How many disco's

[TwinCitySparky]

I have a 3 phase converter feeding an air compressor. There is one disconnect before the converter that effectively shuts down both units.

I am adding a second compressor that will use power from same system/converter and sits right next to the first compressor to assist it when pressure drops low. (small auto shop)

Do I absolutely need to place individual disconects for both compressors or am I Ok with just using one properly sized/fused disco to shutdown everything (the converter and 2 compressors).

Thanks

 

[nakulak]

I believe that what you want to do is ok
430.112 ex c
670 doesn't seem to contradict this

 

[RUWired]

I think you need to go to article 455.8(C)1 or 2 for your answer. The rating of the disconnect must be sized for the new combo of loads.

(C) Rating. The ampere rating of the disconnecting means shall not be less than 115 percent of the rated maximum single-phase input full-load amperes or, for specific fixed loads, shall be permitted to be selected from 455.8(C)(1) or (C)(2).

(C)(1) Current Rated Disconnect. The disconnecting means shall be a circuit breaker or molded-case switch with an ampere rating not less than 250 percent of the sum of the following:

(1) Full-load, 3-phase current ratings of the motors
(2) Other loads served

(C)(2) Horsepower Rated Disconnect. The disconnecting means shall be a switch with a horsepower rating. The equivalent locked rotor current of the horsepower rating of the switch shall not be less than 200 percent of the sum of the following:

(1) Nonmotor loads
(2) The 3-phase, locked-rotor current of the largest motor as determined from Table 430.251(B)
(3) The full-load current of all other 3-phase motors operating at the same time