3.4 Feeder and Transformer Tap Rules

3.4 Feeder and Transformer Tap Rules

Quick Answer: As a fundamental baseline (NEC 240.21), overcurrent protection must be provided at the point where a conductor receives its supply. The 10-Foot Tap Rule (240.21(B)(1)) allows tap conductors up to 10 feet long to be protected by the downstream equipment main breaker, provided they are enclosed in a raceway and have ampacity not less than the calculated load. The 25-Foot Tap Rule (240.21(B)(2)) allows runs up to 25 feet, provided the tap conductor ampacity is at least one-third (1/3) the rating of the upstream feeder OCPD and terminates in a single circuit breaker or fuse set. Tap conductors are never permitted to supply another tap.

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1. The Core Overcurrent Rule and Anatomy of a Tap (NEC 240.21)

The foundational bedrock of electrical safety in NEC 240.21 states:

"Overcurrent protection shall be provided in each ungrounded circuit conductor and shall be located at the point where the conductors receive their supply, except as specified in 240.21(A) through (H)."

Normally, if you connect a 100A-rated conductor to a busway or feeder protected by a 400A circuit breaker, the 100A conductor is completely unprotected against sustained overloads between 100A and 400A. If a 300A load is applied, the conductor will melt its insulation and cause a structural fire long before the 400A breaker trips.

Definition of a Tap Conductor (NEC 240.2):

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 NEC 240.4 for similar conductors.

The Golden Cardinal Rule of Taps:

Taps cannot be tapped! You cannot tap a feeder with a 25-foot tap, and then take a 10-foot tap off that tap conductor. Every tap must originate from a conductor that has upstream overcurrent protection conforming to its ampacity, and must terminate directly in a single overcurrent protective device.

   FEEDER BUS OR CONDUCTOR (Protected at 400A upstream)
   ========================================================================
                 |                                         |
                 | [10-Foot Tap]                           | [25-Foot Tap]
                 | Length <= 10 ft                         | Length <= 25 ft
                 | Must be in raceway                      | Ampacity >= 1/3 of 400A (133.3A)
                 | Ampacity >= Load & Panel Rating         | Protected from physical damage
                 v                                         v
          +--------------+                          +--------------+
          | 100A Main    |                          | 150A Main    |
          | Panelboard   |                          | Disconnect   |
          +--------------+                          +--------------+

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

The 10-foot tap rule is the most versatile provision in industrial and commercial distribution, allowing electricians to tap large feeder troughs and wireways to supply machine disconnects or small lighting panelboards.

Mandatory Requirements (NEC 240.21(B)(1)):

To legally install a 10-foot tap, ALL of the following five conditions must be satisfied:

1. Length: The total length of the tap conductors must not exceed 10 feet (3.0 m).
2. Calculated Load Ampacity: The tap conductor ampacity must not be less than the combined calculated loads on the circuits supplied by the tap conductors.
3. Termination Device Rating: The tap conductor ampacity must not be less than the rating of the equipment supplied (such as a disconnect switch or panelboard bus rating), OR not less than the rating of the single overcurrent protective device at the termination of the tap.
4. Physical Boundary: The tap conductors must not extend beyond the switchboard, switchgear, panelboard, disconnecting means, or control devices they supply.
5. Raceway Enclosure: The tap conductors must be enclosed in an approved raceway (EMT, RMC, IMC, etc.) extending from the tap point to the enclosure of the equipment supplied.

Notice what is NOT in the 10-Foot Rule: There is no minimum percentage requirement relative to the upstream feeder OCPD! A 10 AWG copper conductor (30A) can legally tap a 1,200-ampere feeder, provided the run is under 10 feet, enclosed in conduit, and terminates directly in a 30A circuit breaker or fused switch.

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

When routing conductors across an electrical room or machine floor to supply a subpanel, 10 feet is often insufficient. The 25-foot tap rule permits longer distances, but compensates for the increased fire risk by introducing a strict one-third ampacity floor.

Mandatory Requirements (NEC 240.21(B)(2)):

To legally install a 25-foot tap, ALL of the following four conditions must be satisfied:

1. Length: The total length of the tap conductors must not exceed 25 feet (7.5 m).
2. The 1/3 Ampacity Rule: The ampacity of the tap conductors must NOT BE LESS THAN ONE-THIRD (1/3 or 33.3%) of the ampere rating of the overcurrent protective device protecting the feeder conductors!
3. Single OCPD Termination: The tap conductors must terminate in a single circuit breaker or single set of fuses that limits the load to the ampacity of the tap conductors.
4. Protection from Physical Damage: The conductors must be protected from physical damage by being enclosed in an approved raceway or cable armor (such as Type MC cable).

