4.4 Feeder & Transformer Tap Rules (NEC 240.21)

Feeder & Transformer Tap Rules (NEC 240.21)

Under the general rule of NEC 240.21, overcurrent protection must be provided in each ungrounded conductor at the exact point where the conductor receives its electrical supply. However, running a full-size feeder directly to every small disconnect or subpanel is economically and physically impractical in commercial and industrial facilities.

To accommodate practical electrical distribution, the Code provides specific, strictly regulated Tap Rules under NEC 240.21(B) (Feeder Taps) and NEC 240.21(C) (Transformer Secondary Taps). Mastery of these tap rules and their underlying safety mechanics is a cornerstone of the Idaho Journeyman Electrician examination.

1. Tap Conductor Definition & General Prohibitions

+-----------------------------------------------------------------------------+
|                        WHAT IS A TAP CONDUCTOR? (NEC 240.2)                 |
|                                                                             |
|   "A conductor, other than a service conductor, that has overcurrent        |
|    protection ahead of its point of supply exceeding the value permitted    |
|    for similar conductors in Section 240.4."                                |
|                                                                             |
|   [FUNDAMENTAL TAP RULES]                                                   |
|   1. A tap conductor CANNOT supply another tap (NO TAPS FROM TAPS!).        |
|   2. Tap conductors must be enclosed in raceway or approved cable from the   |
|      tap point to the termination enclosure.                                |
|   3. Tap conductors must terminate in a single OCPD or group of OCPDs as    |
|      specifically permitted by the individual tap section.                 |
|   4. The 'Next Higher Standard Device' rule (240.4(B)) CANNOT be applied to |
|      tap conductor terminations!                                            |
+-----------------------------------------------------------------------------+

Why Do Tap Rules Exist?

When a small conductor is tapped off a large feeder (e.g., a 10 AWG wire tapped off a 400A feeder), the 400A upstream breaker cannot protect the 10 AWG wire from sustained thermal overloads. The tap rules ensure safety by:

• Limiting Length: Minimizing the physical exposure distance of the undersized conductor.
• Mandating Mechanical Protection: Requiring raceways or metal enclosures to prevent physical damage and short circuits.
• Mandating Downstream OCPD: Ensuring that a downstream overcurrent device limits maximum current to the conductor's rated ampacity before an overload can occur.

2. Feeder Tap Rules (NEC 240.21(B))

+-----------------------------------------------------------------------------+
|                     THE 10-FT VS. 25-FT FEEDER TAP RULES                    |
|                                                                             |
|   [10-FOOT FEEDER TAP (NEC 240.21(B)(1))]                                   |
|   * Max Length: 10 Feet (3.0 m)                                             |
|   * Minimum Ampacity: Not less than the computed load on the circuit AND    |
|     not less than the rating of the device/panelboard supplied.             |
|   * Enclosure: Enclosed in raceway, cable tray, or metal cabinet.           |
|   * Extent: Cannot extend beyond switchboard, panelboard, or control devices|
|                                                                             |
|   [25-FOOT FEEDER TAP (NEC 240.21(B)(2)) - THE 1/3 RULE]                    |
|   * Max Length: 25 Feet (7.5 m)                                             |
|   * Minimum Ampacity: At least 1/3 (33.3%) of the upstream feeder OCPD!     |
|   * Protection: Enclosed in raceway or protected from physical damage.      |
|   * Termination: Must terminate in a SINGLE circuit breaker or set of fuses!|
+-----------------------------------------------------------------------------+

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

Conductors are permitted to be tapped from a feeder without overcurrent protection at the tap point where ALL of the following conditions are satisfied:

1. The length of the tap conductors does not exceed 10 feet (3.0 m).
2. 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 device supplied by the tap conductors, or not less than the rating of the overcurrent protective device at the termination.
3. The tap conductors do not extend beyond the switchboard, switchgear, panelboard, disconnecting means, or control devices they supply.
4. Except at the point of connection to the feeder, the tap conductors are enclosed in a raceway, which extends from the tap to the enclosure of an enclosed switchboard, switchgear, a panelboard, or control devices, or to the back of an open switchboard.
5. For field installations where the tap conductors leave the enclosure or vault in which the tap is made, the rating of the overcurrent device on the line side of the tap conductors cannot exceed 10 times the tap conductor's ampacity.

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

Conductors are permitted to be tapped from a feeder without overcurrent protection at the point of supply where ALL of the following conditions are met:

1. The length of the tap conductors does not exceed 25 feet (7.5 m).
2. The ampacity of the tap conductors is not less than 1/3 of the rating of the overcurrent device protecting the feeder conductors.
3. The tap conductors are suitably protected from physical damage by being enclosed in an approved raceway or by other approved means.
4. The tap conductors terminate in a single circuit breaker or a single set of fuses that limits the load to the ampacity of the tap conductors.

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

Feeder conductors installed outside of a building or structure are permitted to be tapped without overcurrent protection at the tap point if:

1. The conductors are suitably protected from physical damage.
2. The conductors terminate at a single circuit breaker or a single set of fuses that limits the load to the ampacity of the conductors.
3. The overcurrent device for the conductors is an integral part of a disconnecting means or located immediately adjacent thereto.
4. The disconnecting means is installed at a readily accessible location either outside of a building or structure, or inside nearest the point of entrance of the conductors.

3. Transformer Secondary Tap Rules (NEC 240.21(C))

Under NEC 240.21(C), secondary conductors connected to a transformer are considered tap conductors because primary overcurrent devices generally cannot protect secondary conductors against overcurrent due to phase transformation and winding ratios (especially in 3-phase delta-wye transformations).

