Fused disconnect protected by fuse?

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caryj

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Seattle, WA
So here is the situation:

An existing MCC bucket is a 250A fused switch.
I want to install a new Package Roof Top Unit, fed by this 250A feed and 250A wire and fused down to the necessary 175A. Can I install a 200A rated disconnect fed from 250A feed?

With the 175A fuses, there is no potential of more than 175A so the 200A rated parts and pieces of the disconnect are not in danger of seeing a normal condition load of more than 200A. And for that matter, the terminals of the disconnect will not accept fuses greater than 200A.

I know I can install 200A fuses in the MCC or a bigger disconnect, but do I have to? It seems that the 175A fuses are protecting the downstream wires as well as the disconnect.

What do you think?
 
It is permissible. Your theory is on target.
In fact, since you need 175 amp fuses, it is probably better than using fuse reducers.
 
1. OK, I'll bite... Where do you get a 250A rated fusible switch?
2. Why not use 175A fuses in the MCC and a non-fused switch at the unit?
3. What is the MCA and MOCP on the rooftop unit nameplate?
 
1. The fusible switch has 250A fuses in it, not necessarily a 250A rated switch (in other words prob 400A rated bucket with 250A fuses).

2. Partially because I want to know if I am compromising the NEC with the installation that I described.

3. MOCP = 175A, don't know the MCA off hand.

I have asked this question to a couple of electricians and an engineer and I get different answers. Most people just suggest replacing fuses at the MCC bucket but this is costly and undesirable because it would require the use of fuse reducers.

The engineer said that providing a 250A feed to a 200A rated piece of equipment , fused down or not, would violate the UL listing. I don't know if I buy that.

Is this a grey area or can I take a cut and dry answer away and use it on the next time this happens too?
 
caryj said:
So here is the situation:

It seems that the 175A fuses are protecting the downstream wires as well as the disconnect.

What do you think?
Click to expand...

The 200a disconnect more than likely needs to be HP rated to the Tables in 430.248,

430.249, and 430.250.
 
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