Fire pump disconnect.

[Andy1099]

Fire pump disconnect.
A disconnect is being installedbetween the fire pump controller and the power source (a utility transformer)as allowed by NEC 695.4(B)(1). The codereads “A single disconnecting means and associated overcurrent protectivedevices shall be permitted…” Does this requirement mean that a disconnect aheadof the controller must have overcurrent protection? If overcurrent protectionis not required, is 125% the size of the full load current the size for thedisconnect rating and not locked rotor current like it would be for overcurrentprotection?

 

[gadfly56]

Fire pump disconnect.
A disconnect is being installedbetween the fire pump controller and the power source (a utility transformer)as allowed by NEC 695.4(B)(1). The codereads “A single disconnecting means and associated overcurrent protectivedevices shall be permitted…” Does this requirement mean that a disconnect aheadof the controller must have overcurrent protection? If overcurrent protectionis not required, is 125% the size of the full load current the size for thedisconnect rating and not locked rotor current like it would be for overcurrentprotection?

The disconnect doesn't have to be fused, but it does have to be rated for the LRC. IFF it is fused, then the OCPD must carry the LRC indefinitely. The 125% is for conductor sizing.

 

[necGuru]

Most people that design and/or install a disconnect for a fire pump will do it incorrectly and then get burned by the inspector. Why do you need this disconnect? Most of the time, a fire pump service doesn't need a disconnect. If you choose to use one it must be:

• SUSE (Suitable for use as service equipment)
• Lockable in the closed position (this would be the "ON" position) and no, you can't drill your own hole for this purpose.
• Independent and sufficiently far away from other service disconnects to make it obvious that this is an entirely different animal.
• Marked with 1-inch lettering that states, FIRE PUMP DISCONNECTING MEANS.
• Signage placed at the controller stating the location of the fire pump disconnecting means.

As the previous person mentioned, if there is overcurrent protection it must be rated at locked-rotor. The value comes out to be very LARGE and most architects and electrical engineers usually get this value wrong on the plan review. By "most" I would estimate that more than 50% of plans are incorrect. For more information, visit NFPA-20. It is very interesting.