Protecting a Fused Disconnect

[charlie b]

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.

 

[iwire]

No you do not have to protect the fused disconnect.

 

[jim dungar]

If you have to protect fused disconnects, then how can they be used as service entrance devices?

 

[electrofelon]

If you have to protect fused disconnects, then how can they be used as service entrance devices?

Panel boards must be protected (408.36) , but there is an exception for mlo used as service equipment.

 

[jap]

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


JAP>

 

[jap]

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>

 

[jim dungar]

Panel boards must be protected (408.36)

Can you quote similar language for a switch?

Article 240 specifically requires a disconnect on the supply side of fuses, except for a few items at some services (e.g. fuses for metering equipment)

 

[charlie b]

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.

 

[electrofelon]

Can you quote similar language for a switch?

Article 240 specifically requires a disconnect on the supply side of fuses, except for a few items at some services (e.g. fuses for metering equipment)

I was just nitpicking your pedagogical logic

 

[jap]

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>

 

[Electrobe]

This would be a feeder tap and the rules in Section 240.21 would apply to this installation.

 

[Cow]

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]

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]

This would be a feeder tap and the rules in Section 240.21 would apply to this installation.

The tap would start on the line side of the disconnect, assuming that you are running the 500s to the disconnect.

 

[don_resqcapt19]

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]

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]

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>

 

[don_resqcapt19]

So your the guy who does that kind of stuff...

JAP>

My job is to provide a safe, efficient, and cost effective installation for the client...so yes I do that kind of stuff.

 

[ggunn]

My job is to provide a safe, efficient, and cost effective installation for the client...so yes I do that kind of stuff.

You left out "NEC compliant".

 

[kwired]

You left out "NEC compliant".

He didn't say to what standards "safe" actually means. All kidding aside sometimes NEC may not apply.