6.2 Feeder and Transformer Tap Rules

6.2 Feeder and Transformer Tap Rules

Quick Reference: The general principle of overcurrent protection codified in NEC 240.21 dictates that an overcurrent protective device must be installed at the exact point where a conductor receives its electrical supply. However, routing full-sized feeder conductors into small subpanels, disconnects, or motor starters is often physically and economically impracticable. To resolve this, NEC 240.21(B) (Feeder Taps) and NEC 240.21(C) (Transformer Secondary Taps) provide strictly defined statutory exceptions known as Tap Rules. Plans examiners must verify that every tap conductor satisfies precise length, ampacity ratio, raceway enclosure, and termination requirements to prevent catastrophic conductor burnout during overload conditions.

1. Foundational Tap Principles & The Point-of-Supply Rule (NEC 240.21)

The Point of Supply Mandate (NEC 240.21)

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

Definition of a Tap Conductor (NEC Article 100 & 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 that exceeds the value permitted for similar conductors in NEC 240.4.

+---------------------------------------------------------------------------------------------------+
|                         FEEDER TAP VS. STANDARD BRANCH/FEEDER TOPOLOGY                            |
+---------------------------------------------------------------------------------------------------+
|  STANDARD INSTALLATION:                                                                           |
|  [ 400A OCPD ] =================> 400A Rated Feeder Conductors =================> [ 400A Panel ] |
|  (OCPD rating matches or protects conductor ampacity per NEC 240.4)                               |
|                                                                                                   |
|  FEEDER TAP INSTALLATION (NEC 240.21(B)):                                                         |
|  [ 400A OCPD ] ======= 400A Feeder Conductor =======+===========================> [ 400A Main ]  |
|                                                     | (Tap Point)                                 |
|                                                     | (Tap Conductor Ampacity < 400A)             |
|                                                     v (Max Length: 10 ft, 25 ft, or Outside)      |
|                                            [ Single 100A OCPD ]                                   |
|                                            [ (e.g., Subpanel) ]                                   |
+---------------------------------------------------------------------------------------------------+

The Universal Prohibition: Taps from Taps (NEC 240.21)

[!CAUTION] Plan Review Absolute Prohibition: A tap conductor is never permitted to supply another tap conductor (colloquially called "tapping a tap"). Once a tap is made from a feeder, it must terminate directly in an overcurrent protective device. Creating secondary sub-taps off an existing tap conductor is a critical code violation that must result in immediate plan rejection.

2. Feeder Tap Rules: 10-Foot, 25-Foot, and Outside Taps (NEC 240.21(B))

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

Conductors are permitted to be tapped from a feeder without overcurrent protection at the point of supply where the length of the tap conductors does not exceed $10\text{ feet}$ ($3.0\text{ m}$) and all of the following four conditions are satisfied:

1. Ampacity Floor (NEC 240.21(B)(1)(1)): The ampacity of the tap conductors is not less than:
   • The combined calculated loads on the circuits supplied by the tap conductors, AND
   • The rating of the equipment containing an overcurrent device supplied by the tap conductors, OR the rating of the overcurrent protective device at the termination of the tap conductors.
2. Enclosure Boundary (NEC 240.21(B)(1)(2)): The tap conductors do not extend beyond the switchboard, switchgear, panelboard, disconnecting means, or control devices that they supply.
3. Raceway Enclosure (NEC 240.21(B)(1)(3)): Except at the point of connection to the feeder, the tap conductors are enclosed in a raceway, which extends from the tap point to the enclosure of an enclosed switchboard, switchgear, panelboard, or control devices, or to the back of an open switchboard.
4. The 1/10th Rule for Field Taps (NEC 240.21(B)(1)(4)): For field installations where the tap conductors leave the enclosure or vault in which the tap is made, the rating of the upstream overcurrent device protecting the feeder shall not exceed 10 times the tap conductor's ampacity ($I_{tap} \ge \frac{1}{10} I_{upstream-OCPD}$).

2. The 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 the length of the tap conductors does not exceed $25\text{ feet}$ ($7.5\text{ m}$) and all of the following three conditions are met:

1. The 1/3 Ampacity Rule (NEC 240.21(B)(2)(1)): 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: $I_{tap} \ge \frac{1}{3} \times I_{upstream-OCPD}$
2. Physical Protection (NEC 240.21(B)(2)(2)): The tap conductors are suitably protected from physical damage by being enclosed in an approved raceway (e.g., EMT, IMC, RMC) or by other approved means.
3. Single OCPD Termination Mandate (NEC 240.21(B)(2)(3)): 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. (The OCPD may supply any number of additional overcurrent devices on its load side).

