Feeder Tap Rule Calculator — NEC 240.21(B)
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Select the NEC edition adopted by your AHJ. Affects article references and label text.
Auto selects the rule by length (10 ft → 10-ft rule; 10–25 ft → 25-ft rule; >25 ft requires confirmed conditions or manual selection). Transformer rule is always manual.
Rating of the breaker or fuse protecting the feeder on the line side of the tap. The tap ampacity fraction is based on this OCPD rating, not on feeder conductor ampacity.
Tap conductor ampacity at conditions of use — after temperature correction (NEC 310.15) and conduit-fill adjustment. Not required in transformer mode.
Length of the tap conductor run from the supply point to the termination. Not required in transformer mode.
Select feet or meters. Internal comparison uses exact ft conversion (1 m = 3.280839895 ft). NEC states rounded metric equivalents — near-limit runs should be confirmed against the adopted edition.
Load per Article 220 (2017–2023) or Article 120 (2026). Tap conductor must be ≥ calculated load. If not entered, the load condition is unverified.
Rating of the OCPD or equipment at the tap termination. For the 10-ft rule, tap ampacity must be ≥ end OCPD. For 25-ft / high-bay rules, end OCPD must be ≤ tap ampacity.
Required for all tap rules. 10-ft rule: raceway required where tap leaves the enclosure. Other rules: protected throughout the run.
Required for 25-ft, transformer, high-bay, and outside rules. The tap must terminate in a single overcurrent device, not in a distribution panel or multiple OCPDs.
How to Use the Feeder Tap Rule Calculator
Select NEC Edition: choose the edition adopted by your AHJ. Affects article references for load calculations.
Select Tap Rule or leave it on Auto. Auto chooses by length: ≤10 ft selects the 10-ft rule, 10–25 ft selects the 25-ft rule. Runs over 25 ft require a confirmed high-bay or outside condition, or a manual rule selection. The transformer rule is always selected manually.
Enter the Feeder OCPD Rating in amperes. This is the breaker or fuse protecting the feeder on the line side of the tap. The required fraction is based on this OCPD rating, not on feeder conductor ampacity.
Enter the Tap Conductor Ampacity at conditions of use — after temperature correction (NEC 310.15) and conduit-fill adjustment. Not the raw Table 310.16 value.
Enter the Tap Length and select feet or meters. Enter the Calculated Load (required); omitting it leaves the load condition unverified. Optionally enter the End OCPD rating to refine the required minimum ampacity.
Confirm the installation conditions using the attestation dropdowns: raceway/protection, single-OCPD termination, not-extended-beyond-equipment (10-ft rule), high-bay conditions (B)(4), or outside conditions (B)(5). An unconfirmed condition produces UNVERIFIED-CONDITIONS, not a false pass.
For the transformer rule, enter primary and secondary ampacity, voltages, and lengths in the transformer fields.
Click Calculate and read the badge. COMPLIANT also shows headroom above the NEC minimum as a planning indicator.
This calculator screens NEC 240.21(B) feeder tap conductor sizing only. It does not size the feeder conductor, the OCPD, or compute the load. Tap ampacity must already reflect all derating. PV/source-side interconnections under Article 705 and full transformer protection under Article 450 are out of scope.
