Gera Tools

NEC Tap Rule Calculator (210% and 100 A Rules)

Check NEC 240.21 tap rules for 10-ft, 25-ft, and outside feeder taps

Checks the NEC 240.21(B) tap rules — 10-ft rule at 10% of OCPD, 25-ft rule at one-third of OCPD, and the unlimited outside-tap rule — against a proposed tap conductor ampacity and feeder rating. Runs in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

Last updated Source: Gera Tools

What is a tap conductor?

A tap is a conductor connected to a feeder that is smaller than the feeder and is protected at its origin only by the feeder's overcurrent device. NEC 240.21(B) carves out specific exceptions that let these unprotected taps exist within strict length and ampacity limits.

Feeder taps let a large feeder branch off into a smaller conductor without its own overcurrent device — but only inside the narrow exceptions NEC 240.21(B) allows. This calculator checks a proposed tap against the 10-ft, 25-ft, and outside-tap rules at once so you know whether the conductor is legal before you pull it.

How it works

Each rule sets a minimum tap ampacity tied to the feeder’s overcurrent device rating, with a length ceiling:

10-ft rule:   length ≤ 10 ft   tap ampacity ≥ 10% of feeder OCPD
25-ft rule:   length ≤ 25 ft   tap ampacity ≥ 1/3 of feeder OCPD
outside rule: any length       single OCPD at building entry, damage-protected

The tool computes both thresholds from the feeder rating and reports which rules the tap satisfies. Passing any one rule, with its termination and enclosure conditions met, makes the tap compliant.

Worked examples

Example 1 — 25-ft rule pass: A 175 A tap off a 400 A feeder, run 20 ft. One-third of 400 A is 133 A. The tap conductor at 175 A exceeds 133 A, so it passes the 25-ft ampacity threshold. The 20 ft length is within the 25 ft limit. This tap is compliant under the 25-ft rule, provided it terminates in a single overcurrent device rated 175 A or less and the conductor is contained in a raceway or cable assembly.

Example 2 — 10-ft rule: Shorten that same tap to 8 ft. Now the 10-ft rule applies: the conductor must carry at least 10% of 400 A, which is 40 A. At 175 A it exceeds that threshold comfortably. Either rule could apply here, but an inspector will typically look for which rule is being cited in the design documentation.

Example 3 — outside tap: A tap connects to a feeder inside a building but immediately exits to run along the building exterior for 60 ft before entering a panel room. Its entire outdoor run must be protected against physical damage, and it must terminate in a single overcurrent device at the point it re-enters the building. If those conditions are met, the unlimited-length outside rule under 240.21(B)(5) allows the otherwise-too-long run.

Common mistakes with feeder taps

  • Using table ampacity instead of derated ampacity. Tap compliance is measured against the conductor’s actual usable ampacity after all derating for ambient temperature and conduit fill, not the bare table value. An undersized tap that appears to pass because the table ampacity looks large enough may fail once derating is applied.
  • Missing the single-device termination requirement. The 25-ft and outside rules both require the tap to terminate in a single overcurrent device, not fan out to a multi-way panel. If the tap feeds a panelboard with a main breaker, the main breaker is the single device. If there is no main breaker, the tap does not meet the rule.
  • Confusing the tap point with the origin of the feeder. The length is measured from the point where the tap is made to the point where it terminates in overcurrent protection, not from the origin of the feeder.

Always confirm the termination and physical-protection conditions in the actual code edition adopted in your jurisdiction, and use the derated conductor ampacity when checking compliance.