire wont fit in the lugs the clock starts ticking and that cost along with the replacement lugs, labor or pin adapters to make what you've already got, generally gets thrown back at the person who designed it to begin with...

Oh well.

JAP>
 

Cow

Senior Member
Location
Eastern Oregon
I would not use a 200 amp disconnect in this installation. I would use a 400 amp with fuse reducers. Like Jap said, the lugs in the 200 amp disconnect will be too small and it would be silly trying to bend 500's into the space of a 200 amp disconnect.
 

Electrobe

Member
You could also Use the properly sized Polaris type connectors to convert to 4/0 and use the 200 amp switch. Will need to follow the feeder tap rules of 240.21. The Polaris connectors could be installed in a separate enclosure and route the 4/0 to the switch. I think this would need to be the 25ft rule but would need to verify.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
The EC is telling me that the difference between a 200 amp and a 400 amp fused disconnect is about $1,500. At the moment, I am planning on telling him that a 200 amp rated disconnect is acceptable. Many thanks to all for your responses.
I would use the 200 amp disconnect and if there is an issue with wire bending space for the 500 kcmil in the 200 amp disconnect, I would use a splice box and 200 amp tap conductors on the line side of the 200 amp disconnect. I can do that for way less than $1500.
 

kwired

Electron manager
Location
NE Nebraska
I would use the 200 amp disconnect and if there is an issue with wire bending space for the 500 kcmil in the 200 amp disconnect, I would use a splice box and 200 amp tap conductors on the line side of the 200 amp disconnect. I can do that for way less than $1500.
In the OP's situation all that is needed is 152 amp conductors (unless feeder tap conditions would require more then that, but that is not likely from a 400 amp feeder) and 175 amp fuses in the 200 amp disconnect.
 

jap

Senior Member
I would use the 200 amp disconnect and if there is an issue with wire bending space for the 500 kcmil in the 200 amp disconnect, I would use a splice box and 200 amp tap conductors on the line side of the 200 amp disconnect. I can do that for way less than $1500.
So your the guy who does that kind of stuff... :)

JAP>
 
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