6.3 Feeder & Transformer Tap Rules (NEC 240.21)

6.3 Feeder & Transformer Tap Rules (NEC 240.21)

Exam Focus: Point of supply rule (240.21), definition of tap conductor (240.2), 10-foot feeder tap rule (240.21(B)(1)), 25-foot feeder tap rule (240.21(B)(2)), 100-foot high-bay tap rule (240.21(B)(3)), and transformer secondary tap rules (240.21(C)).

The general overcurrent protection rule in NEC 240.21 states that an overcurrent protective device must be installed at the point where a conductor receives its supply. If a smaller conductor is spliced or tapped onto a larger feeder conductor, the upstream breaker is too large to protect the smaller wire under standard rules. To avoid installing costly main fused disconnects at every splice point, the NEC provides specific Tap Rules under NEC 240.21(B) and 240.21(C).

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What is a Tap Conductor? (NEC 240.2 Definition)

NEC 240.2 Definition: A tap conductor is a conductor, other than a service conductor, that has overcurrent protection ahead of its point of supply that exceeds the value specified in Part III of Article 240 for the conductor ampacity.

                         ANATOMY OF A FEEDER TAP
+-------------------------------------------------------------------------+
| Main Distribution Feeder (e.g., 400A Breaker / 500 kcmil Wire)         |
| ========================= SPLICED TAP POINT ========================== |
|                                  |                                      |
|                       Tap Conductor (e.g., 1/0 AWG Wire)               |
|                       Length <= 10 ft or <= 25 ft                       |
|                                  |                                      |
|                        Single Terminating OCPD                          |
|                        (e.g., 125A Main Breaker Panel)                 |
+-------------------------------------------------------------------------+

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

Feeder tap conductors not over 3.0 meters (10 feet) in length are permitted without overcurrent protection at the point of supply, provided ALL five of the following conditions are met:

1. Calculated Load: The ampacity of the tap conductor is not less than the combined calculated loads on the circuits supplied by the tap conductors.
2. 10% Feeder Rating Rule: The ampacity of the tap conductor is not less than 10% (1/10th) of the rating of the overcurrent device protecting the feeder conductors.
3. Raceway Enclosure: The tap conductors do not extend beyond the switchboard, panelboard, disconnecting means, or control devices they supply, and are enclosed in an approved raceway (conduit, tubing, or wireway).
4. Termination Device: The tap conductors terminate in a single circuit breaker or single set of fuses that limits the load to the ampacity of the tap conductors (or a main breaker in a panelboard).
5. Physical Limits: The tap conductors do not leave the enclosure or room except when enclosed in a raceway.

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

Feeder tap conductors not over 7.5 meters (25 feet) in length are permitted without overcurrent protection at the point of supply, provided ALL four of the following conditions are met:

1. 1/3 Feeder Rating Rule: The ampacity of the tap conductor is not less than 1/3 (33.33%) of the rating of the overcurrent device protecting the feeder conductors.
2. Physical Protection: The tap conductors are protected from physical damage by being enclosed in an approved raceway or by other approved means.
3. Single OCPD Termination: The tap conductors terminate in a single circuit breaker or set of fuses having a rating NOT greater than the ampacity of the tap conductors.
4. Length Restriction: The total length of the tap conductor does not exceed 25 feet.

                      10-FOOT VS. 25-FOOT TAP RULE COMPARISON
+-----------------------------------+-----------------------------------+
| 10-FOOT TAP RULE (240.21(B)(1))   | 25-FOOT TAP RULE (240.21(B)(2))   |
+-----------------------------------+-----------------------------------+
| Max Length: 10 Feet (3.0 m)       | Max Length: 25 Feet (7.5 m)       |
| Min Ampacity: >= 10% Feeder OCPD  | Min Ampacity: >= 33.33% (1/3) OCPD|
| Min Ampacity: >= Calculated Load  | Physical damage protection mandatory|
| Enclosed in raceway/enclosure     | Terminates in single OCPD <= Wire |
| Terminates in single OCPD         | Ampacity                          |
+-----------------------------------+-----------------------------------+

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Specialized Feeder Tap Rules

1. 100-Foot High-Bay Industrial Tap Rule (NEC 240.21(B)(3))

In high-bay industrial manufacturing buildings with walls over 35 feet high, tap conductors up to 100 feet long are permitted IF:

• Conditions of maintenance and supervision ensure only qualified persons service the system.
• The tap ampacity is at least 1/3 (33.33%) of the feeder OCPD rating.
• The horizontal length of the tap does not exceed 25 feet, and total length does not exceed 100 feet.
• Conductors are enclosed in raceways and terminate in a single OCPD.

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

Outside taps may be of unlimited length without OCPD at the supply point IF:

• Conductors are suitably protected from physical damage.
• Conductors terminate in a single OCPD that limits load to wire ampacity.
• The OCPD is located outdoors or inside near the point of conductor entrance.

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Transformer Secondary Tap Rules (NEC 240.21(C))

[!WARNING] Primary Overcurrent Protection Trap (NEC 240.4(F)): Primary overcurrent devices CANNOT protect secondary conductors unless the transformer is 2-wire primary to 2-wire secondary, or 3-phase delta-delta (3-wire to 3-wire). For standard 480V to 208Y/120V 3-phase 4-wire transformers, primary breakers DO NOT protect secondary conductors! Secondary conductors must follow 240.21(C).

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

Secondary conductors of a transformer do not require overcurrent protection at the transformer secondary terminals if length is $\le 10\text{ ft}$ and:

1. Secondary conductor ampacity $\ge$ calculated load.
2. Secondary ampacity multiplied by the primary-to-secondary voltage ratio is $\ge 10% (1/10\text{th})$ of the primary OCPD rating: $\text{Min Secondary Ampacity} \ge \frac{\text{Primary OCPD Rating}}{10} \times \left( \frac{V_{\text{primary}}}{V_{\text{secondary}}} \right)$
3. Secondary conductors terminate in a single OCPD sized $\le$ secondary conductor ampacity.

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Step-by-Step Worked Tap Calculation Examples

Example 1: 10-Foot Feeder Tap Calculation

Scenario: A 400A circuit breaker protects a 3-phase feeder. A 10-foot tap is made to supply a small subpanel with a calculated load of 32A. What is the minimum size copper THHN conductor (75°C terminals) required for the tap?

• Rule 1: Check 10% Feeder OCPD Rule: $\text{Min Ampacity} = 400\text{ A} \times 0.10 = 40\text{ Amperes}$
• Rule 2: Check Calculated Load: Load = 32A.
• Select Conductor: The required ampacity is the larger of 40A or 32A $\rightarrow$ 40 Amperes.
• Table 310.16 (75°C Column):
  • 10 AWG Cu = 35A (too small).
  • 8 AWG Cu = 50A (compliant).
• Terminating OCPD: The subpanel must be equipped with a main breaker rated 50A or less to protect the 8 AWG wire.

Example 2: 25-Foot Feeder Tap Calculation

Scenario: A 400A feeder breaker supplies a distribution trough. A 22-foot feeder tap is run in EMT to supply a disconnect switch. What is the minimum required copper THHN wire size and maximum terminating fuse size?

• Rule 1: Check 1/3 Feeder OCPD Rule: $\text{Min Ampacity} = 400\text{ A} \times \frac{1}{3} = 133.33\text{ Amperes}$
• Select Conductor (Table 310.16 75°C Column):
  • 1/0 AWG Cu = 150A (exceeds 133.33A requirement).
• Terminating OCPD: The disconnect switch fuses must be rated 150A or less (matching 1/0 AWG ampacity).