Inputs & Outputs
Inputs
- •NEC Edition — Options: NEC 2017, NEC 2020, NEC 2023 (default), NEC 2026
- •Tap Rule — Options: Auto (select by length), 10-ft tap rule [240.21(B)(1)], 25-ft tap rule [240.21(B)(2)], Transformer primary+secondary [240.21(B)(3)], Over-25-ft high-bay [240.21(B)(4)], Outside unlimited length [240.21(B)(5)]
- •Feeder OCPD Rating (A)
- •Tap Conductor Ampacity (A)
- •Tap Length
- •Length Unit — Options: Feet (ft), Meters (m)
- •Calculated Load (A)
- •End OCPD / Equipment Rating (optional) (A)
- •In Raceway or Protected from Physical Damage — Options: Unknown — not yet confirmed, Confirmed — in raceway or protected, Not confirmed — does not apply
- •Terminates in a Single OCPD — Options: Unknown — not yet confirmed, Confirmed — terminates in a single OCPD, Not confirmed — does not apply
- •Not Extended Beyond Supplied Equipment (10-ft rule) — Options: Unknown — not yet confirmed, Confirmed — tap ends at supplied equipment, Not confirmed — does not apply
- •High-Bay Conditions Met [240.21(B)(4)] (Over-25 / Auto >25 ft) — Options: Unknown — not yet confirmed, Confirmed — all (B)(4) high-bay conditions met, Not confirmed — conditions not met
- •Outside Conditions Met [240.21(B)(5)] (Outside / Auto >25 ft) — Options: Unknown — not yet confirmed, Confirmed — all (B)(5) outside conditions met, Not confirmed — conditions not met
- •Primary Conductor Ampacity (transformer mode) (A)
- •Secondary Conductor Ampacity (transformer mode) (A)
- •Primary Voltage (transformer mode) (V)
- •Secondary Voltage (transformer mode) (V)
- •Primary Conductor Length (transformer mode)
- •Secondary Conductor Length (transformer mode)
Outputs
- •Compliance Status
- •Tap Rule Applied (auto / manual)
- •Required Minimum Ampacity (A)
- •Required Fraction (1/10, 1/3, or none)
- •Tap Length vs Length Limit
- •Headroom above NEC minimum (or Deficit) (A / %)
- •Condition-by-condition PASS / FAIL / UNVERIFIED table
Formula
Inputs & Outputs
Inputs (standard mode — 10-ft, 25-ft, high-bay, outside):
- NEC Edition — NEC 2017 / 2020 / 2023 (default) / 2026. Affects article references for load calculations.
- Tap Rule — Auto (select by length), 10-ft tap rule [240.21(B)(1)], 25-ft tap rule [240.21(B)(2)], Transformer primary+secondary [240.21(B)(3)], Over-25-ft high-bay [240.21(B)(4)], Outside unlimited length [240.21(B)(5)].
- Feeder OCPD Rating (A) — breaker or fuse protecting the feeder on the line side of the tap.
- Tap Conductor Ampacity (A) — after temperature correction and conduit-fill adjustment (non-transformer rules).
- Tap Length — run length from supply point to termination (non-transformer rules).
- Length Unit — feet or meters.
- Calculated Load (A) — required; per Article 220 (NEC 2017–2023) or Article 120 (NEC 2026). Leaving blank leaves the load condition unverified.
- End OCPD / Equipment Rating (optional) (A) — OCPD or equipment rating at the tap termination. Refines required minimum ampacity.
- In Raceway or Protected from Physical Damage — attestation required for all rules.
- Terminates in a Single OCPD — attestation required for 25-ft, transformer, high-bay, and outside rules.
- Not Extended Beyond Supplied Equipment (10-ft rule) — attestation; shown for 10-ft rule and Auto mode when length ≤ 10 ft.
- High-Bay Conditions Met [240.21(B)(4)] (Over-25 / Auto >25 ft) — attestation; shown for high-bay rule and Auto mode when length > 25 ft.
- Outside Conditions Met [240.21(B)(5)] (Outside / Auto >25 ft) — attestation; shown for outside rule and Auto mode when length > 25 ft.
Additional inputs (transformer mode [240.21(B)(3)] only):
- Primary Conductor Ampacity (transformer mode) (A) — after correction and adjustment.
- Secondary Conductor Ampacity (transformer mode) (A) — used with voltage ratio for reflected check.
- Primary Voltage (transformer mode) (V) — transformer primary voltage.
- Secondary Voltage (transformer mode) (V) — transformer secondary voltage.
- Primary Conductor Length (transformer mode) — primary run length.
- Secondary Conductor Length (transformer mode) — secondary run length; primary + secondary must not exceed 25 ft (7.5 m).
Outputs:
- Compliance Status — INFEASIBLE / INVALID-INPUT / NON-COMPLIANT / UNVERIFIED-CONDITIONS / COMPLIANT (with headroom band).
