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Tuesday, May 30, 2023

Question:

Hello CQD, I am doing a project where the electrical engineer specifies a 400 amp three pole breaker at the MDP. Parallel 4/0 copper conductors to a main lug only panel. The MLO panelboard has lugs rated at 250 amps. The engineers load calculations are 444 amps. I told the engineer this can't work. She said she can de-rate the load calculations by 43% and the MLO panel will be fine. I told the engineer I cannot hook up 400 amp wire, protected by a 400 amp breaker, to a 250 amp rated lug, this could cause a fire. Is this code compliant? Thanks, Don Giles
A

Answer:

Hello Don, thank you for your question. Section 110.10 requires overcurrent protection devices to be selected to insure the circuit/feeder is not damage from overcurrent or faults conditions. In general, the equipment and conductors would need to be sized based on the calculated load served. The overcurrent protection device would then be sized off of the calculated load served, and in accordance to Section 240.4(B). Also see Section 110.14 for temperature rating of connections.

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