OCP-2 — Overcurrent-Device Location and Application
Key Takeaways
- 2017 NEC 240.21 generally places overcurrent protection where conductors receive their supply; a tap rule is a conditional exception, not a blanket allowance for unprotected conductors.
- The 10 ft and 25 ft feeder-tap rules use different ampacity, termination, length, enclosure, and physical-protection conditions, all of which must be satisfied.
- A series rating is valid only for an identified tested combination or permitted engineered application, with required marking and the 240.86(C) motor-contribution limit.
- Current limitation, arc-energy reduction, selective coordination, interrupting rating, and equipment-specific overcurrent protection are distinct checks and should never be treated as synonyms.
Exam checkpoints
| Checkpoint | What to verify |
|---|---|
| 1 | 2017 NEC 240.21 generally places overcurrent protection where conductors receive their supply; a tap rule is a conditional exception, not a blanket allowance for unprotected conductors. |
| 2 | The 10 ft and 25 ft feeder-tap rules use different ampacity, termination, length, enclosure, and physical-protection conditions, all of which must be satisfied. |
| 3 | A series rating is valid only for an identified tested combination or permitted engineered application, with required marking and the 240.86(C) motor-contribution limit. |
Begin at the supply point
Under 2017 NEC 240.21, branch-circuit and feeder conductors generally must have overcurrent protection connected at the point where they receive their supply. The device protects conductors on its load side, so moving it downstream leaves the intervening conductors exposed unless a specific 240.21 permission applies. A permitted tap is not simply an "unprotected wire" or any conductor shorter than 25 ft; it is an installation that satisfies every condition of the selected subsection.
Section 240.15 generally requires protection in each ungrounded conductor. For a multiwire circuit, common-trip or simultaneous-disconnect requirements must also be checked. Section 240.22 generally prohibits an overcurrent device in a grounded conductor unless it opens all conductors of the circuit simultaneously or a stated exception applies. Opening only the neutral can leave equipment energized.
Section 240.24 requires overcurrent devices to be readily accessible. The center of the grip of the operating handle in its highest position generally cannot exceed 6 ft 7 in. above the floor or working platform. Devices cannot be located near easily ignitible material, such as in clothes closets; dwelling-unit overcurrent devices cannot be in bathrooms; and they cannot be located over stairway steps. Apply the listed exceptions only to the equipment they actually describe.
Apply the 10-foot feeder-tap rule
For 240.21(B)(1), the tap-conductor length cannot exceed 10 ft. Its ampacity must be at least the combined calculated loads it supplies and at least the rating of the equipment containing the terminating overcurrent device, or the terminating device rating as the rule permits. The tap cannot extend beyond the supplied switchboard, panelboard, disconnect, or control enclosure. Except at the feeder connection, it must be enclosed in the prescribed raceway or enclosure path. For a field-installed tap that leaves the enclosure or vault where the tap is made, ampacity must also be at least one-tenth of the feeder overcurrent-device rating.
Example: a field tap leaves the feeder enclosure and is protected upstream by a 400 A feeder breaker. It supplies a 30 A calculated load and terminates at a 60 A breaker in equipment subject to the rule. The separate ampacity floors are 30 A for load, 40 A from 400 A ÷ 10, and 60 A for the termination. The governing minimum is 60 A, before applying temperature, bundling, terminal, or other ampacity limitations. A 35 A conductor fails even though the load is only 30 A. The measured length is the conductor length from tap point to termination, not a straight-line distance between enclosures.
Apply the 25-foot feeder-tap rule
For 240.21(B)(2), tap length cannot exceed 25 ft. Ampacity must be at least one-third of the feeder overcurrent-device rating. The conductors must terminate in a single circuit breaker or single set of fuses that limits the load to the tap-conductor ampacity, and the tap must be protected from physical damage by an approved raceway or other approved means. These requirements are cumulative.
Example: a 300 A feeder breaker supplies a 25 ft-or-shorter tap. The one-third floor is 300 A ÷ 3 = 100 A. A 90 A conductor fails even for a 70 A calculated load. A conductor with 100 A allowable ampacity terminating in a 100 A breaker can satisfy the numerical conditions, but only if length, physical protection, termination, calculated load, equipment, and all other rules also comply. Do not use the 240.4(B) next-size-up rule to round a tap's one-third ampacity requirement downward.
Transformer-secondary conductors, outdoor taps of unlimited length, high-bay taps, motor feeder taps, service conductors, busway taps, and generator conductors have their own provisions. A primary-side device does not automatically protect transformer-secondary conductors. Identify the conductor category before borrowing a feeder-tap rule.
Keep series ratings and current limitation in bounds
Section 240.86 permits a downstream circuit breaker on a fault-current level above its individual interrupting rating only as part of a compliant series-rated system. Under 240.86(B), the specific line-side device and load-side breaker combination is tested and marked for the end-use equipment. Section 240.86(A) permits a documented engineered selection under its professional-engineering and calculation conditions. Two devices in series do not acquire a series rating by adding their interrupting ratings. The equipment requires the series-combination field marking, and replacement components must be the identified components.
The motor screen in 240.86(C) matters. Series ratings cannot be used where motor circuits are connected between the higher-rated line-side device and the lower-rated breaker and the sum of those motor full-load currents exceeds 1 percent of the lower breaker's interrupting rating. If that breaker is rated 10,000 A interrupting, 1 percent is 100 A. A total of 120 A motor full-load current in the described location fails the condition; a total below 100 A does not by itself approve the combination—it merely passes this one screen.
A current-limiting fuse or breaker reduces peak let-through current and clearing time only when fault current is within its current-limiting range. That characteristic can support a tested or engineered application, but it does not by itself create a series rating, raise equipment SCCR, or prove selective coordination. Selective coordination seeks to open the protective device nearest the fault; a series-rated upstream device may also open.
Section 240.87 separately addresses arc-energy reduction where the highest continuous-current trip setting for which the actual overcurrent device installed in a circuit breaker is rated or can be adjusted is 1200 A or higher. The 2017 methods include zone-selective interlocking, differential relaying, maintenance switching with local indication, active arc-flash mitigation, qualifying instantaneous settings or overrides, and approved equivalent means. "Current limiting" is not a substitute phrase for satisfying 240.87. Finally, motor, HVAC, transformer, appliance, fire-pump, and other equipment articles can change protection selection. Always finish with voltage rating, available fault current, interrupting rating, equipment SCCR, conductor protection, device listing, and the equipment-specific rule.
A 10 ft field-installed feeder tap leaves the tap enclosure. The feeder breaker is 400 A, the calculated load is 30 A, and the terminating breaker is 60 A. What is the minimum tap-conductor ampacity from these stated 240.21(B)(1) numerical conditions?
Which installation satisfies the numerical part of the 25 ft feeder-tap rule for a 300 A feeder overcurrent device?
A series combination uses a lower-rated breaker with a 10 kA interrupting rating. Motors connected between the higher-rated device and that breaker total 120 A full-load current. What does 240.86(C) indicate?
Which statement correctly describes a current-limiting overcurrent device?