- Tap Rule Applied (auto / manual) — which of the five rules was applied.
- Required Minimum Ampacity (A) — max(rule fraction × feeder OCPD, calculated load, end OCPD).
- Required Fraction (1/10, 1/3, or none) — fraction applied from the governing rule.
- Tap Length vs Length Limit — actual length versus the rule's length cap.
- Headroom above NEC minimum (or Deficit) (A / %) — planning indicator for compliant non-transformer taps; deficit amount when non-compliant.
- Condition-by-condition PASS / FAIL / UNVERIFIED table — each required condition evaluated individually.
Length normalization (internal comparison in feet):
length_ft = length_m × 3.280839895 (when entered in meters)
NEC prints rounded metric equivalents (3 m, 7.5 m, 30 m); the calculator displays both and flags near-limit lengths.
Required minimum ampacity by rule:
10-ft [240.21(B)(1)]: req = max( feeder_OCPD / 10, calc_load, end_OCPD )
25-ft [240.21(B)(2)]: req = max( feeder_OCPD / 3, calc_load, end_OCPD )
high-bay[240.21(B)(4)]: req = max( feeder_OCPD / 3, calc_load, end_OCPD )
outside [240.21(B)(5)]: req = max( calc_load, end_OCPD ) (no length cap)
Transformer rule [240.21(B)(3)] — two-conductor test:
primary: primary_ampacity ≥ feeder_OCPD / 3
secondary: secondary_ampacity × (Vsecondary / Vprimary) ≥ feeder_OCPD / 3
length: primary_length + secondary_length ≤ 25 ft (7.5 m)
Compliance check (non-transformer rules):
compliant if tap_ampacity ≥ req
deficit_A = req − tap_ampacity (when short)
Headroom (compliant non-transformer — screening/planning only):
headroom_% = (tap_ampacity − req) / req × 100
Headroom is a planning indicator; the code requirement is met at 0% headroom.
Decision model (evaluated in priority order):
| Priority | Status | Condition |
|---|---|---|
| 1 | INFEASIBLE | Required input missing |
| 2 | INVALID-INPUT | Impossible value (≤ 0 rating, negative length) |
| 3 | NON-COMPLIANT | A confirmed condition fails |
| 4 | UNVERIFIED-CONDITIONS | A required condition is not confirmed |
| 5 | COMPLIANT | All conditions confirmed and pass |
Key Facts
- The 10-ft rule allows a conductor as small as 1/10 of the feeder OCPD; the 25-ft rule requires at least 1/3 — a jump from 40 A to 133.3 A on a 400 A feeder.
- The fraction is based on the feeder OCPD rating, not on the feeder conductor ampacity.
- The tap ampacity used in the check must be after temperature correction (NEC 310.15) and conduit-fill adjustment — not the raw Table 310.16 value.
- The next-size-up allowance of NEC 240.4(B) does not apply to tap conductors; the conductor must meet the fraction outright at its corrected ampacity.
- An unconfirmed condition produces UNVERIFIED-CONDITIONS, not COMPLIANT — the calculator never assumes an unknown condition is satisfied.
- The transformer rule [240.21(B)(3)] uses a two-conductor test; headroom above the minimum is not reported for transformer taps.
- The outside rule [240.21(B)(5)] has no length limit but still requires physical protection, a single-OCPD termination, and an accessible disconnect near the point of entrance.
- Load-calculation article references change by NEC edition: Article 220 (2017–2023) becomes Article 120 in 2026.
Applications
- Feeding a nearby panelboard or disconnect from a switchboard or busway with a short tap conductor
- Sizing a 25-ft tap to machinery set away from the main distribution in an industrial plant
- Checking a transformer feeder tap under 240.21(B)(3) — the tap condition only, not full transformer protection
- Tapping an overhead busway down to floor equipment in a high-bay manufacturing building
- Running an outside tap of unlimited length to a separate structure under the (B)(5) conditions
- Verifying an existing tap before adding load or re-feeding equipment
- Plan review, inspection pre-screening, and exam preparation for the NEC tap rules
Example Calculation
Example 1: 25-ft Tap — COMPLIANT / MINIMAL HEADROOM
Inputs: Feeder OCPD 400 A, tap ampacity 150 A, length 20 ft, end OCPD 150 A, single OCPD: Confirmed, raceway: Confirmed.
