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Subject: MCC sizing/upgrade
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Date: 03/13/01 at 10:06 AM
Posted by: Stevan Simmons
I need to add a fusible branch switch unit to an existing MCC (Sq D Model 5). The unit is rated for 600 amps horizontal and 300 amps vertical at 480 VAC. I currently have two units in this 300 amp section. A Size 4 starter with 250 amp short circuit protection and supplying a 75 hp motor at 96 FLA. and a Size 2 starter supplying a 20 hp drive at 25 FLA with a 30 amp(this seems to be too small also) short circuit breaker. I am assuming that the vertical capacity limit is based on supplying 125% of the largest motor + the FLA of the smaller motors. This would be an existing amperage of 120 + 25 = 145 amps. As the vertical bus is rated at 300 amps I calculate 155 amps of spare capacity. I need 160 amps to feed two 80 amp panels using TW 4/0 conductors. Am I getting close here or am I way off base with this calculation? Thanks for the help!
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