225A fused disconnect

[Dennis Alwon]

I can feed a 200 amp panel with a 1000 amp breaker as long as the wire going to it is properly sized AND the overcurrent protective device in the panel is sized to protect the panel. So a 200 amp panel that has a 200 amp main breaker is protected because even if 1000 amp overcurrent protective device is ahead the breaker in the panel will trip before the current gets too high for the panel (or disconnect). The circuit is limited by the 200 amp (or 175 amp) fuse or breaker

 

[kwired]

I can feed a 200 amp panel with a 1000 amp breaker as long as the wire going to it is properly sized AND the overcurrent protective device in the panel is sized to protect the panel. So a 200 amp panel that has a 200 amp main breaker is protected because even if 1000 amp overcurrent protective device is ahead the breaker in the panel will trip before the current gets too high for the panel (or disconnect). The circuit is limited by the 200 amp (or 175 amp) fuse or breaker

But we are trying to convince OP that the switch in a fused disconnect doesn't need supply side OCPD at or below the rating of the unit.

Pretty certain there isn't any NEC section that requires OCP for the supply side to the switch. It inherently is protected from overloading by the fuse(s) though.

 

[jim dungar]

How? 225A breaker protects 200A fused disco? Its 200A breaker is 225A???

How does a 200A main breaker protect a 20A branch breaker?

An feeder circuit overcurrent protective device protects its load side conductors not the device at the end of the conductors.

 

[hhsting]

How does a 200A main breaker protect a 20A branch breaker?

An feeder circuit overcurrent protective device protects its load side conductors not the device at the end of the conductors.

Below is the situation:

How is the 200A disco line side fuse 175A terminals, contact terminals make when mounted on enclosure disco protected by 225A trip breaker?

 

[Rock86]

Below is the situation:

How is the 200A disco line side fuse 175A terminals, contact terminals make when mounted on enclosure disco protected by 225A trip breaker?

As stated... the line side is protected by the 225A circuit breaker.

 

[kwired]

Below is the situation:

How is the 200A disco line side fuse 175A terminals, contact terminals make when mounted on enclosure disco protected by 225A trip breaker?

The terminals and the switch do not require any specific ocpd level on the supply side of them. The fuses installed will still protect them from overloading, the supply side device will still protect them in short circuit/ground fault situations.

 

[hhsting]

The terminals and the switch do not require any specific ocpd level on the supply side of them. The fuses installed will still protect them from overloading, the supply side device will still protect them in short circuit/ground fault situations.

I see line side termination of the fuse is like tap conductors so that should be protected by 240.21(B)(1) less than 10 feet. 4/0awg copper conductor is being tapped by 175A fuse line side terminal.

But I dont follow why switch itself is not need protection?

 

[jap]

Its a 200A rated disconnect. I could install 30A fuses if I wanted. If I install 225A fuses, now there is a problem. Read section 240.

And that's what he's asking.

You cant install 225 amp fuses in a 200 amp disconnect, and, for good reason.

The disconnect is not rated above 200 amps.

JAP>

 

[don_resqcapt19]

You need to look at this like you look at a motor circuit. Think of the upstream breaker as being the branch circuit short circuit and ground fault protective devices and the fuses as being the motor overload protection.
The 225 amp breaker will provide the protection for a short circuit or ground fault on the conductors between the breaker and the disconnect and for the same type of fault on the line side of the fuses within the disconnect. The fuses will provide overload protection for the complete circuit.
There is no issue here.
Switches only need to have a current rating equal to or greater than the rating of the load that they supply.

 

[jim dungar]

I see line side termination of the fuse is like tap conductors so that should be protected by 240.21(B)(1) less than 10 feet. 4/0awg copper conductor is being tapped by 175A fuse line side terminal.

But I dont follow why switch itself is not need protection?

What protection do you think it needs?
Short circuit protection needs to come from the 225A feeder device or the impedance of the 200A conductor. Overcurrent protection is provided by the 175A fuses in the switch (there is no way for current to flow through the switch without flowing through the fuses).

 

[electrofelon]

But I dont follow why switch itself is not need protection?

Because there is no such requirement. You need to get this idea out of your head that everything is required to be protected at or below it's rating by an OCPD. Conductors and panel boards do, but not much else does.

 

[LarryFine]

But I dont follow why switch itself is not need protection?

Because the switch itself is not a (225 amp) load. It will never be asked to break more than its rating.

Switches are rated for the current they can safely interrupt, not for their continuous load capacity.

 

[hhsting]

Because the switch itself is not a (225 amp) load. It will never be asked to break more than its rating.

Switches are rated for the current they can safely interrupt, not for their continuous load capacity.

What about switch enclosure? Wont it melt like lets say 210A?

 

[kwired]

Because the switch itself is not a (225 amp) load. It will never be asked to break more than its rating.

Switches are rated for the current they can safely interrupt, not for their continuous load capacity.

Motor rated switches including these typical fused disconnect type, kind of sort of are rated both those ways.

 

[don_resqcapt19]

What about switch enclosure? Wont it melt like lets say 210A?

Do you have any idea how temperature rise from current works?

How is your question any different from the installation of a 20 amp breaker on the bus of a panelboard with a 400 amp OCPD?

 

[hhsting]

Do you have any idea how temperature rise from current works?

How is your question any different from the installation of a 20 amp breaker on the bus of a panelboard with a 400 amp OCPD?

I meant melt or burn. Speaking of that i bet the 20A breaker enclosure would be rated for 400A in that panelboard.

Anyways why in your opinion it would not melt or burn in disco case?

 

[jim dungar]

What about switch enclosure? Wont it melt like lets say 210A?

How will it ever see more current than the 175A fuses that are installed in it?

 

[augie47]

IMO (and based somewhat on experience), if you subjected a 200 amp fusible disconnect to a 210 amp load over long period of time there might be some heat damage (internal) especially with a general duty switch. (Not much different from subjecting a breaker to continuous overload).
As noted above, in your specific case the 175 amp fuse would prevent any switch damage

 

[hhsting]

How will it ever see more current than the 175A fuses that are installed in it?

These may not be instantaneous fuse you know. Fuses may have Time current curves where trip is delayed. So that can see more than 175A

 

[Dennis Alwon]

These may not be instantaneous fuse you know. Fuses may have Time current curves where trip is delayed. So that can see more than 175A

So what the wire and breaker can handle that short time period.