+---------------------------------------------------------------------------------------------------+
|              CRITICAL COMPARISON: 10-FOOT VS. 25-FOOT FEEDER TAP RULES                            |
+----------------------------+-----------------------------------+----------------------------------+
| Parameter                  | 10-Foot Feeder Tap (240.21(B)(1)) | 25-Foot Feeder Tap (240.21(B)(2))|
+----------------------------+-----------------------------------+----------------------------------+
| Maximum Conductor Length   | 10 feet (3.0 m)                   | 25 feet (7.5 m)                  |
+----------------------------+-----------------------------------+----------------------------------+
| Minimum Conductor Ampacity | Load rating & device rating;      | Exactly 1/3 (33.3%) of upstream  |
|                            | 1/10th upstream OCPD if leaving   | feeder OCPD rating.              |
|                            | enclosure/vault.                  |                                  |
+----------------------------+-----------------------------------+----------------------------------+
| Termination Requirement    | Must terminate in panelboard,     | MUST terminate in a SINGLE       |
|                            | switchboard, or disconnect.       | circuit breaker or set of fuses! |
+----------------------------+-----------------------------------+----------------------------------+
| MLO Panelboard Permitted?  | YES (provided panel bus rating is | NO! (MLO panel with multiple     |
|                            | >= tap conductor ampacity).       | branch breakers violates rule).  |
+----------------------------+-----------------------------------+----------------------------------+
| Raceway Mandate            | Entire length in raceway.         | Protected from physical damage.  |
+----------------------------+-----------------------------------+----------------------------------+

[!WARNING] Plan Review Trap — MLO Panelboards on 25-Foot Taps: Designers frequently attempt to terminate a 25-foot tap into a Main-Lug-Only (MLO) panelboard containing multiple branch circuit breakers (e.g., ten 20A breakers totaling 200A of load), arguing that the calculated load is under the tap ampacity. This is a severe code violation under NEC 240.21(B)(2)(3). A 25-foot tap must terminate in a single main overcurrent protective device.

3. Outside Feeder Taps (NEC 240.21(B)(5))

Where feeder tap conductors are installed outside of a building or structure, NEC 240.21(B)(5) permits an unlimited physical length for the tap conductors, provided all of the following conditions are satisfied:

1. Physical Protection: The tap conductors are suitably protected from physical damage in an approved raceway or cable armor.
2. Single OCPD Termination: 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.
3. Disconnecting Means Location: The terminating overcurrent device is an integral part of a disconnecting means or is located immediately adjacent thereto.
4. Readily Accessible Disconnect: The disconnecting means is installed at a readily accessible location either outside the building or structure, or inside nearest the point of entrance of the conductors.

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

A critical principle in electrical engineering is that transformer secondary conductors are NOT protected by the transformer primary overcurrent device in standard 3-phase systems.

Why does a primary breaker fail to protect secondary conductors? Consider a $480\text{V}$ primary to $208\text{Y}/120\text{V}$ secondary 3-phase delta-wye transformer. A line-to-ground or line-to-neutral secondary fault on phase A of the wye secondary draws current through only two primary winding phases. Due to the transformer phase transformation ratio ($480 / 120 = 4:1$), a high secondary fault current reflects back to the primary at a greatly reduced ampere magnitude, failing to trip the primary breaker before secondary conductors overheat. Therefore, under NEC 240.21(C), transformer secondary conductors are treated as tap conductors and must comply with specific secondary tap rules.

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

Secondary conductors of a transformer are permitted to have an unprotected length not exceeding $10\text{ feet}$ ($3.0\text{ m}$) provided:

1. The ampacity of the secondary conductors is not less than the calculated load on the circuits supplied.
2. The ampacity is not less than the rating of the equipment containing an OCPD supplied, OR the rating of the terminating OCPD.
3. For field installations where the secondary conductors leave the enclosure or vault, the secondary conductor ampacity must satisfy the Primary OCPD Voltage Transformation Ratio Formula:

$I_{sec} \ge \frac{1}{10} \times I_{pri-OCPD} \times \left( \frac{V_{primary}}{V_{secondary}} \right)$

1. The secondary conductors are enclosed in a raceway from the transformer to the terminating panelboard or switchboard.

2. Secondary Conductors Not Over 25 Feet: Industrial Installations (NEC 240.21(C)(6))

In industrial occupancies where servicing is performed only by qualified persons, secondary conductors are permitted up to $25\text{ feet}$ ($7.5\text{ m}$) without secondary OCPD at the transformer provided:

1. The secondary conductor ampacity is not less than the secondary full-load current rating of the transformer.
2. The secondary conductors are protected from physical damage.
3. The secondary conductors terminate in a single circuit breaker or set of fuses sized to the conductor ampacity.

