Motor Protection

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coryp

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I have a 15 hp motor with a local 30 amp non-fused disconnect being protected by a adjustable motor protector adjustable from 25 to 32 amps.

I am questioning weather the local disconnect needs to be 60 amps since the protection could be set at 32 amps or if 430.110 allows the disconnect to remain at 30 amps.

Opinions??
 
you don't state a voltage and the 25-32 amp range fits no 430.250 values. From looking at a SquareD catalog it would appear a no-fuse
heavy duty disconnect would be rated 15hp @ 480, but any lower voltage would requirea 60 amp nfd.
IMHO 430.110 seems to read accordingly.
 
15hp at 480v is 21A as per table 430.250.

  • So the OL's = ??A @ 125% of name plate (430.32)
  • the OCPD = 50A @ 250% of table 430.250 or 40A fuses at 175% of table 430.250 (430.52)
  • & the disconnect = 30A at 115% of table 430.250 (430.110(A))
 
So I can have a 30 amp non-fused disconnect downstream from a 40 amp fuse? I see based on quoted code sections but does that really make sense? I guess thats the beauty of the code???
 
The voltage is 480V.

Seems a bit odd to have a 40 amp OCPD and a 30 amp local disconnect switch.

Should the wire size be #8 or #10? I am assuming #8 based on OCPD.

Thanks for you input.
 
coryp said:
So I can have a 30 amp non-fused disconnect downstream from a 40 amp fuse? I see based on quoted code sections but does that really make sense? I guess thats the beauty of the code???
Click to expand...

The non-fused disconnect sounds like it is only there for "Disconnecting Means" see 430.102, it may not even be needed if the OCP is within sight or the motor complies with the exception to 430.102(B).

The OCP (source breaker or fuses) is for the circuit conductor protection and are sized to accommodate the LRA (lock rotor ampere) for start up only, and the OL's are sized for the motor protection.

The disconnect is only required to be at least 115% of the table ampere it can be more though.

The circuit conductors are required to be at least 125% of the table ampere so #10 THHN is fine, the OL?s will not allow this conductor to over load yet the OCP will allow start up as well as short circuit and ground fault protection.
It really is all safe.
 
coryp said:
So I can have a 30 amp non-fused disconnect downstream from a 40 amp fuse? I see based on quoted code sections but does that really make sense? I guess thats the beauty of the code???
Click to expand...

The motor overloads will prevent overloading the disconnect as well.
 
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