225A fused disconnect

[jim dungar]

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 if there was a 175A breaker mounted upstream of the 200A switch you would consider it as not being protection.

All UL listed switches have been tested with specific fuse types, such as Class H, R, and J which are typically time delay styles.

There is almost no general purpose protective device which operates instantaneously, in fact most overcurrent protection devices will allow 133% of current to flow for many 10s of minutes.

 

[hhsting]

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

Would you say 20 to 25 minutes short time? Because time delay 175A fuse at 180A can take that long

 

[Dennis Alwon]

Would you say 20 to 25 minutes short time? Because time delay 175A fuse at 180A can take that long

It will not affect the wire at all. Do you really think that a conductor rated at 175 is going to melt with 180 amps for 25 minutes?

 

[hhsting]

It will not affect the wire at all. Do you really think that a conductor rated at 175 is going to melt with 180 amps for 25 minutes?

No I meant the disconnect enclosure which is rated at 200A if the current moves up to 210A

 

[don_resqcapt19]

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?

There is no special rating for a breaker that is based on the rating of the upstream OCPD. The same exact breaker is installed in a panel with a 100 amp main as in one with a 400 amp or even larger main.

If what you are trying to say is correct, there would be no way to ever make a 240.21(B) installation, because based on what you are saying the load end OCPD equipment would have to have a rating equal to the upstream OCPD.

As far as it not melting or burning, the fuse in the disconnect limits the running current to the rating of the fuses. Even 250 amps of current on the 4/0 feeder conductors would not reach a temperature that exceeds 90°C. You can run 629 amps through those conductors without their temperature exceeding 200°C. While that will damage the conductor insulation, it is no where near a temperature that would cause the metal parts of the disconnect to melt.

The only things that will damage your disconnect would be poor connections made by the installer, or some damage that causes a high current short circuit or ground fault. With those types of fault there would be little to no difference in the damage at the disconnect between a 200 amp or a 225 amp upstream OCPD.

 

[retirede]

This may be the most bizarre thread I’ve ever seen here!

 

[Tulsa Electrician]

See if this helps a little.
Just an example

 

[LarryFine]

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

Not before the 175a fuses start blowing. They're designed to melt first.

 

[augie47]

This may be the most bizarre thread I’ve ever seen here!

consider the source

 

[jim dungar]

It seems like we may need to take a break on this thread.