6.2 Overcurrent Protection Location & Feeder Tap Rules

6.2 Overcurrent Protection Location & Feeder Tap Rules

Quick Reference:

• General Rule (NEC 240.21): An overcurrent device must be connected at the point where conductors receive their supply.
• Definition of Tap Conductor (NEC 240.2): 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 Table 310.16.
• 10-Foot Feeder Tap (240.21(B)(1)): Max length 10 ft; ampacity $\ge$ computed load; ampacity $\ge$ rating of device supplied or terminating OCPD; enclosed in raceway; does not extend beyond switchboard, switchgear, panelboard, or control devices.
• 25-Foot Feeder Tap (240.21(B)(2)): Max length 25 ft; ampacity $\ge \mathbf{1/3}$ of upstream feeder OCPD rating; protected from physical damage (raceway/cable); terminates in a single circuit breaker or set of fuses.
• Outside Feeder Taps (240.21(B)(5)): Unlimited length outdoors; protected from physical damage; terminates in a single OCPD; disconnect located outside or immediately inside building entrance.
• Transformer Secondary Taps (240.21(C)): Primary OCPD does NOT protect secondary conductors (except 2-wire single-phase and 3-wire delta-delta per 240.4(F)); secondary conductors must satisfy 10-ft (240.21(C)(2)) or 25-ft (240.21(C)(6)) rules.

In standard electrical design, every conductor is protected at its origin by an overcurrent device sized to match its allowable ampacity. However, in commercial, industrial, and large residential distribution systems, installing a bulky circuit breaker or disconnect switch at every point where conductor size changes—such as high up in a warehouse raceway, inside an industrial wireway, or directly on a main switchgear bus—is physically impractical and economically prohibitive. To provide engineered flexibility while safeguarding premises against electrical fires, NEC 240.21 establishes Tap Rules. Mastery of tap calculations is among the highest-frequency testing areas on the Connecticut E-2 Unlimited Journeyperson exam.

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1. The General Location Rule & Tap Conductor Definition

The Foundational Rule: NEC 240.21

NEC 240.21 establishes that overcurrent protection shall be provided in each ungrounded circuit conductor at the point where the conductor receives its supply, except as specified in 240.21(A) through (H). Under the baseline rule, if a 400A feeder is spliced to smaller conductors, an overcurrent device rated for those smaller conductors must be installed right at the splice junction.

What Legally Constitutes a "Tap Conductor"?

NEC 240.2 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 ampacity value permitted for similar conductors in Table 310.16.

   400A Feeder Overcurrent Device
   ====================[ 400A OCPD ]====================
                                |
             500 kcmil Copper   | 400A Feeder Conductors
             (Ampacity = 380A)  |
                                v
                 Trough / Wireway Splice Junction
                                |
        ========================+========================
        |                                               |
        | Tap Conductors (e.g., 1 AWG Cu = 130A)        | Tap Conductors
        | (Protected upstream by 400A breaker!)         |
        v                                               v
   Must meet 10-ft or 25-ft                        Must meet 10-ft or 25-ft
   feeder tap requirements!                        feeder tap requirements!

Core Principle of Tap Safety: Because tap conductors are protected upstream by a device sized far above their ampacity, a sustained overload on the tap could burn the conductor without tripping the upstream breaker! Therefore, tap rules enforce a three-part safety net:

1. Strict length limits to minimize fault exposure;
2. Continuous raceway or physical protection to prevent mechanical damage and short circuits;
3. Mandatory termination in an OCPD to eliminate downstream overloads.

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

Under NEC 240.21(B)(1), conductors are permitted to be tapped from a feeder without overcurrent protection at the point of supply if all five of the following conditions are satisfied:

1. Length Limitation: The total length of the tap conductors does not exceed 10 feet (3.0 m).
2. Load Ampacity: The ampacity of the tap conductors is not less than the combined computed loads on the circuits supplied by the tap conductors.
3. Termination Equipment Rating: The ampacity of the tap conductors is not less than the rating of the equipment containing an overcurrent device being supplied, or not less than the rating of the overcurrent protective device at the termination of the tap conductors.
4. Raceway Protection: The tap conductors are enclosed in an approved raceway, which extends from the tap point to the enclosure of an enclosed switchboard, switchgear, panelboard, or control device, or are enclosed in a box or metal gutter.
5. Enclosure Boundary: For field installations where tap conductors leave the enclosure or raceway in which the tap is made, the tap conductors shall not extend beyond the switchboard, switchgear, panelboard, or control devices they supply.

                           10-FOOT FEEDER TAP RULE
   Feeder OCPD: 400A
   -------------------------------------------------------------------------
   [ 400A Feeder ] ===== Splice in Trough =====
                                  |
                                  | Max 10 ft length
                                  | Enclosed in rigid conduit / EMT
                                  |
                                  v
                     [ 100A Main Breaker Panel ]
   Requirement:
   - Tap conductor ampacity must be >= 100A (the rating of the terminating OCPD).
   - 3 AWG Copper THHN (100A @ 75°C) or 1 AWG Copper THHN (130A @ 75°C) is fully compliant.
   - Notice: The tap conductor ampacity does NOT have to be 1/3 or 1/10 of the 400A feeder!

