7.2 Feeder & Branch Circuit Tap Rules (NEC 240.21)

Feeder & Branch Circuit Tap Rules (NEC 240.21)

In standard electrical distribution, every conductor must be protected at the exact point where it receives its supply. However, running full-sized conductors throughout a large industrial plant or commercial building to feed small subpanels, motor controllers, or disconnects is economically prohibitive and physically impractical. NEC 240.21 provides specific Tap Rules that permit conductors to be tapped from larger feeders without an overcurrent protective device at the tap point, provided rigorous physical protection, length, and termination criteria are met.

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1. The Fundamental Principle & Tap Conductor Definition

The Point of Supply Rule (NEC 240.21)

NEC 240.21 General Rule: Overcurrent protection shall be provided in each ungrounded circuit conductor at the point where the conductor receives its supply, except as specifically permitted in 240.21(A) through (H).

Definition of a Tap Conductor (NEC 240.2)

A Tap Conductor is defined as a conductor, other than a service conductor, that has overcurrent protection ahead of its point of supply exceeding the value permitted for similar conductors that are protected as described in NEC 240.4.

+-----------------------------------------------------------------------------+
|                     ANATOMY OF A FEEDER TAP INSTALLATION                    |
|                                                                             |
|   [ 400A Main Feeder Breaker ]                                              |
|                |                                                            |
|   ============== 500 kcmil Copper Feeder (Ampacity = 380A) =============    |
|                |                                                            |
|                +--- [ TAP POINT (No OCPD here!) ]                           |
|                |                                                            |
|                |    <-- Tap Conductor (Length <= 10 ft or 25 ft)            |
|                |        (Enclosed in approved raceway / protected)          |
|                v                                                            |
|   [ Subpanel Main OCPD (Terminating Breaker / Fuses) ]                      |
|   (Limits current to the allowable ampacity of the tap conductor)           |
+-----------------------------------------------------------------------------+

The Golden Rule of Tap Sizing: NO Round-Up Permitted!

The Next Higher Standard Rating Rule (NEC 240.4(B)) NEVER applies to tap conductors. The overcurrent protective device at the termination of a tap conductor must have a rating that is less than or equal to the allowable ampacity of the tap conductor.

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2. The 10-Foot Feeder Tap Rule (NEC 240.21(B)(1))

Feeder conductors are permitted to be tapped, without overcurrent protection at the tap point, for runs not exceeding 10 feet (3.0 m) if ALL of the following conditions are met:

1. Length Limitation: The total length of the tap conductors does not exceed 10 feet (3.0 m).
2. Ampacity Requirements: The ampacity of the tap conductors is:
   • Not less than the combined computed loads on the circuits supplied by the tap conductors, AND
   • Not less than the rating of the equipment containing an overcurrent protective device supplied by the tap conductors, OR not less than the rating of the overcurrent protective device at the termination of the tap conductors.
3. Physical Protection: The tap conductors are enclosed in an approved raceway, cable, or within the enclosure of an enclosed switchboard, switchgear, panelboard, or control devices.
4. No Extension: The tap conductors do not extend beyond the switchboard, switchgear, panelboard, disconnecting means, or control devices they supply.

+-----------------------------------------------------------------------------+
|                   10-FOOT FEEDER TAP RULE (NEC 240.21(B)(1))                |
|                                                                             |
|   MAXIMUM LENGTH:      10 Feet (3.0 meters)                                 |
|   MINIMUM AMPACITY:    1. >= Combined computed load                         |
|                        2. >= Rating of terminating OCPD or panelboard bus   |
|   ENCLOSURE:           Must be in raceway, cable, or equipment enclosure    |
|   TERMINATION:         Must terminate directly in panel/switchboard/device  |
|   RATIO RULE:          No 1/10th ratio required for FEEDER taps             |
|                        (Contrast with Transformer Secondary Taps!)          |
+-----------------------------------------------------------------------------+

Worked Example (10-Foot Tap):

A 600A main feeder in a wireway is tapped to supply a 100A subpanel located 6 feet away. The calculated load on the subpanel is 78 amperes, and the subpanel contains a 100A main circuit breaker.

• Length = 6 ft $\le 10\text{ ft}$ (Compliant).
• Enclosed in rigid metallic conduit (Compliant).
• Minimum conductor ampacity required = 100 Amperes (must equal terminating OCPD).
• Minimum copper conductor: #3 AWG THHN Cu (rated 100A at 75°C).
• Exam Trap: An apprentice selects #4 AWG Cu (rated 85A at 75°C) arguing that 85A exceeds the 78A computed load and rounds up to 100A under 240.4(B). This is a code violation: tap conductors cannot round up; ampacity must be $\ge 100\text{A}$.

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3. The 25-Foot Feeder Tap Rule (NEC 240.21(B)(2))

Feeder conductors are permitted to be tapped for runs not exceeding 25 feet (7.5 m) if ALL of the following conditions are met:

1. Length Limitation: The total length of the tap conductors does not exceed 25 feet (7.5 m).
2. The One-Third (1/3) Ampacity Rule: The ampacity of the tap conductors is not less than one-third (1/3) of the rating of the overcurrent protective device protecting the feeder conductors from which they are supplied: $I_{\text{tap}} \ge \frac{I_{\text{feeder\_OCPD}}}{3}$
3. Physical Protection: The tap conductors are protected from physical damage by being enclosed in an approved raceway, cable tray, or other approved cable method.
4. Single OCPD Termination: The tap conductors terminate in a single circuit breaker or a single set of fuses that limits the load to the allowable ampacity of the tap conductors.

