Load calculations rarely land on a clean, standard breaker size. If your math comes out to 47 amps, there's no 47A breaker on the shelf — so what do you do? NEC 240.4(B) answers that exact question, and it's narrower than most people assume.
NEC Table 240.6(A) defines standard overcurrent protection device (breaker and fuse) ratings. When a calculation results in a non-standard ampacity, the code generally allows rounding up to the next standard size — but only under specific conditions.
The broader principle behind Article 240 is that overcurrent devices exist to protect conductors: a breaker's rating should match, or stay below, what the conductor can safely carry. 240.4(B) is a practical exception to that principle, built specifically to handle the mismatch between real-world calculated loads and the fixed lineup of manufactured breaker sizes. Without it, a conductor sized for a load that falls between two standard breaker ratings would have no valid overcurrent device available at all.
The Rule
Under NEC 240.4(B), you can use the next standard overcurrent device rating (rounding up) if the calculated conductor ampacity does not correspond to a standard size, provided:
- The rating does not exceed 800 amps, and
- The circuit is not a multioutlet branch circuit supplying receptacles for cord-and-plug connected loads.
In practice, this is what lets an electrician land a conductor's calculated ampacity onto the next standard breaker size, rather than requiring a custom, non-standard overcurrent device that doesn't exist commercially. It's a narrow allowance, though — it exists to handle the gap between a calculated number and a manufactured breaker rating, not as a general license to oversize protection on a circuit.
Find the Right Standard Breaker Size
Reference the full NEC 240.6(A) standard amperage chart and check your calculated load.
Worked Example
Say a load calculation determines a conductor must carry 47A. There's no standard 47A breaker on the chart — the closest sizes are 45A and 50A. Because 45A is below the calculated load, it can't be used. NEC 240.4(B) is what allows the installer to move up to the next standard size, 50A, instead of being stuck without a compliant overcurrent device.
Calculated ampacity: 47A → No standard 47A breaker exists → Round up to 50A (next standard size, per 240.4(B))
This is why the round-up rule matters in practice: real-world load calculations rarely produce a number that lines up exactly with a standard breaker rating, and without 240.4(B), every non-standard result would be a dead end.
Standard Breaker Sizes (NEC 240.6(A))
These are the standard inverse-time circuit breaker ratings the round-up rule draws from:
When the Round-Up Rule Doesn't Apply
- Above 800 amps: The allowance stops at the 800A threshold — larger services need exact or conservative sizing.
- Multioutlet branch circuits: Circuits supplying receptacles for cord-and-plug connected loads are excluded from the round-up allowance.
- When conductor ampacity already matches a standard size: The rule only applies when the calculated ampacity falls between two standard sizes — if it already lands on one, that's your answer.
Frequently Asked Questions
Q: What is the NEC 240.4(B) round-up rule?
A: It allows you to use the next standard overcurrent device rating when a calculated conductor ampacity doesn't match a standard breaker size, provided the rating doesn't exceed 800 amps and the circuit isn't a multioutlet branch circuit supplying cord-and-plug loads.
Q: What are the standard breaker sizes under NEC 240.6(A)?
A: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000, and 6000 amps.
Q: Does the round-up rule apply above 800 amps?
A: No. The allowance is limited to overcurrent devices rated 800 amps or less.
Q: Can I round up on a multioutlet branch circuit?
A: No. NEC 240.4(B) specifically excludes multioutlet branch circuits supplying receptacles for cord-and-plug connected loads from the round-up allowance.
Disclaimer: While our articles and tools are designed to align with the National Electrical Code, always check with local building inspectors and licensed electrical contractors before initiating physical installations.