Is it allowed to use a molded case switch (magnetic only) for the fire pump disconnecting means? If so, how is it sized since LRC (lock rotor current applies to overcurrent protection only.
It would be easier and better, IMHO, if you don't use a disconnecting means. You will avoid a bunch of code violations that way.
But if you are going to use a disconnecting means, and probably create all sorts of issues, it must be sized per locked rotor. It's all there. Check out 2017 695.4(B)(1)(2)(1)....
"Overcurrent protective device(s) shall be rated to carry
indefinitely the sum of the locked-rotor current of the
largest fire pump motor and the pressure maintenance
pump motor(s) and the full-load current of all of the
other pump motors and associated fire pump accessory
equipment when connected to this power supply. Where
the locked-rotor current value does not correspond to a
standard overcurrent device size, the next standard overcurrent
device size shall be used in accordance with
240.6...." (it goes on)
8 out of 10 times that someone submits plans showing a disconnect for fire pump, the plans are wrong. Just don't do it. Also check out 2017 695.4(B)(3) for a bunch of other requirements.
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