NEC 240.4(B) Next-Size-Up Lookup
NEC 240.4(B) is a limited overcurrent-protection permission. It can allow the next higher standard overcurrent device only when the conductor ampacity falls between standard device ratings and the rule's conditions are satisfied.
| Field Item | NEC Reference | Field Meaning |
|---|---|---|
| Final conductor ampacity | NEC 310.16, 310.15, 110.14(C) | Start with the conductor's usable ampacity after ampacity adjustment and correction, temperature column, and terminal-limit checks. Do not use a raw table value that has not been reviewed. |
| Standard overcurrent device sizes | NEC 240.6(A) | The rule depends on whether the conductor ampacity lands between standard breaker or fuse ratings. It is not permission to skip the standard-size comparison. |
| Ampacity does not match a standard device | NEC 240.4(B) | The next higher standard size may be considered only when the conductor ampacity does not correspond with a standard overcurrent device rating. |
| Next higher standard size limit | NEC 240.4(B) | The selected next higher standard rating must stay within the allowed limit for this rule and must not be blocked by another NEC section or equipment requirement. |
| Multiple receptacles for cord-and-plug portable loads | NEC 240.4(B)(1) | This condition can block use of the next-size-up path for certain branch circuits supplying more than one receptacle for portable cord-and-plug loads. |
| Small-conductor protection | NEC 240.4(D) | Small-conductor rules can impose fixed maximum overcurrent protection and prevent using 240.4(B) as a general round-up shortcut. |
| Equipment or article-specific rules | NEC 240.4(G), Articles 430, 440, 445, 695, and related equipment rules | Motors, HVAC equipment, generators, fire pumps, and other specialized equipment may use different overcurrent rules or nameplate limits instead of the general next-size-up path. |
Field Example
Applying the Next-Size-Up Check
NEC 240.4(B) starts after the usable conductor ampacity is known. The field sequence is:
Adjusted conductor ampacity → compare to NEC 240.6(A) standard sizes → check whether NEC 240.4(B) is allowed or blocked
Field example: if the final usable ampacity lands at 37A, the next standard 40A device may be reviewed only if the 240.4(B) conditions are met and no smaller-conductor, equipment, motor, HVAC, or manufacturer rule blocks it. This is different from the fixed limits in NEC 240.4(D) Small Conductor Rule. Use the Breaker Size Calculator after the conductor ampacity basis is known, and use the Wire Size Calculator when conductor selection still needs review.
Field Workflow
Next-Size-Up Workflow
Use the next-size-up rule only after the conductor has already passed the load and ampacity review. The rule is a narrow overcurrent-device selection path, not a way to make an undersized conductor serve a larger load.
Find usable ampacity first. Confirm conductor material, insulation, derating, ambient correction, terminal limits, and wiring method before comparing overcurrent sizes.
Check the load basis. The conductor still has to carry the calculated load, including continuous-load treatment where applicable.
Compare standard sizes. Use standard overcurrent device ratings to confirm whether the conductor ampacity actually falls between two standard sizes.
Check rule blockers. Stop the round-up path when small-conductor limits, receptacle restrictions, equipment markings, nameplates, or article-specific rules control.
Breaker Sizing Boundary
Breaker Sizing Boundary
The conductor must still pass the load calculation. NEC 240.4(B) does not increase conductor ampacity. If the calculated load is higher than the conductor's usable ampacity, choosing the next higher breaker or fuse does not correct the conductor-sizing problem.
The rule only solves a standard-size gap. The common field use case is a conductor ampacity that lands between standard overcurrent device ratings. If the ampacity already matches a standard rating, the next larger device is not justified by this rule.
Blocked Applications
Blocked Applications
NEC 240.4(B) is a permission rule, not a default breaker-sizing method. Before using it, confirm that another conductor-protection rule, equipment marking, or nameplate requirement has not taken control.
Small-conductor protection, multioutlet receptacle branch-circuit restrictions, HVAC equipment nameplates, motor circuit rules, and over-800-amp applications are common places where the general round-up idea can be wrong or incomplete.
Field Sequence
Load basis → conductor ampacity → standard OCPD size → 240.4(B) conditions → article-specific blockers
If any controlling rule blocks the round-up path, use the lower standard overcurrent device or resize the conductor as required by the application.
Calculator Use
TradeHub Calculator Application
TradeHub calculators treat NEC 240.4(B) as a limited overcurrent-protection check after usable conductor ampacity and blocked-rule conditions are reviewed.
Related TradeHub Calculators
Field Checks
Common Field Misses
Rounding up when the conductor ampacity already matches a standard device rating. If the ampacity corresponds to a standard breaker or fuse size, this rule does not justify the next larger size.
Using breaker round-up to cover an overloaded conductor. The calculated load still has to fit the usable conductor ampacity before overcurrent-device selection is trusted.
Ignoring small-conductor rules. Small-conductor protection limits can block a general next-size-up assumption, especially on common branch-circuit conductor sizes.
Applying the rule above its permitted range. Overcurrent devices above the rule limit need a different conductor-protection review.
Overriding equipment nameplates or article-specific rules. HVAC, motor, generator, and other specialized equipment can use different OCPD logic that must be checked directly.
Related References
Related NEC Field References
Source Scope
Source Alignment and Use Scope
This NEC 240.4(B) page is a field reference based on next-size-up overcurrent protection conditions, NEC 240.4(D), NEC 240.4(G), NEC 240.6(A), conductor ampacity context, and related TradeHub source alignment records. It is intended for screening and planning only; it does not reproduce the NEC, approve an installation, override equipment markings, or replace local AHJ review. Verify final application against the adopted NEC cycle, local amendments, equipment markings, manufacturer instructions, and project documents.
Review the TradeHub Code Citation & Source Log for source alignment records and the TradeHub Methodology page for how field references are scoped.
Field FAQ
Next-Size-Up Rule FAQ
Can I use NEC 240.4(B) whenever conductor ampacity is lower than the breaker?
No. The next-size-up rule is limited to specific conditions. The conductor ampacity must not already match a standard overcurrent device size, the selected next higher standard size must stay within the rule limit, and other NEC sections or equipment instructions must not prohibit the application.
Does NEC 240.4(B) apply when the conductor ampacity already matches a standard breaker size?
No. When the allowable conductor ampacity already corresponds to a standard overcurrent device rating, NEC 240.4(B) is not a permission to move to the next larger breaker or fuse.
Can the next-size-up rule override small-conductor protection limits?
No. Small-conductor protection limits and other specific NEC rules can block the general next-size-up path. Check the applicable conductor, equipment, and article-specific requirements before applying 240.4(B).