Checks a feeder tap conductor's length, ampacity, physical protection, and downstream overcurrent device rating against one of the three most commonly applied NEC 240.21(B) tap rules — the exceptions that let a tap be sized smaller than the feeder's own overcurrent device would otherwise require, provided every condition of the specific rule is met.
Standards followed
NEC 240.21(B)(1) — 10-ft tap ruleNEC 240.21(B)(2) — 25-ft tap ruleNEC 240.21(B)(5) — outside taps of unlimited length
What it can do
- 10-ft tap: checks length ≤10ft, tap ampacity ≥1/10 of the feeder OCPD, physical protection, and terminal OCPD rating.
- 25-ft tap: checks length ≤25ft, tap ampacity ≥1/3 of the feeder OCPD, physical protection, and terminal OCPD rating.
- Outside unlimited-length tap: checks the location/disconnect conditions of 240.21(B)(5) with no minimum ampacity ratio requirement.
- Itemized pass/fail for every condition of the selected rule, plus an overall compliant/non-compliant verdict.
Example problem
400A feeder OCPD, 8ft tap run, tap conductor rated 60A, terminates in a single 60A breaker, enclosed in raceway.
- Minimum tap ampacity = 400/10 = 40A — the 60A tap conductor satisfies this.
- 8ft ≤ 10ft, enclosed in raceway, and the 60A terminal OCPD ≤ 60A tap ampacity — every 240.21(B)(1) condition is met.
COMPLIANT with the 10-ft tap rule.
This tool covers the three most frequently used tap rules only — the transformer primary/secondary tap rules of 240.21(B)(3)/(C) and the over-25-ft high-bay manufacturing building rule of 240.21(B)(4) have their own distinct conditions and are not covered here. Always verify the complete rule text and any local amendments before relying on a tap installation.