Rule (auto): 25-ft, because length is over 10 ft and not over 25 ft
Required minimum: max(400 ÷ 3, 0, 150) = max(133.3, 150) = 150 A
Length: 20 ft ≤ 25 ft — PASS
Ampacity: 150 A ≥ 150 A — PASS
End OCPD: 150 A ≤ 150 A — PASS
Headroom: (150 − 150) / 150 = 0% → MINIMAL HEADROOM
Result: COMPLIANT / MINIMAL HEADROOM. The end OCPD rating drives the required minimum to 150 A, leaving no headroom above the NEC minimum.
Example 2: 25-ft Tap — NON-COMPLIANT (fraction)
Inputs: Feeder OCPD 400 A, tap ampacity 125 A, length 20 ft, single OCPD: Confirmed, raceway: Confirmed. No load or end OCPD entered.
Required minimum: 400 ÷ 3 = 133.3 A
Ampacity: 125 A < 133.3 A — FAIL
Deficit: 8.3 A
Result: NON-COMPLIANT. Tap is 8.3 A short of the 1/3 minimum. Increase tap to ≥ 134 A (next practical size), or reduce feeder OCPD to ≤ 375 A.
Example 3: Transformer Tap [240.21(B)(3)] — COMPLIANT
Inputs: Feeder OCPD 300 A, primary ampacity 110 A, secondary ampacity 200 A, 480 V primary / 240 V secondary, primary 5 ft + secondary 15 ft = 20 ft.
Required fraction: 300 ÷ 3 = 100 A
Primary: 110 A ≥ 100 A — PASS
Reflected secondary: 200 × (240/480) = 100 A ≥ 100 A — PASS
Length: 5 + 15 = 20 ft ≤ 25 ft — PASS
Result: COMPLIANT. No single headroom value for a two-conductor tap.
Example 4: 10-ft Tap — NON-COMPLIANT (fraction always active)
Inputs: Feeder OCPD 600 A, tap ampacity 50 A, length 8 ft, not-extended: Confirmed, raceway: Confirmed. No load.
Required minimum: max(600/10, 0, 0) = 60 A
Ampacity: 50 A < 60 A — FAIL
Deficit: 10 A
Result: NON-COMPLIANT. The 1/10 fraction is always active for 10-ft taps regardless of other conditions.
Standards & References
- NFPA 70 (NEC) — Product page with edition links — Official NFPA 70 page; links to all active editions and supplements.
- NFPA 70 Free Online Access — Free read-only NEC access after creating a free NFPA account.
- NEC 2023 — NFPA link.nfpa.org portal — NEC 2023 publication portal.
- NEC 2026 List-Change Reference Table (NFPA) — Covers 240.21(B)(1)–(3) changes.
- NEC 240.21(B) — Feeder tap conductor overcurrent protection: (B)(1) 10-ft, (B)(2) 25-ft, (B)(3) transformer primary+secondary, (B)(4) over-25-ft high-bay, (B)(5) outside unlimited.
- NEC 240.4 — Conductor protection context for tap conductors.
- NEC 310.15 / 310.16 — Conductor ampacity, temperature correction, and conduit-fill adjustment. Tap ampacity must be entered after these adjustments.
- NEC 240.4(B) — Next-standard-size-up allowance. Does NOT apply to tap conductors.
- EC&M — Understanding the Rules for Feeder Taps — Background reading on feeder tap rule application.
- Electrical Contractor Magazine — Applied Tap Rules — Practical 10-ft and 25-ft tap rule application.
Units
Currents are in amperes (A) representing conductor or device ratings. Lengths accept feet (ft) or meters (m); internal comparison uses the exact factor 1 m = 3.280839895 ft. The NEC prints rounded metric equivalents such as 3 m for 10 ft, 7.5 m for 25 ft, and 30 m for 100 ft, which differ slightly from exact conversion — lengths entered in meters very close to a limit should be confirmed against the adopted edition. Ampacity values display as whole numbers when inputs are whole, otherwise to one decimal place.
