Disconnect Switch Sizing

[faresos]

Hello Everyone,

What size of disconnect can be used for 300HP air compressor (NP FLA= 325A, with S.F.: 1.15)? My understanding the 400A HD disconnect is only rated for 250HP. The disconnect is not used for controls (On/Off) as it has it's own controller. The NEC tables indicate the FLA for 300HP is 361A. Questions:

1. Can I size the disconnect switch based the name plat of the motor X S.F. (325A X 1.15 = 374A- which means I can use 400A HD Disc sw) or it needs to be rated for the motor HP, whch means I have to use 600A since the 400A disc is only rated at 250 HP?

2. Can I use the FLA name plate (325A) for sizing or I must use the NEC FLA (361A)


Thank you,

 

[don_resqcapt19]

You are stuck using the table current value. See 430.6(A)(1).

1) Table Values.
Other than for motors built for low speeds (less than 1200 RPM) or high torques, and for multispeed motors, the values given in Table 430.247, Table 430.248, Table 430.249, and Table 430.250 shall be used instead of the actual current rating marked on the motor nameplate to determine the following:
(1) Ampacity of conductors
(2) Current ratings of switches
(3) Current ratings of branch-circuit short-circuit and ground-fault protection

The disconnect does not always have to have a horsepower rating equal or greater than the motor. See 430/109(E).

(E) Isolating Switches.
For stationary motors rated at more than 40 hp dc or 100 hp ac, the disconnecting means shall be permitted to be a general-use or isolating switch where plainly marked “Do not operate under load.”

General use switches do not have a horsepower rating, but you are still stuck with a 600 amp disconnect for this application.

 

[faresos]

Thank you Don for your response, i'm looking at exception #3 under (A)(1) where it does say I can use the amps name plate value if both the Amps and HP is listed unless i'm missing reading it :
Exception No. 3: For a listed motor-operated appliance that
is marked with both motor horsepower and full-load current,
the motor full-load current marked on the nameplate of the
appliance shall be used instead of the horsepower rating on
the appliance nameplate to determine the ampacity or rating
of the disconnecting means

 

[JoeStillman]

Mind your AIC. That disco is only rated 10k.

 

[faresos]

Mind your AIC. That disco is only rated 10k.

This is actually a fused disconnect with R type fuses, so we should be good here, but if it was Non fused then it will have SCCR rating but it will have a higher rating when it's in series with an upstream overcurrent protection (breaker or fuse).

 

[don_resqcapt19]

Thank you Don for your response, i'm looking at exception #3 under (A)(1) where it does say I can use the amps name plate value if both the Amps and HP is listed unless i'm missing reading it :
Exception No. 3: For a listed motor-operated appliance that
is marked with both motor horsepower and full-load current,
the motor full-load current marked on the nameplate of the
appliance shall be used instead of the horsepower rating on
the appliance nameplate to determine the ampacity or rating
of the disconnecting means

That applies to listed motor-operated appliances like garage door openers and whole house vacuums where some fictitious horsepower, such as peak horse power, is used for marketing purposes. This provides relief from the drastically oversized disconnect that would be required if you sized them to the nameplate horsepower.

I doubt that your 300 hp compressor is listed as an appliance.