Step-by-Step Calculation: 10-Foot Feeder Tap

Problem: An existing 400-ampere commercial distribution panel feeds a feeder trough. An electrician taps this feeder to supply a new lighting subpanel containing a 100-ampere main circuit breaker. The distance from the tap splices inside the gutter through an EMT nipple to the subpanel main breaker terminals is 6 feet. What is the minimum size copper THHN conductor permitted for this tap, assuming 75°C terminals?

• Step 1: Check length: 6 feet $\le$ 10 feet (Complies).
• Step 2: Check raceway: Installed in EMT from gutter to panel (Complies).
• Step 3: Check termination: Terminates in a 100A main circuit breaker (Complies).
• Step 4: Determine minimum conductor ampacity: Per 240.21(B)(1)(b), ampacity must not be less than the rating of the terminating overcurrent device (100A). Per Table 310.16 (75°C column), 3 AWG copper has an allowable ampacity of 100 amperes.
• Conclusion: Minimum size conductor = 3 AWG Copper THHN.

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

Where the tap length exceeds 10 feet but does not exceed 25 feet, the requirements become significantly more stringent to mitigate the increased risk of conductor damage.

The Mandatory 25-Foot Conditions (NEC 240.21(B)(2))

1. Length Limitation: The total length of the tap conductors does not exceed 25 feet (7.5 m).
2. The 1/3 Ampacity Rule: The ampacity of the tap conductors is not less than one-third (33.3%) of the rating of the overcurrent device protecting the feeder conductors from which they are tapped.
3. Overcurrent Termination: The tap conductors terminate in a single circuit breaker or a single set of fuses that limits the load to the allowable ampacity of the tap conductors.
4. Physical Protection: The tap conductors are suitably protected from physical damage by being enclosed in an approved raceway or by other approved cable methods.
5. No Tapping a Tap: The tap conductors must originate directly from a feeder; tapping a tap conductor is strictly forbidden.

                           25-FOOT FEEDER TAP RULE
   Feeder OCPD: 600A
   -------------------------------------------------------------------------
   [ 600A Feeder ] ===== Splice in Wireway =====
                                  |
                                  | Max 25 ft length (Enclosed in EMT/Raceway)
                                  | Conductor Ampacity MUST BE >= 600A / 3 = 200A
                                  |
                                  v
                     [ 200A Main Breaker Panel ]
   Requirement:
   - Conductor ampacity >= 200A (1/3 of 600A feeder OCPD).
   - Terminating breaker <= Conductor ampacity (e.g., 200A breaker protects 200A wire).

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

Problem: A 600-ampere circuit breaker protects a feeder installed in an industrial building. A tap is made in an auxiliary gutter to supply an enclosed disconnect switch located 20 feet away. The tap conductors are installed in EMT. Assuming 75°C terminals, calculate:

1. The minimum allowable conductor ampacity for the tap conductors;
2. The minimum size copper THHN conductor;
3. The maximum size overcurrent device permitted at the termination.

• Step 1: Calculate minimum 1/3 ampacity threshold:
  $\text{Minimum Ampacity} = \frac{\text{Feeder OCPD Rating}}{3} = \frac{600\text{ A}}{3} = 200\text{ Amperes}$
• Step 2: Size conductor from NEC Table 310.16 (75°C column):
  • 2/0 AWG Cu = 175A (Insufficient; $175 < 200$)
  • 3/0 AWG Cu = 200A (Meets the exact 200A requirement)
• Step 3: Determine terminating overcurrent protection:
  The tap conductors must terminate in an overcurrent device that limits load to the ampacity of the conductors. Since 3/0 AWG Cu has an ampacity of 200A, the terminating OCPD cannot exceed 200 amperes.

Exam Trap Alert: Next Higher Standard Device Does NOT Apply to Tap Terminations!
Under NEC 240.21(B)(2), the terminating device must limit load to the ampacity of the tap conductors. If you install a tap conductor with an ampacity of 185 amperes, you CANNOT round up to a 200A terminating breaker under 240.4(B). You must install an overcurrent device rated at 175A or increase the conductor ampacity to 200A!

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4. Specialized Feeder Taps: High-Bay & Outside Taps

25-Foot Industrial High-Bay Feeder Taps (NEC 240.21(B)(3))

In high-bay industrial manufacturing spaces where structural ceilings exceed 35 feet, installing electrical subpanels at floor level from high-overhead feeders requires specialized provisions:

• The building walls must exceed 35 feet (11 m) in height at the point of installation.
• The installation is maintained only by qualified persons.
• The tap conductors are not over 25 feet (7.5 m) long horizontally and not over 100 feet (30 m) long total.
• Tap conductor ampacity is at least one-third (1/3) of the feeder overcurrent device rating.
• Tap conductors terminate in a single breaker or set of fuses.
• Conductors are protected from damage and installed in continuous raceways.

