sizing a non fused safety switch

[gk351]

If I was sizing a 75 hp 3ph motor rated at 96 FLA> My wire size would need to meet 125% rule, which the amperage is 120Amps, which 1 AWG. My question is, if I need a non fused disco at the motor, the manufactures rating is 100a rated for 75 hp. But since I sized my wire to 120A (125%) do I need to bump to the next sized disco, which is 200Amp/125Hp? I dont believe I Have to, but just looking for some opinions.

 

[Smart $]

If I was sizing a 75 hp 3ph motor rated at 96 FLA> My wire size would need to meet 125% rule, which the amperage is 120Amps, which 1 AWG. My question is, if I need a non fused disco at the motor, the manufactures rating is 100a rated for 75 hp. But since I sized my wire to 120A (125%) do I need to bump to the next sized disco, which is 200Amp/125Hp? I dont believe I Have to, but just looking for some opinions.

430.110...

Exception: A listed unfused motor-circuit switch having a
horsepower rating not less than the motor horsepower shall
be permitted to have an ampere rating less than 115 percent
of the full-load current rating of the motor.

 

[ron]

On a related but yet unrelated directly to the OP question, will an un-fused disco have an adequate short circuit rating for the available fault current? Usually on larger motors, the system is big enough to offer >5kA at the line side terminals of the disco. That would mean it needs to be fused to get a higher short circuit rating.

 

[Smart $]

On a related but yet unrelated directly to the OP question, will an un-fused disco have an adequate short circuit rating for the available fault current? Usually on larger motors, the system is big enough to offer >5kA at the line side terminals of the disco. That would mean it needs to be fused to get a higher short circuit rating.

Non-fusible disconnects are not intended to interrupt fault current. Are there even any non-fusible disconnects that have a SCCR by themselves? I'm of the impression the only scenario where a non-fusible disconnect even has an SCCR is when they are used in conjunction with fuses... and the SCCR is dependent on the class of fuse used.

 

[GoldDigger]

Would not an SCCR also apply to the fault current that the disconnect can pass without damage until an upstream or downstream protection opens? It does not necessarily just tell you what current it can interrupt.

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[Smart $]

Would not an SCCR also apply to the fault current that the disconnect can pass without damage until an upstream or downstream protection opens? It does not necessarily just tell you what current it can interrupt.

Yes, but is correlated with the OCPD. For that info you would have to refer to manufacturer documentation.

I briefly looked at SqD HD-NF-DS's. Only rated with fuses of 4 class types giving 10kA, 50kA, and 100KA SCCR's... and not evaluated for using MCCB's.

 

[jim dungar]

Non-fusible disconnects are not intended to interrupt fault current.

That is correct but they must be able to be closed on a fault on withstand the fault current until an upstream device operates. That is why they have an SCCR and not an AIC.

Most UL listed 'safety switches' have a 10kA SCCR, although the majority of them have some type of series rating.
Most motor disconnects (eg. rotary design) have a 5kA SCCR.