5.5 Overcurrent Device Location, Fuses, Circuit Breakers & Transformer Secondary Conductors (Article 240)
Key Takeaways
- Overcurrent devices must be readily accessible, with the center of the operating handle grip no more than 6 ft 7 in. above the floor or working platform in its highest position (NEC 240.24(A)).
- Overcurrent devices may not be located near easily ignitible material such as clothes closets, in dwelling unit bathrooms, or over the steps of a stairway (NEC 240.24(D)-(F)).
- A slash-rated breaker such as 120/240 V may be used only on a solidly grounded system where the voltage to ground does not exceed the lower value and the voltage between conductors does not exceed the higher value (NEC 240.85).
- Circuit breakers rated or adjustable to 1200 amperes or more need a documented arc energy reduction method (NEC 240.87); Washington requires test reports at inspection.
- Transformer secondary conductors up to 10 ft long need an ampacity of at least the computed load and at least the rating of the device they terminate in (NEC 240.21(C)(2)).
Last updated: September 2026
Where overcurrent devices may go (240.24)
| Rule | 2020 NEC |
|---|---|
| Readily accessible, installed so the center of the operating handle grip in its highest position is no more than 2.0 m (6 ft 7 in.) above the floor or working platform (exceptions include busway plug-ins, supplementary devices, and devices next to the equipment they serve) | 240.24(A) |
| Each occupant must have ready access to the overcurrent devices serving that occupancy, except where building management supervises the service | 240.24(B) |
| Not exposed to physical damage | 240.24(C) |
| Not in the vicinity of easily ignitible material, such as clothes closets | 240.24(D) |
| Not in bathrooms of dwelling units, dormitory units, or guest rooms and suites of hotels and motels (supplementary devices excepted) | 240.24(E) |
| Not over the steps of a stairway | 240.24(F) |
Washington amendments tighten these rules:
- Panelboards may not be installed in clothes closets, toilet rooms, or shower rooms (WAC 296-46B-408).
- Outdoor enclosures containing overcurrent devices must be at least 24 inches above grade unless pad-mounted or over paving (WAC 296-46B-240).
- A panelboard being repaired or replaced in an existing location over steps may stay there (WAC 296-46B-240).
Adjustable-trip devices (240.6(B), (C))
- If the long-time pickup setting has an external adjusting means, the device's ampere rating is the maximum setting possible.
- If the setting is behind restricted access (a removable and sealable cover, a bolted equipment door, a locked door accessible only to qualified persons, or password protection), the rating may be the adjusted setting.
This matters because conductor protection (240.4) and equipment grounding conductor sizing (Table 250.122) are based on the device's rating.
Plug fuses (240.50-240.54)
- Plug fuses may be used only where the voltage between conductors does not exceed 125 V, or on systems with a grounded neutral where the voltage to ground does not exceed 150 V (240.50(A)).
- Edison-base fuses may be used only as replacements in existing installations where there is no evidence of overfusing or tampering (240.51(B)).
- Type S fuses and adapters are classified at not over 125 V and 0 to 15 A, 16 to 20 A, and 21 to 30 A. Adapters are designed so they can't be removed once installed, which prevents overfusing (240.53, 240.54).
Cartridge fuses (240.60, 240.61)
- 300-volt-type cartridge fuses may be used on single-phase line-to-neutral circuits supplied from a solidly grounded system where the line-to-neutral voltage does not exceed 300 V.
- Fuseholders must reject fuses of a lower voltage rating or higher ampere rating than the fuseholder's class (noninterchangeable).
- Markings (240.60(C)): ampere rating, voltage rating, interrupting rating where other than 10,000 A, "current limiting" where applicable, and the manufacturer's name or trademark.
