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SESK: Stuff Everyone Should Know · sesk.info/elec-next-size-up-800

The 240.4(B) 'next size up' rule allows rounding up the overcurrent device only up to what rating?

800A or less and three conditions must be met.

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  1. 100A or less
  2. 800A or less ✔
  3. 2,000A or less

Why: If a conductor's ampacity doesn't match a standard breaker size, 240.4(B) lets you use the next higher standard rating. This works only at 800A or less and only when three conditions are met.

Huh, didn't know that: Above 800A, conductor ampacity must be at least the overcurrent device rating, per 240.4(C).

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Why did the breaker want a bigger size? It needed some room to grow.
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References

Answer: 800A or less. If a conductor's ampacity doesn't match a standard breaker size, 240.4(B) lets you use the next higher standard rating. This works only at 800A or less and only when three conditions are met.

  1. EC&M: Protecting Conductors Part 3 www.ecmweb.com/maintenance-repair-operations/article/20901392/protecting-conductors-part-3
    if you’re working with 800A or less, you can use the next higher standard OCPD if you meet three conditions [240.4(B)]
    Checked 2026-10-10.

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