430.110(c)

[charlie b]

The process for sizing the disconnect for a combination load includes consideration of full load and locked rotor currents. I have been asked about Variable Air Volume (VAV) units with 3-phase fused disconnects, with 480 volt 3-phase heaters, and with 277 volt 1-phase motors. The contractor wants to down size the disconnect (as shown on the drawings) from 60 amp to 30 amp. Two questions:

(1) The process takes me to Table 430.248 for the single phase motor. But the table does not address 277 volt motors. What do I do? Can I take the tabulated value and apply a ratio of 230/277, to get the equivalent current?

(2) The process has me size the disconnect no smaller than 115% of the calculated full load amps. But it also has me calculate an equivalent load at locked rotor conditions. How, if at all, does the locked rotor current come into play, in the process of selecting the disconnect rating?

 

[Dennis Alwon]

Charlie I don't see why you can't take the ratio. I tried it with some other voltages and applying the ratio seems to work just fine.

If you had a 1 hp motor @ 115 volts (16 amps) and you want to find the load at 277 volts then I would say 277/115 = 16/x . Thus 277x = 115*16.
x = 6.4

 

[RB1]

Charlie:

The sum of the locked rotor currents of the motors must be added together. This ampere value is converted back to horsepower (430.110(C) (1)). The disconnecting means must have this horsepower rating in order safely interrupt the locked rotor current of the motors and other loads..