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

Where feeder conductors run outdoors, different physical risk profiles apply:

                      OUTSIDE FEEDER TAP (UNLIMITED LENGTH)
   Industrial Plant Main Building                  Detached Storage Facility
   ==============================                  =========================
   [ 800A Main Switchboard ]
              |
              v
   Tapped Conductors Run Outdoors Overhead / Underground
   Distance: 50 ft, 150 ft, 300 ft... (NO LENGTH RESTRICTION!)
   Protected against physical damage per Article 300
              |
              +-----------------------------------> [ Building Disconnect ]
                                                    (Single OCPD <= Wire Ampacity)
                                                    Located outside or immediately
                                                    inside point of entrance (225.32)

1. Length Limitation: There is no maximum length limit (unlimited footage).
2. Outdoor Routing: Tap conductors are installed outdoors, exterior to buildings or structures.
3. Physical Protection: Conductors are shielded against physical damage.
4. Overcurrent Termination: Conductors terminate in a single circuit breaker or set of fuses that limits the load to the ampacity of the conductors.
5. Disconnecting Means: The overcurrent device is installed as part of a building disconnecting means located at a readily accessible location either outside the building or structure, or immediately inside nearest the point of entrance of the conductors per NEC 225.32.

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

One of the most dangerous misconceptions on electrical licensing exams is assuming that a transformer's primary overcurrent device automatically protects the secondary conductors. It does not.

Why Primary Breakers Do Not Protect Secondary Conductors

Except for specialized two-wire single-phase and delta-to-delta three-phase systems meeting NEC 240.4(F), magnetic flux transformation means secondary fault currents reflect back to the primary divided by the turns ratio. A secondary phase-to-ground fault can severely overheat secondary conductors without ever drawing enough primary current to trip the primary breaker!

Therefore, transformer secondary conductors are legally classified as taps governed under NEC 240.21(C).

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

Secondary conductors of a transformer are permitted without secondary overcurrent protection at the transformer terminals if all of the following are satisfied:

1. Conductor length does not exceed 10 feet (3.0 m).
2. Conductor ampacity is not less than the combined computed loads on the circuits supplied.
3. Conductor ampacity is not less than the rating of the equipment containing an overcurrent device or the rating of the terminating overcurrent device.
4. For field-installed conductors, the secondary conductor ampacity must satisfy the primary-to-secondary ratio requirement:
   $\text{Ampacity}_{\text{sec}} \ge \frac{V_{\text{primary}}}{V_{\text{secondary}}} \times \left(0.10 \times \text{Rating}_{\text{pri OCPD}}\right)$ (Secondary ampacity must be at least one-tenth of the primary OCPD rating multiplied by the primary-to-secondary voltage ratio).
5. Conductors are enclosed in a raceway and do not extend beyond the panelboard or switchboard they supply.

25-Foot Transformer Secondary Tap Rule (NEC 240.21(C)(6))

Secondary conductors are permitted up to 25 feet (7.5 m) from the transformer if:

1. Length does not exceed 25 feet.
2. Tap conductor ampacity is at least equal to the secondary full-load current rating of the transformer, or at least one-third of the primary OCPD rating multiplied by the primary-to-secondary voltage ratio: $\text{Ampacity}_{\text{sec}} \ge \frac{V_{\text{primary}}}{V_{\text{secondary}}} \times \left(\frac{1}{3} \times \text{Rating}_{\text{pri OCPD}}\right)$
3. Conductors terminate in a single circuit breaker or set of fuses limiting load to conductor ampacity.
4. Conductors are suitably protected from physical damage.

Calculation Walkthrough: 480V to 208Y/120V Transformer Secondary Tap

Problem: A 75 kVA dry-type transformer has a 480V 3-phase primary and a 208Y/120V 3-phase secondary. The primary is protected by a 100A circuit breaker. An electrician installs secondary conductors from the transformer to a main breaker panelboard located 8 feet away in EMT. Calculate the minimum secondary conductor ampacity required under NEC 240.21(C)(2).

• Step 1: Calculate voltage ratio:
  $\text{Ratio} = \frac{V_{\text{pri}}}{V_{\text{sec}}} = \frac{480\text{ V}}{208\text{ V}} \approx 2.308$
• Step 2: Calculate the 10% primary OCPD reflection:
  $\text{Minimum Ampacity} = 2.308 \times (0.10 \times 100\text{ A}) = 2.308 \times 10\text{ A} = 23.08\text{ Amperes}$
• Step 3: Check transformer full-load current rating:
  $I_{\text{sec}} = \frac{75,000\text{ VA}}{\sqrt{3} \times 208\text{ V}} = \frac{75,000}{360.27} = 208.2\text{ Amperes}$
• Step 4: Practical application: While 23.08A satisfies the ratio check, the secondary conductors must supply the panelboard load and match the panelboard main breaker (e.g., 200A or 225A). A 225A breaker panelboard requires 4/0 AWG copper THHN (230A @ 75°C).

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6. Comprehensive Tap Rules Comparison Matrix