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JMK

Proper Sizing for an MSR

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Hello all - I am looking for some assistance on how to size a safety contactor that will 'Disconnect' the supply voltage from (4) motors. This is what confuses me - Do I need to rate the contactor, as a disconnect (per NEC)? The four motors are as follows - (1) 1/2 HP, 208 VAC, 3PH (3) 3/4 HP, 208 VAC, 1PH Now, from what I gather out of the NEC: Disconnecting Means (Art. 100) A device or group of devices, or other means by which the conductors of a circuit can be disconnected from their source of supply. Ok, that's what the safety contactor is doing... Disconnect Means, General (Art 430.101) Part IX is intended to require disconnecting means capable of disconnecting motors and controllers from the circuit. Again, I interpret the safety contactor to do just that. Ampere rating and interrupting capacity for combo loads Art 430.110©(1) The rating of the disconnect means shall be determined from the sum of all currents...at the full load condition and also at the locked rotor condition. The combined full load and locked rotor current so obtained shall be considered as a single motor for the purpose of this requirement. The next paragraphs go on to refer the user to the tables .248-.251 for the FLC and LRC values associated with the motor. Ampere Rating in 430.110©(2) refers back to© (1), but adds the 115%... Here’s my example: Motor 1 (from above) has an FLC of 2.4 Amps (T430.250), and an LRC of 22.1 Amps (T430.251(B)), for a total of 24.5 Amps Motors 2-4 from above have an FLC of 7.6 Amps (T430.248), and an LRC of 45.8 Amps (T430.251(A)), for a subtotal of 53.4 Amps, (*3 motors) for a total of 160.2 Amps Total Motor Load (430.110©(1) is 184.7 Amps, *1.15 per 430.110©(2) = 212.4 Amps. Refer back to T430.251(B) at 208 VAC and 212 Amps would be 15 HP, which is rated at 257 Amps. So, the morale of my long winded story - Is this the correct method? I would need a Safety Contactor rated for 15 HP at 208 VAC.

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