5. Worked Plan Review Calculations: Feeder & Transformer Taps

Worked Example 1: Sizing a 25-Foot Feeder Tap

• Parameters: A $480\text{V}$, 3-phase feeder is protected by an upstream $600\text{ A}$ circuit breaker in the main switchboard. A $22\text{-foot}$ tap is routed in EMT to supply a new lighting distribution panelboard equipped with a $150\text{ A}$ main circuit breaker.
• Step 1: Check 25-Foot Feeder Tap Ampacity Floor (NEC 240.21(B)(2)(1)) $\text{Minimum Tap Ampacity} = \frac{1}{3} \times I_{upstream-OCPD} = \frac{1}{3} \times 600\text{ A} = \mathbf{200\text{ A}}$
• Step 2: Check Termination Device Rating (NEC 240.21(B)(2)(3))
  • Tap terminates in a single $150\text{ A}$ main breaker $\rightarrow$ Complies with the "single OCPD" requirement.
• Step 3: Select Tap Conductor Size from Table 310.16 (75°C THHN Copper)
  • A $150\text{ A}$ load would normally use $1/0\text{ AWG Cu}$ ($150\text{ A}$ at 75°C).
  • However, the 1/3 tap rule requires an ampacity of at least $200\text{ A}$!
  • Table 310.16 requires $3/0\text{ AWG Cu}$ ($200\text{ A}$ at 75°C).
• Conclusion: The plans examiner must verify that the tap conductors are sized at least $3/0\text{ AWG Cu}$, even though the downstream panel main breaker is only $150\text{ A}$. Using $1/0\text{ AWG Cu}$ is a code deficiency.

Worked Example 2: Sizing 10-Foot Transformer Secondary Conductors

• Parameters: A $75\text{ kVA}$ transformer ($480\text{V}$ primary Delta to $208\text{Y}/120\text{V}$ secondary Wye) is protected by a $125\text{ A}$ primary circuit breaker. The secondary conductors leave the transformer enclosure and run $8\text{ feet}$ in EMT to a secondary panelboard with a $225\text{ A}$ main circuit breaker.
• Step 1: Calculate Secondary Transformer Full-Load Amperes (FLA) $I_{sec-FLA} = \frac{75\text{ kVA} \times 1000}{\sqrt{3} \times 208\text{ V}} = \frac{75,000}{360.27} = 208.2\text{ A}$
• Step 2: Check 10-Foot Secondary Tap Voltage Ratio Floor (NEC 240.21(C)(2)) $I_{sec-min} = \frac{1}{10} \times I_{pri-OCPD} \times \left( \frac{V_{pri}}{V_{sec}} \right) = 0.10 \times 125\text{ A} \times \left( \frac{480\text{ V}}{208\text{ V}} \right) = 12.5 \times 2.308 = \mathbf{28.85\text{ A}}$
• Step 3: Check Equipment Termination Rating Requirement (NEC 240.21(C)(2)(1)(b))
  • Conductor ampacity cannot be less than the terminating OCPD rating: $225\text{ A}$.
• Step 4: Select Conductor Size (Table 310.16 at 75°C)
  • Conductor must have an ampacity $\ge 225\text{ A}$.
  • $4/0\text{ AWG Cu}$ ($230\text{ A}$ at 75°C) satisfies the requirement.

6. Plans Examiner Verification Checklist: Tap Rules

• Point-of-Supply Verification: Identify all points where conductor size decreases without upstream OCPD; confirm each qualifies under a specific 240.21 rule.
• No Sub-Tapping (Taps from Taps): Verify that no tap conductor feeds another downstream tap conductor.
• 10-Foot Feeder Taps (240.21(B)(1)): Verify total length $\le 10\text{ ft}$, raceway enclosure from end-to-end, ampacity matches load/device, and $\ge 1/10\text{th}$ upstream OCPD if leaving enclosure.
• 25-Foot Feeder Taps (240.21(B)(2)): Verify total length $\le 25\text{ ft}$, tap ampacity $\ge 1/3$ upstream OCPD, and tap terminates in a SINGLE main circuit breaker or fuse set (REJECT MLO panels).
• Outside Taps (240.21(B)(5)): Confirm conductors are outdoors and terminate in a single disconnect/OCPD at the building point of entry.
• Transformer Secondary Taps (240.21(C)): Confirm primary voltage-ratio formula check and terminating OCPD sizing.