Circuit breakers (240.80-240.87)
| Topic | Rule |
|---|---|
| Trip-free | Breakers trip free of the operating handle and cannot be held closed against a fault (240.80) |
| Indicating | Must clearly show open (off) and closed (on). When the handle operates vertically, up is on (240.81) |
| Interrupting rating marking | Required where the rating is other than 5,000 A (240.83(C)) |
| Used as switches | Breakers used to switch 120 V and 277 V fluorescent lighting must be listed and marked SWD or HID; high-intensity discharge lighting needs HID (240.83(D)) |
| Straight vs. slash voltage ratings | A straight-rated breaker (for example 240 V or 480 V) may be used where the voltage between any two conductors does not exceed its rating. A slash-rated breaker (120/240 V or 480Y/277 V) is limited to solidly grounded systems where the voltage to ground does not exceed the lower number and the voltage between conductors does not exceed the higher number (240.85). A 480Y/277 V breaker may not be used on a 480 V corner-grounded or ungrounded delta system. |
| Series ratings | A downstream breaker may be applied above its own interrupting rating only as part of a tested, listed series combination. The end-use equipment must be field-marked "CAUTION - SERIES COMBINATION SYSTEM RATED ___ AMPERES" (240.86) |
| Arc energy reduction | Breakers rated or adjustable to 1200 A or more must have a documented method to reduce clearing time. Options include zone-selective interlocking, differential relaying, an energy-reducing maintenance switch with a local status indicator, an active arc-flash mitigation system, an instantaneous trip or override set below the available arcing current, or an approved equivalent. The 2020 NEC also requires performance testing when the system is installed (240.87) |
240.67 imposes similar arc-energy documentation for fuses rated 1200 A or more. Washington requires the arcing-current calculations and signed performance test reports for these fuses and breakers to be available to the inspector before the equipment is energized (WAC 296-46B-240).
Transformer secondary conductors (240.21(C))
240.4(F): transformer secondary conductors are not considered protected by the primary overcurrent device. The exceptions are single-phase transformers with a two-wire (single-voltage) secondary and three-phase delta-delta transformers with three-wire secondaries, when the primary protection complies with 450.3 and does not exceed the secondary conductor ampacity multiplied by the secondary-to-primary voltage ratio.
Otherwise, secondary conductors must meet one of these rules:
| Rule | Length | Main conditions |
|---|---|---|
| 240.21(C)(2) | 10 ft or less | Ampacity at least the computed load and at least the rating of the equipment or overcurrent device at the termination. The conductors may not extend beyond the equipment they supply and are enclosed in a raceway. For field installations where the conductors leave the enclosure or vault, the primary device rating times the primary-to-secondary voltage ratio, divided by 10, may not exceed the conductor ampacity. |
| 240.21(C)(3) | 25 ft or less | Industrial installations only, with qualified maintenance and supervision. Ampacity at least the transformer's secondary current rating, the sum of the downstream overcurrent device ratings not over the conductor ampacity, all devices grouped, and conductors protected in a raceway or other approved means. |
| 240.21(C)(4) | Unlimited | Outside secondary conductors, protected from damage, terminating in a single breaker or set of fuses at the building disconnect |
| 240.21(C)(5) | 25 ft total, primary plus secondary | Secondary conductors from a feeder-tapped transformer (240.21(B)(3)) |
| 240.21(C)(6) | 25 ft or less | Secondary ampacity at least one-third of (primary OCPD rating times the primary-to-secondary voltage ratio); terminate in a single breaker or set of fuses sized to the conductor ampacity; protected from physical damage |
Worked example: 240.21(C)(6)
A 480 V to 208Y/120 V transformer has a 100 A primary breaker. The voltage ratio is 480 / 208 = 2.31.
- 100 A x 2.31 = 231 A of secondary-equivalent protection
- 231 / 3 = 77 A minimum secondary conductor ampacity for a run of 25 ft or less
- The run must end in a single breaker or set of fuses no larger than the conductors' ampacity (240.4 allows the next standard size where applicable).
Test Your Knowledge
Under NEC 240.24(A), what is the maximum height of the center of the grip of an overcurrent device's operating handle in its highest position?
A
B
C
D
Test Your Knowledge
Can a breaker marked 480Y/277 V protect a circuit on a 480 V, 3-phase corner-grounded delta system?
A
B
C
D
Test Your Knowledge
Where may overcurrent devices not be located under NEC 240.24?
A
B
C
D