5.4 Feeder & Transformer Tap Rules

5.4 Feeder & Transformer Tap Rules

Exam Fast Fact: When performing tap calculations on the exam, remember the two cornerstone ratios: the 10-foot feeder tap has no minimum 1/3 or 1/10 ampacity ratio when contained inside an enclosure/panelboard (it only needs to handle the calculated load and the rating of the device supplied), but must be at least 1/10 of the feeder OCPD if it leaves the enclosure. The 25-foot feeder tap strictly requires conductor ampacity to be at least one-third (1/3) of the feeder overcurrent device rating and must terminate in a single OCPD.

In standard electrical circuit design, every wire must be protected at the exact point where it receives its supply. If a 400A feeder supplies a distribution gutter, connecting a 10 AWG or 1 AWG conductor directly to that 400A busbar would create a catastrophic hazard: a short circuit or sustained overload on that smaller conductor would incinerate the wire long before the 400A upstream breaker tripped. However, installing a large, separate disconnect at every single junction point is physically and economically impractical. NEC 240.21 provides specific, rigorously engineered exceptions known as the Tap Rules.

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The Supply Point Mandate & Definition of a Tap — NEC 240.21

NEC 240.21 states the general rule: Overcurrent protection shall be provided in each ungrounded circuit conductor at the point where the conductor receives its supply.

What is a Tap Conductor?

Article 240 defines a Tap Conductor as a conductor, other than a service conductor, that has overcurrent protection ahead of its point of supply that exceeds the value permitted for that conductor in NEC 240.4.

The Ironclad Anti-Tap Rule: You Cannot "Tap a Tap"

One of the most critical structural rules in electrical design is that a tap conductor cannot be tapped again. Every tap must originate from a full-sized conductor protected at its rated ampacity. Tapping a tap conductor results in cumulative impedance that prevents upstream overcurrent devices from clearing faults, and is an immediate Code violation under NEC 240.21.

COMPLIANT FEEDER TAP CONFIGURATION:
[400A Feeder Overcurrent Device]
              │
              ▼ (Full 400A Rated Feeder Conductors)
══════════════════════════════════════════════════════ (Distribution Gutter / Bus)
       │                                       │
       ▼ (10-ft Tap ≤ 10')                     ▼ (25-ft Tap ≤ 25')
 [Enclosed in Raceway]                   [Min. 1/3 Ampacity (≥ 133.3A)]
       │                                       │
       ▼                                       ▼
[Terminates in Panelboard Main]          [Terminates in Single Breaker ≤ Wire Ampacity]

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

Conductors are permitted to be tapped to a feeder without overcurrent protection at the point of supply, provided the tap length does not exceed 10 feet (3.0 m) and satisfies all of the following conditions:

1. Ampacity Floor: The ampacity of the tap conductors must not be less than:
   • The calculated load on the circuits supplied by the tap conductors, AND
   • The rating of the device supplied by the tap conductors, or the rating of the overcurrent protective device at the termination of the tap conductors.
2. Physical Boundary: The tap conductors must not extend beyond the switchboard, switchgear, panelboard, disconnecting means, or control devices that they supply.
3. Physical Protection: The tap conductors must be enclosed in an approved raceway (such as EMT, RMC, or IMC) extending from the point of tap to the equipment enclosure, or installed within an enclosed switchboard, switchgear, or panelboard.
4. Field Installation 1/10th Rule: For field installations where the tap conductors leave the enclosure or vault in which the tap is made, the rating of the overcurrent device protecting the feeder must not exceed 10 times the tap conductor's ampacity (i.e., tap conductor ampacity $\ge 1/10$ of feeder OCPD).

Worked Example: Sizing a 10-Foot Tap

• Problem: An electrical contractor taps a 400A commercial feeder in an auxiliary wireway to supply a 100A main breaker lighting panelboard located 6 feet away in EMT raceway. The calculated load is 78 amperes. What is the minimum conductor size required for this tap?
• Analysis:
  • Check 1: Length is 6 feet $\le 10$ feet.
  • Check 2: Tap conductor ampacity must be at least the calculated load (78A) and at least the rating of the device supplied (100A main breaker).
  • Check 3: The wireway tap leaves the enclosure, so the 1/10th rule applies: $400\text{ A} \div 10 = 40\text{ A}$.
  • Evaluation: The 100A main breaker rating is the highest threshold ($100\text{ A} > 78\text{ A} > 40\text{ A}$).
• Conductor Selection: From Table 310.16 (75°C column), a 3 AWG copper conductor is rated 100 amperes (or 1 AWG aluminum rated 100A). The tap conductors must have an ampacity of at least 100A.

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

Where the distance between the feeder tap and the terminating overcurrent device is between 10 feet and 25 feet (7.5 m), the requirements become significantly more restrictive. To prevent severe overcurrent damage across extended wire runs, the Code enforces the famous One-Third (1/3) Ampacity Rule:

1. Length Limit: Conductor length must not exceed 25 feet (7.5 m).
2. The 1/3 Ampacity Mandate: The ampacity of the tap conductors must be not less than one-third (1/3) of the rating of the overcurrent protective device protecting the feeder conductors: $\text{Tap Conductor Ampacity} \ge \frac{\text{Feeder OCPD Rating}}{3}$
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 allowable ampacity of the tap conductors.
4. Physical Protection: The tap conductors must be protected from physical damage by being enclosed in an approved raceway or by other approved means.

