NEC 240.21(B): Feeder Tap Rules Explained for Commercial Installations

Last updated: October 4, 2026

NEC 240.21(B) permits tap conductors to connect to a feeder without overcurrent protection at the tap point — but only under four strict rules: the 10-foot rule, the 25-foot rule, transformer secondary taps, and outside taps. Each rule has its own length, ampacity, and termination limits, and missing any one condition fails the tap.

Feeder taps are one of the most misunderstood and most failed areas of the NEC. The section exists to allow flexibility in commercial electrical design without compromising safety — and misapplying it almost guarantees red tags, rework, and delays. If you are new to the Code framework, start with our NEC overview; for system fundamentals, review Services and Feeders Explained before diving into this highly technical section.

What does NEC 240.21(B) cover?

NEC 240.21(B) governs tap conductors connected to feeder conductors without overcurrent protection at the point of connection. Normally, conductors must be protected at their ampacity; 240.21(B) allows exceptions — but only under strict conditions. This section applies almost exclusively to commercial and industrial installations.

Why do feeder tap rules exist?

Feeder taps let contractors supply remote equipment or panels efficiently, avoid excessive breaker sizes, and reduce unnecessary conductor runs.

The risk they manage: improperly installed tap conductors can overheat, create fire hazards, and bypass critical overcurrent protection. NEC 240.21(B) balances design flexibility with safety.

What are the four feeder tap rules in NEC 240.21(B)?

The four rules contractors use are the 10-foot tap rule, the 25-foot tap rule, transformer secondary taps, and outside tap conductors. Each has its own conditions, and all conditions of the rule you use must be met.

What is the 10-foot tap rule — NEC 240.21(B)(1)?

A tap up to 10 feet long is permitted when all of the following are met:

• Length does not exceed 10 feet.
• Ampacity is at least the load served, and not less than the rating of the device protecting the feeder multiplied by applicable adjustment factors.
• The tap conductors terminate in a single overcurrent device.
• The conductors are installed in a manner that minimizes damage.

📌 Common use: feeding a nearby panelboard from a switchboard.

What is the 25-foot tap rule — NEC 240.21(B)(2)?

A tap up to 25 feet long is permitted when the tap conductor's ampacity is not less than one-third of the feeder overcurrent device rating, the conductors terminate in a single overcurrent device, and they are protected from physical damage.

This is one of the most misapplied rules in the field, and inspectors scrutinize it heavily.

When do transformer secondary tap rules apply — NEC 240.21(B)(3)?

When tap conductors supply a transformer secondary. Key requirements:

• Length does not exceed 25 feet (primary plus secondary).
• The conductors have sufficient ampacity for the load.
• Secondary conductors terminate in an overcurrent device sized per NEC 240.4.
• Proper grounding and bonding are provided.

This rule ties directly into service and feeder coordination.

When are outside tap conductors allowed — NEC 240.21(B)(5)?

Tap conductors run outside a building are permitted when the conductors are protected from physical damage, terminate in a single disconnect, and enter the building immediately. This rule is most often used in utility-fed or campus-style installations.

Which other NEC articles affect feeder taps?

Feeder taps cannot be evaluated in isolation — inspectors expect coordination with:

• NEC 215.2 — Feeder Conductor Sizing
• NEC 215.10 — Ground-Fault Protection of Feeders
• NEC 225.31 — Disconnecting Means for Buildings
• NEC 230.71 — Service Disconnecting Means
• NEC 220.87 — Existing Load Calculations

If one of these is wrong, the tap rule often fails too.

What are the most common feeder tap mistakes?

The failures inspectors flag most often — and they are among the top commercial inspection failures overall:

• ❌ Exceeding tap length limits.
• ❌ Undersizing tap conductors.
• ❌ Feeding multiple overcurrent devices from a single tap.
• ❌ Poor physical protection of tap conductors.
• ❌ Confusing feeder taps with service conductors.

What are the labeling requirements for feeder taps?

While NEC 240.21(B) focuses on conductor protection, NEC 110.22 still applies. Inspectors expect tap-fed equipment clearly identified, disconnects labeled with source information, and panels marked accurately.

✅ At Print Pro, we supply NEC-compliant feeder, panel, and disconnect labels that help contractors pass inspections without delays.

How do you install feeder taps that pass inspection?

• If you are unsure which tap rule applies, don't guess — verify with the AHJ.
• Keep tap conductors as short as physically possible.
• Protect tap conductors like they're unprotected — because they are.
• Document tap calculations and lengths during rough-in.
• Label tap-fed equipment clearly to avoid inspection confusion.

NEC 240.21(B) frequently asked questions

How long can a feeder tap be?

Up to 10 feet under NEC 240.21(B)(1) or up to 25 feet under 240.21(B)(2), each with its own ampacity and termination conditions. Outside taps under 240.21(B)(5) follow separate rules for conductors run outside a building.

What size must a 25-foot tap conductor be?

Under NEC 240.21(B)(2), the tap conductor's ampacity must be not less than one-third of the rating of the overcurrent device protecting the feeder, and the tap must terminate in a single overcurrent device.

Can one feeder tap supply multiple breakers?

No. The 10-foot and 25-foot tap rules both require the tap conductors to terminate in a single overcurrent device. Feeding multiple devices from one tap is a common inspection failure.

Do tap conductors and tap-fed equipment need labels?

Yes. NEC 110.22 requires disconnecting means to be legibly marked to indicate their purpose, so inspectors expect tap-fed equipment identified, disconnects labeled with source information, and panels marked accurately.

Conclusion

NEC 240.21(B) allows feeder taps only when strict conditions are met. Success comes down to length limits, conductor sizing, physical protection, and coordination with the rest of the electrical system.

To see how this fits into the larger NEC framework, review the NEC overview, Services and Feeders, NEC 215.2 — Feeder Sizing, and NEC 225.31 — Building Disconnects.

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