Step-by-Step Calculation: 25-Foot Tap Sizing

Problem: A 400-ampere feeder routed through an industrial manufacturing plant is tapped with a 22-foot run of copper conductors to feed a new distribution panelboard. What is the minimum permitted conductor size (75°C THHN copper) and what is the maximum rating of the main circuit breaker at the panelboard?

• Step 1: Calculate the 1/3 Ampacity Floor: $\text{Minimum Ampacity} = \frac{\text{Feeder OCPD Rating}}{3} = \frac{400\text{ A}}{3} = 133.33\text{ Amperes}$

• Step 2: Select the Conductor from NEC Table 310.16 (75°C Column):

  • A 1 AWG copper conductor is rated at 130A (Too small! Fails the 133.33A threshold).
  • A 1/0 AWG copper conductor is rated at 150 Amperes (Satisfies the 1/3 rule: $150\text{ A} \ge 133.33\text{ A}$).

• Step 3: Select the Termination OCPD:

  • The tap must terminate in a single breaker that limits the load to the ampacity of the tap conductor.
  • The 1/0 AWG conductor has an ampacity of 150A. Under NEC 240.21(B)(2), the termination breaker must not exceed the conductor ampacity. A 150-ampere main breaker must be installed.

Exam Trap Alert: The "Next Higher Standard Rating Rule" (NEC 240.4(B)) cannot be used to protect tap conductors at their termination! The termination device must strictly limit load to the conductor's ampacity.

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

In agricultural, campus, and heavy industrial facilities, feeders often run outdoors between buildings. NEC 240.21(B)(5) permits outside feeder taps of unlimited length without upstream overcurrent protection at the tap point.

Conditions for Outside Taps:

1. The conductors must be located entirely outside of a building or structure, except at the point of entrance to the building served.
2. The conductors must be protected from physical damage in an approved raceway or cable.
3. The conductors must terminate at a single circuit breaker or single set of fuses that limits the load to the ampacity of the conductors.
4. The overcurrent device must be an integral part of a disconnecting means or immediately adjacent thereto.
5. The disconnecting means must be installed at a readily accessible location either outside the building, or inside nearest the point of entrance of the conductors.

Because the tap conductors reside outside the building envelope, an electrical failure will not ignite structural framing, justifying the absence of a length restriction.

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

Electricians routinely install dry-type step-down transformers (e.g., 480V three-phase primary to 208Y/120V three-phase secondary). A common misconception is that the primary circuit breaker protects the secondary conductors. It does not!

Except for 2-wire single-phase and 3-wire delta-delta transformers satisfying 240.21(C)(1), secondary conductors are tap conductors and must be protected under NEC 240.21(C).

   PRIMARY SUPPLY (480V 3-Phase, Protected by 100A Breaker)
   =========================================================
                              |
                              v
                  +-----------------------+
                  |  TRANSFORMER          |
                  |  480V Delta Primary   |
                  |  208Y/120V Secondary  |
                  +-----------------------+
                              |
                              | [Secondary Conductors (<=10 ft or <=25 ft)]
                              v
                  +-----------------------+
                  | Secondary Main Panel  |
                  | (Single Main Breaker) |
                  +-----------------------+

Comparison: 10-Foot vs. 25-Foot Secondary Taps

Step-by-Step Transformer Secondary Sizing Calculation:

Problem: A 45 kVA, 480V three-phase primary to 208Y/120V three-phase secondary dry-type transformer is protected on the primary side by a 60-ampere circuit breaker. The secondary conductors run 18 feet in EMT to an indoor distribution panelboard. What is the minimum permitted secondary conductor ampacity under the 25-foot secondary tap rule?

1. Determine Primary-to-Secondary Voltage Ratio: $\text{Voltage Ratio} = \frac{V_{\text{primary}}}{V_{\text{secondary}}} = \frac{480\text{ V}}{208\text{ V}} = 2.308$

2. Calculate Full Reflected Primary OCPD Current on Secondary: $I_{\text{secondary\_reflected}} = 60\text{ A} \times 2.308 = 138.48\text{ Amperes}$

3. Apply the 1/3 Secondary Tap Rule (NEC 240.21(C)(6)): $\text{Minimum Secondary Ampacity} = \frac{138.48\text{ A}}{3} = 46.16\text{ Amperes}$

4. Conductor and Termination Selection:

   • Under Table 310.16 (75°C copper), an 8 AWG conductor is rated for 50 Amperes ($50\text{ A} \ge 46.16\text{ A}$). However, the secondary conductors must also carry the actual calculated load and terminate in an OCPD matching their rating.

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6. Comprehensive Tap Rules Summary Table