Worked Example: Sizing a 25-Foot Tap

• Problem: An electrician needs to run a 22-foot feeder tap from an 800-ampere feeder in a manufacturing plant to supply an enclosed circuit breaker. What is the minimum allowable conductor size and what is the maximum rating of the terminating circuit breaker?
• Step 1: Calculate Minimum Tap Ampacity: $\text{Minimum Ampacity} = \frac{800\text{ A}}{3} = 266.67\text{ Amperes}$
• Step 2: Select Conductor from Table 310.16 (75°C Column):
  • 250 kcmil copper is rated 255A. (255A < 266.67A — INSUFFICIENT!)
  • 300 kcmil copper is rated 285A. (285A $\ge$ 266.67A — COMPLIANT!)
• Step 3: Select Terminating Overcurrent Device:
  • Condition 2 of 240.21(B)(2) states that the tap conductors must terminate in a single OCPD that limits the load to the ampacity of the tap conductors.
  • The tap conductor ampacity is 285A. Standard breaker sizes in 240.6(A) are 250A and 300A.
  • Can the electrician install a 300A breaker? NO! Under 240.21, you cannot use the next-size-up rule (240.4(B)) to protect tap conductors at their termination! The terminating OCPD cannot exceed the conductor ampacity.
  • Therefore, the maximum breaker rating for 300 kcmil (285A) is 250 amperes. If a 300A breaker is required for the load, the tap conductor must be upsized to 350 kcmil copper (rated 310A)!

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

Where feeder taps are installed completely outside of a building, the risk of structural building fire caused by conductor overheating is substantially reduced. NEC 240.21(B)(5) allows outside taps of unlimited length, provided:

1. The tap conductors are protected from physical damage in an approved raceway, cable tray, or underground duct bank.
2. The tap conductors terminate at a single circuit breaker or single set of fuses that limits the load to the allowable ampacity of the conductors.
3. The overcurrent device is an integral part of a disconnecting means or immediately adjacent thereto.
4. The disconnecting means is installed outside the building or inside nearest the point of entrance of the conductors.

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Transformer Secondary Tap Rules — NEC 240.21(C) & 240.4(F)

A common misconception among apprentice electricians is assuming that a circuit breaker on the primary side of a transformer protects the secondary conductors. Under NEC 240.4(F), transformer secondary conductors are generally NOT considered protected by the primary overcurrent device.

Why Primary Breakers Do Not Protect Secondary Wires

In a standard 3-phase Delta-Wye transformer (e.g., 480V Delta primary to 208Y/120V Wye secondary):

• A single-phase line-to-neutral fault on the secondary side draws current through only two conductors of the primary delta winding.
• Due to the square root of 3 ($\sqrt{3}$) vector relationship and transformation ratios, the primary current does not rise proportionately to the secondary fault current.
• A severe 150% or 200% continuous overload on a secondary neutral or phase wire may register as a normal operating load on the primary breaker, allowing secondary conductors to overheat and ignite insulation without tripping the primary device.

THE DELTA-WYE SECONDARY PROTECTION DILEMMA (NEC 240.4(F)):

  480V Primary (Delta)            208Y/120V Secondary (Wye)
      ┌─────────┐                     ┌─────────┐
      │ ┌─────┐ │                     │    A    ├─── Secondary Phase A
Phase ├─┤ W1  ├─┤                     ├─▲───────┤
      │ └─────┘ │   Magnetic Flux     │ │ W1    ├─── Secondary Phase B
Phase ├─┤ W2  ├─┼────────────────────►│ ├─▲─────┤
      │ └─────┘ │                     │ │ │ W2  ├─── Secondary Phase C
Phase ├─┤ W3  ├─┤                     │ │ │ ├───┤
      │ └─────┘ │                     └─┼─┼─┼───┘
      └─────────┘                       │ │ │
Primary Breaker CANNOT detect           └─┴─┴─────── NEUTRAL (Ground Fault / Unbalance)
phase-to-neutral secondary overloads!

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

Secondary conductors from a transformer are permitted to be installed without secondary overcurrent protection at the transformer terminals if the length does not exceed 10 feet (3.0 m) and:

1. Conductor ampacity is not less than the calculated load supplied.
2. Conductor ampacity is not less than the rating of the device supplied or the terminating overcurrent device.
3. For field installations, the secondary conductor ampacity is not less than the primary-to-secondary voltage ratio formula: $\text{Secondary Ampacity} \ge \frac{\text{Primary OCPD Rating} \times \left(\frac{V_{\text{primary}}}{V_{\text{secondary}}}\right)}{10}$
4. Conductors are enclosed in an approved raceway.

Worked Calculation: Sizing Secondary Conductors for a 45 kVA Transformer

• Transformer Specifications: 45 kVA, 3-phase, 480V Delta primary to 208Y/120V Wye secondary. The secondary feeds a panelboard located 7 feet away.
• Step 1: Calculate Secondary Full-Load Amperes (FLA): $I_{\text{secondary}} = \frac{45{,}000\text{ VA}}{208\text{ V} \times 1.732} = \frac{45{,}000}{360.26} = 124.9\text{ Amperes}$
• Step 2: Apply Continuous Sizing (125%): $124.9\text{ A} \times 1.25 = 156.1\text{ Amperes}$
• Step 3: Select Conductor from Table 310.16 (75°C Column):
  • 2/0 AWG THHN copper is rated 175 amperes at 75°C ($175\text{ A} \ge 156.1\text{ A}$).
• Step 4: Select Terminating Main Breaker:
  • The panelboard main circuit breaker must not exceed the conductor ampacity: install a 175A main circuit breaker (or a 150A breaker if load permits).

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