+-----------------------------------------------------------------------------+
|                 TRANSFORMER SECONDARY TAPS (NEC 240.21(C))                  |
|                                                                             |
|   [WHY PRIMARY OCPD CANNOT PROTECT SECONDARY CONDUCTORS]                    |
|   * In a 480V Delta to 208Y/120V Wye Transformer, a 100A primary OCPD       |
|     corresponds to 231A on the secondary (Ratio = 480 / 208 = 2.31).        |
|   * A single-phase line-to-neutral secondary fault draws only ~58% of       |
|     balanced 3-phase current on the primary, leaving the primary breaker    |
|     unable to trip before secondary wires burn up!                          |
|                                                                             |
|   [10-FOOT SECONDARY TAP (NEC 240.21(C)(2))]                                |
|   * Length: Max 10 Feet.                                                    |
|   * Ampacity: Primary-to-secondary ratio equivalent + terminates in single  |
|     OCPD or panelboard with main OCPD matching tap ampacity.                |
|                                                                             |
|   [25-FOOT INDUSTRIAL SECONDARY TAP (NEC 240.21(C)(3))]                     |
|   * Length: Max 25 Feet in industrial installations with qualified personnel.|
|   * Secondary conductor ampacity must be at least 1/3 of secondary FLC.     |
+-----------------------------------------------------------------------------+

Transformer Secondary Tap Code Requirements Summary

4. Master Comparison: 10-Foot vs. 25-Foot Feeder Tap Sizing Rules

5. Worked Step-by-Step Examination Tap Sizing Calculations

Example 1: 10-Foot Feeder Tap Sizing

Question: A 400-ampere, 480V, 3-phase feeder supplies a large industrial distribution trough. An electrician wishes to install an 8-foot raceway tap to supply a 100-ampere, 3-phase enclosed circuit breaker disconnect serving a pump controller. The computed continuous load on the pump is 68A (68A x 1.25 = 85A). What is the minimum size copper THHN conductor (75°C terminals) required for this 10-foot tap?

+-----------------------------------------------------------------------------+
|                        10-FOOT TAP SCHEMATIC DIAGRAM                        |
|                                                                             |
|   [400A Main Feeder Breaker]                                                |
|              |                                                              |
|              v (400A Feeder Conductor)                                      |
|   +------------------------------------+                                    |
|   |    Feeder Wireway / Trough         |                                    |
|   +------------------------------------+                                    |
|              |                                                              |
|              | <--- 8-foot Tap Conductor in EMT                             |
|              v                                                              |
|   [100A Enclosed Disconnect Breaker]                                        |
+-----------------------------------------------------------------------------+

Step-by-Step Solution:

1. Verify Tap Length (NEC 240.21(B)(1)): Length is 8 feet (<= 10 feet). Condition met.
2. Determine Minimum Required Ampacity:
   • Must not be less than computed load: 85 Amperes.
   • Must not be less than the rating of the termination device / OCPD: The disconnect contains a 100A circuit breaker.
   • Therefore, minimum tap ampacity is 100 Amperes.
3. Check 10-to-1 Upstream Ratio Rule:
   • Feeder OCPD (400A) <= 10 x 100A = 1000A. Condition met.
4. Select Conductor from Table 310.16 (75°C Column):
   • 4 AWG Cu = 85 A (Insufficient for 100A termination)
   • 3 AWG Cu = 100 A (Complies exactly with 100A requirement)
5. Conclusion: Minimum size conductor is 3 AWG Copper THHN.

Example 2: 25-Foot Feeder Tap Sizing (The 1/3 Rule)

Question: A 600-ampere, 208Y/120V feeder protected by a 600A circuit breaker passes through a commercial building. An electrician intends to tap this feeder with a 22-foot run of copper THHN conductors in EMT to supply a new lighting subpanel containing a single 200A main circuit breaker. What is the minimum size copper conductor required for the tap?

Step-by-Step Solution:

1. Verify Tap Length (NEC 240.21(B)(2)): Length is 22 feet (<= 25 feet). Condition met.
2. Apply the 1/3 Feeder OCPD Rule (NEC 240.21(B)(2)(1)):
   • Minimum Tap Ampacity = Feeder OCPD Rating / 3 = 600 A / 3 = 200.0 Amperes
3. Check Termination OCPD:
   • Tap terminates in a single 200A main circuit breaker.
   • Conductor ampacity must be at least 200A.
4. Select Conductor from Table 310.16 (75°C Column):
   • 2/0 AWG Cu = 175 A (Insufficient)
   • 3/0 AWG Cu = 200 A (Exactly meets the 200A requirement)
5. Conclusion: Minimum conductor size is 3/0 AWG Copper THHN.

Example 3: Exam Trap - Violating the 1/3 Rule on 25-Foot Taps

Question: A contractor installs an 18-foot tap consisting of 1 AWG THHN copper conductors (130A ampacity at 75°C) from a 450A feeder breaker, terminating in a 125A fused safety switch. Does this installation comply with NEC 240.21(B)(2)?

Step-by-Step Solution:

1. Check Tap Length: 18 feet (<= 25 ft). Valid.
2. Calculate 1/3 of Upstream Feeder OCPD:
   • Minimum Required Ampacity = 450 A / 3 = 150 Amperes
3. Compare with Conductor Ampacity:
   • 1 AWG Copper has an allowable ampacity of only 130 Amperes at 75°C.
   • 130 A < 150 A (VIOLATION of the 1/3 rule).
4. Conclusion: NON-COMPLIANT. Even though the tap terminates in a 125A switch, the conductor itself must have an ampacity of at least 150A (requiring minimum 1/0 AWG Cu at 150A) to satisfy the 1/3 rule.