Line side rating of disconnect switch

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rbinford

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We have a job where a roof-top HVAC unit is going to be replaced. The drawings show that we are to use the existing feeder to feed the new unit. The existing feeder is 3-#4 CU conductors, from a 3 pole 70 amp circuit breaker, into a 100 amp fused switch at the unit with 70 amp fuses. The new unit will be fused at 25 amps with a 30 amp fused disconnect. Is it ok to feed the 30 amp rated disconnect from a 70 amp breaker? I would think that the disconnect would need to be a 100 amp disconnect. I can't find it in the NEC. A couple of years ago we had an inspector make us install a disconnect with the same rating as the feeder and we ended up using fuse reducers in a similar circumstance.
 
I think not. Current will pass through the disconnect portion of the fused disconnect, before passing through the fuses. Can you keep the existing 100 amp fused disconnect and install 30 amp fuses?
 
Taps are small conductors attached to larger conductors and protected at their ampacity by a device at the load end. This would be a 70 amp breaker serving as the overcurrent device for both a conductor rated at 70 amps and a disconnect switch rated at 30 amps. I don't think the situation is very similar at all.
 
charlie b said:
Taps are small conductors attached to larger conductors and protected at their ampacity by a device at the load end. This would be a 70 amp breaker serving as the overcurrent device for both a conductor rated at 70 amps and a disconnect switch rated at 30 amps. I don't think the situation is very similar at all.
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What is the difference between coming off the load side of the 70A breaker vs tapping the 70A existing conductors and then landing those tap conductors in a set of fuses?

Assuming length and ampacity is correct for the appropriate tap section.
 
I agree with Jumper here. The conditions of 240.21(B)(2) are being met. Only difference is OP has No. 4 conductors instead of No. 10 that would be required. He will likely have to tap the No. 4 anyway as it will be difficult to terminate them on the 30amp switch terminals
 
My view is that 240.21 is not a player in this discussion. We are not connecting conductor (i.e., wire, not just "conductive material") to conductor. The various rules within 240.21 address the ampacity of the tap conductors. In this installation, there are no tap conductors. There is a 70 amp breaker (existing, to remain) feeding 70 amps worth of wire (also existing, to remain). The wire presently lands on a fused disconnect that is rated for 100 amps, and is fused at 70 amps. This will be replaced with a fused disconnect that is rated for 30 amps and will be fused at 25 amps.

In a tap situation, a smaller wire that cannot be protected by the upstream breaker is connected to a larger wire. The smaller wire will be protected by the overcurrent device at its load end. In the current situation, there is no "smaller wire." We would be calling upon the internal "guts" of the fused disconnect to be protected at 70 amps, despite their only being rated for 30 amps. That is fundamentally wrong, and the tap rules do not cover the situation.
 
Gee, why not keep the existing disco and use those little cups on the ends of the 25 amp fuses that fit the fuse clips in the old 100 amp disco?
 
but if the breaker at the feeding panel is changed to a lower value... 30 amps or even 40 amps... would that not satisfy the situation and yet leave the wire in place so that if one needed the higher ampacity later it is simply changing the breakers again? I mean, I would rather have the wires capable, which they are..
 
Russs57 said:
Gee, why not keep the existing disco and use those little cups on the ends of the 25 amp fuses that fit the fuse clips in the old 100 amp disco?
Click to expand...

Those little cups are called Fuse Reducers and yes that would be one way of addressing this issue.

JAP>
 
Adamjamma said:
but if the breaker at the feeding panel is changed to a lower value... 30 amps or even 40 amps... would that not satisfy the situation. . . .
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I like this solution. But you would have to check the EGC size, due to the requirements of 250.122(B).

 
hopefully in the original installation the egc would have been correctly sized for the other wires so should not be a problem. but yes, one should check it first. Just hate replacing wires that are already there.
 
There is no requirement that a switch have a current rating equal or greater than the rating of the upstream protective device. If there was, you could never install a service disconnect.

The fuses limit the overload current through the switch, and as long as the available fault current at the line side of the switch does not exceed the SCCR rating of the switch, I see no code or real world issues.

As others have said this type of issue occurs anytime a fused disconnect is used on the load end of a tap conductor.
 
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