+-----------------------------------------------------------------------------+
|                   25-FOOT FEEDER TAP RULE (NEC 240.21(B)(2))                |
|                                                                             |
|   MAXIMUM LENGTH:      25 Feet (7.5 meters)                                 |
|   MINIMUM AMPACITY:    At least 1/3 of the upstream feeder OCPD rating      |
|   PHYSICAL PROTECTION: Enclosed in raceway, cable tray, or cable armor      |
|   TERMINATION:         MUST terminate in a SINGLE circuit breaker or fuse set|
|                        sized <= tap conductor allowable ampacity            |
+-----------------------------------------------------------------------------+

Worked Example (25-Foot Tap Calculation):

A 400-Ampere feeder protected by a 400A circuit breaker supplies a tap running 22 feet in EMT to a subpanel. What is the minimum conductor size (75°C copper) and maximum terminating circuit breaker permitted?

1. Calculate Minimum Tap Ampacity: $I_{\text{tap\_min}} = \frac{400\text{ A}}{3} = \mathbf{133.33\text{ Amperes}}$
2. Select Conductor from NEC Table 310.16 (75°C Copper):
   • #1 AWG Cu = 130 Amperes (Insufficient: $130\text{ A} < 133.33\text{ A}$)
   • 1/0 AWG Cu = 150 Amperes (Compliant: $150\text{ A} \ge 133.33\text{ A}$)
3. Determine Maximum Terminating OCPD:
   • The terminating breaker cannot exceed the conductor ampacity ($150\text{ A}$).
   • Permitted standard breaker: 150 Amperes.

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4. Outside Feeder Taps of Unlimited Length (NEC 240.21(B)(5))

Where feeder tap conductors are installed completely outdoors, length restrictions are lifted because the open-air environment significantly mitigates fire hazards to building structures:

+-----------------------------------------------------------------------------+
|              OUTSIDE FEEDER TAPS OF UNLIMITED LENGTH (240.21(B)(5))         |
|                                                                             |
|   LOCATION:            Entirely outside buildings (except at entrance point)|
|   LENGTH:              UNLIMITED distance outdoors                          |
|   PHYSICAL PROTECTION: Enclosed in approved raceway or cable                |
|   TERMINATION:         Single circuit breaker or single set of fuses        |
|   DISCONNECT POINT:    Located at a readily accessible location either      |
|                        outside the building or inside nearest point of entry|
|   OCPD SIZING:         Limits load to the allowable ampacity of tap wire    |
+-----------------------------------------------------------------------------+

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5. Transformer Secondary Taps (NEC 240.21(C))

Except for specific 2-wire single-phase and 3-wire Delta-Delta transformers meeting NEC 240.4(F), primary overcurrent protective devices DO NOT protect transformer secondary conductors. Transformer secondary conductors must be protected under the tap rules of NEC 240.21(C):

10-Foot Secondary Tap Rule (NEC 240.21(C)(2))

Secondary conductors not over 10 feet must satisfy:

1. Length $\le 10\text{ feet}$.
2. Ampacity $\ge$ computed load and $\ge$ rating of device supplied or termination OCPD.
3. Ampacity not less than one-tenth (1/10) of the primary OCPD rating multiplied by the primary-to-secondary voltage ratio: $I_{\text{sec\_ampacity}} \ge \left( \frac{I_{\text{pri\_OCPD}}}{10} \right) \times \left( \frac{V_{\text{primary}}}{V_{\text{secondary}}} \right)$
4. Enclosed in an approved raceway.

25-Foot Secondary Tap Rule (NEC 240.21(C)(6))

Secondary conductors not over 25 feet must satisfy:

1. Length $\le 25\text{ feet}$.
2. Ampacity not less than one-third (1/3) of the primary OCPD rating multiplied by the primary-to-secondary voltage ratio: $I_{\text{sec\_ampacity}} \ge \left( \frac{I_{\text{pri\_OCPD}}}{3} \right) \times \left( \frac{V_{\text{primary}}}{V_{\text{secondary}}} \right)$
3. Terminates in a single circuit breaker or set of fuses rated $\le$ secondary conductor ampacity.
4. Protected from physical damage.

+-----------------------------------------------------------------------------+
|                   SUMMARY OF TAP RULES COMPARISON TABLE                     |
|                                                                             |
|   TAP TYPE             MAX LENGTH   MINIMUM AMPACITY RATIO   TERMINATION    |
|   -----------------------------------------------------------------------   |
|   10-Ft Feeder (B)(1)  10 Feet      >= Terminating OCPD      Panel / Switch |
|   25-Ft Feeder (B)(2)  25 Feet      >= 1/3 Feeder OCPD       Single OCPD    |
|   Outside Feeder (B)(5) Unlimited   Sized to disconnect      Single OCPD    |
|   10-Ft Sec. (C)(2)    10 Feet      >= 1/10 Pri OCPD x Ratio Device / OCPD  |
|   25-Ft Sec. (C)(6)    25 Feet      >= 1/3 Pri OCPD x Ratio  Single OCPD    |
+-----------------------------------------------------------------------------+