Limitations
- Screens NEC 240.21(B) feeder tap conductor sizing only. Does not size the feeder conductor, the OCPD, or compute the load.
- Tap ampacity must already include temperature correction (NEC 310.15) and conduit-fill adjustment. The calculator does not apply derating.
- Does not check available fault current, short-circuit rating, SCCR/AIC, ground-fault protection, or voltage drop.
- Does not verify physical routing or the actual measured tap length.
- Does not verify equipment listing or manufacturer instructions; where a tap supplies listed equipment, manufacturer sizing instructions may govern.
- Installation conditions are user confirmations, not verified by the tool.
- PV/source-side interconnections under Article 705 are out of scope — 705.12(A)(3) changes the 1/3 basis.
- Full transformer protection under Article 450 is out of scope beyond the 240.21(B)(3) primary+secondary test.
- The adopted NEC edition and local amendments govern the project; this is a screening aid, not a code approval.
Common Mistakes to Avoid
- Using the raw NEC Table 310.16 ampacity instead of the ampacity after temperature correction and conduit-fill adjustment.
- Applying the 1/10 fraction to a run longer than 10 ft — over 10 ft, the 25-ft rule's 1/3 fraction applies.
- Using the feeder conductor ampacity instead of the feeder OCPD rating as the basis for the fraction.
- Applying the next-standard-size-up allowance of 240.4(B) to a tap conductor, which the Code does not permit.
- Letting the termination OCPD exceed the tap conductor ampacity under the 25-ft rule.
- Assuming a run over 25 ft is allowed without the high-bay (B)(4) or outside (B)(5) conditions actually being met.
- Leaving conditions unconfirmed and reading the result as a pass — unconfirmed conditions produce UNVERIFIED-CONDITIONS, not COMPLIANT.
- Using only one voltage in the transformer rule — the secondary check depends on the secondary-to-primary voltage ratio.
- Treating the feeder OCPD as protecting the tap — the tap conductor is intentionally unprotected at its supply end.
Frequently Asked Questions
What is the difference between the 10-foot and 25-foot tap rules?
How do I calculate the minimum tap conductor ampacity?
Is the one-third fraction based on the feeder OCPD or the feeder conductor ampacity?
Can I use the next-size-up allowance from NEC 240.4(B) for a tap conductor?
What does an unverified result mean?
How is a transformer feeder tap evaluated?
What if my tap run is 25.1 feet?
Does this calculator size the tap conductor?
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Calculate
Select the NEC edition adopted by your AHJ. Affects article references and label text.
Auto selects the rule by length (10 ft → 10-ft rule; 10–25 ft → 25-ft rule; >25 ft requires confirmed conditions or manual selection). Transformer rule is always manual.
Rating of the breaker or fuse protecting the feeder on the line side of the tap. The tap ampacity fraction is based on this OCPD rating, not on feeder conductor ampacity.
Tap conductor ampacity at conditions of use — after temperature correction (NEC 310.15) and conduit-fill adjustment. Not required in transformer mode.
Length of the tap conductor run from the supply point to the termination. Not required in transformer mode.
Select feet or meters. Internal comparison uses exact ft conversion (1 m = 3.280839895 ft). NEC states rounded metric equivalents — near-limit runs should be confirmed against the adopted edition.
Load per Article 220 (2017–2023) or Article 120 (2026). Tap conductor must be ≥ calculated load. If not entered, the load condition is unverified.
Rating of the OCPD or equipment at the tap termination. For the 10-ft rule, tap ampacity must be ≥ end OCPD. For 25-ft / high-bay rules, end OCPD must be ≤ tap ampacity.
Required for all tap rules. 10-ft rule: raceway required where tap leaves the enclosure. Other rules: protected throughout the run.
Required for 25-ft, transformer, high-bay, and outside rules. The tap must terminate in a single overcurrent device, not in a distribution panel or multiple OCPDs.