NEC 240.21(B): Feeder Tap Rules Explained for Commercial Installations
Feeder taps are one of the most misunderstood and most failed areas of the NEC. NEC 240.21(B) exists to allow flexibility in commercial electrical design without compromising safety—but only when very specific conditions are met.
For electrical contractors, misapplying feeder tap rules almost guarantees red tags, rework, and delays.
👉 If you’re new to the Code framework, start with our NEC Overview: What It Is and Why It Matters.
👉 For system fundamentals, review Services and Feeders Explained before diving into this highly technical section.
What NEC 240.21(B) Covers
NEC 240.21(B) governs tap conductors that are connected to feeder conductors without overcurrent protection at the point of connection.
Normally, conductors must be protected at their ampacity. NEC 240.21(B) allows exceptions—but only under strict conditions.
This section applies almost exclusively to commercial and industrial installations.
Why Feeder Tap Rules Exist
Feeder taps allow contractors to:
-
Supply remote equipment or panels efficiently
-
Avoid excessive breaker sizes
-
Reduce unnecessary conductor runs
However, improperly installed tap conductors can:
-
Overheat
-
Create fire hazards
-
Bypass critical overcurrent protection
NEC 240.21(B) balances design flexibility with safety.
The Four Primary Feeder Tap Rules
1. 10-Foot Tap Rule — NEC 240.21(B)(1)
Tap conductors are permitted if 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 OCPD protecting the feeder multiplied by applicable adjustment factors
-
Tap conductors terminate in a single overcurrent device
-
Conductors are installed in a manner that minimizes damage
📌 Common use: feeding a nearby panelboard from a switchboard.
2. 25-Foot Tap Rule — NEC 240.21(B)(2)
This is one of the most misapplied rules in the field.
Requirements include:
-
Length does not exceed 25 feet
-
Tap conductor ampacity is not less than 1/3 of the feeder OCPD rating
-
Conductors terminate in a single overcurrent device
-
Conductors are protected from physical damage
Inspectors scrutinize this rule heavily.
3. Transformer Secondary Taps — NEC 240.21(B)(3)
Applies when tap conductors supply a transformer secondary.
Key requirements:
-
Length does not exceed 25 feet
-
Conductors have sufficient ampacity for the load
-
Secondary conductors terminate in an OCPD sized per NEC 240.4
-
Proper grounding and bonding are provided
This rule ties directly into service and feeder coordination.
4. Outside Tap Conductors — NEC 240.21(B)(5)
Permits tap conductors outside a building under strict conditions:
-
Conductors are protected from physical damage
-
Terminate in a single disconnect
-
Enter the building immediately
Often used in utility-fed or campus-style installations.
How NEC 240.21(B) Connects to Other NEC Articles
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.
Common Contractor Mistakes
❌ Exceeding tap length limits
❌ Undersizing tap conductors
❌ Feeding multiple OCPDs from a single tap
❌ Poor physical protection of tap conductors
❌ Confusing feeder taps with service conductors
These mistakes are among the top commercial inspection failures.
Labeling Requirements (Often Missed)
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
-
Panels marked accurately
✅ At Print Pro, we supply NEC-compliant feeder, panel, and disconnect labels that help contractors pass inspections without delays.
Pro Tips for Contractors
-
If you’re 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.
Conclusion
NEC 240.21(B) allows feeder taps only when strict conditions are met. For contractors, 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:
👉 Need inspection-ready feeder and panel labels? Visit PrintProAZ.com for NEC-compliant labeling solutions trusted by commercial electricians nationwide.
