1 Scope
NOTE This standard covers the materials, ratings, configuration, installation, and testing of individually enclosed low-voltage switches and disconnects rated 600V and below and used as service, feeder, motor, and equipment disconnecting means. (1.1)
NOTE The work includes fusible and non-fusible enclosed safety switches, enclosed molded-case switches, and the door interlocks, lockout provisions, neutral and grounding arrangements, and enclosures that accompany them. (1.2)
NOTE An enclosed switch provides a manually operable, visible, and lockable means of disconnecting a circuit or piece of equipment from its supply; a fusible switch additionally provides branch or feeder overcurrent protection through its fuses. (1.3)
1.4Enclosed switches shall be listed and labeled to UL 98 by a Nationally Recognized Testing Laboratory.
1.5Enclosed switches shall be applied and installed in accordance with NFPA 70, including Article 404 for switches, Article 408 where the switch is part of distribution equipment, Article 430 Part IX for motor disconnecting means, and Article 440 for air-conditioning and refrigerating equipment.
NOTE A non-fusible switch contains no overcurrent device and depends entirely on an upstream overcurrent protective device for short-circuit and overload protection; it provides isolation only. (1.6)
1.7 Assemblies Covered
NOTE Switches covered by this standard are individually enclosed air switches, manually operable by an external handle, with all current-carrying parts enclosed, rated 600V and below and 1200A and below. (1.7.1)
NOTE For assemblies that subdivide a feeder into branch circuits with individual overcurrent devices, see
Low Voltage Panelboards.
(1.7.2) NOTE For drawout power circuit breaker assemblies and higher fault-duty distribution, the switch is part of switchboard or switchgear scope and is not covered here. (1.7.3)
1.8.3Switches serving motor loads fed from motor control centers shall be coordinated with Motor Control Centers. 1.8.4The equipment grounding and any service or system bonding at the switch shall comply with Grounding And Bonding. 1.8.6The available fault current used to select the switch SCCR and any series rating shall be taken from the project arc-flash and short-circuit study per Arc Flash Study. 2 Referenced Standards
2.1Equipment, materials, and installation shall comply with the latest adopted edition of the following standards and codes.
| Standard |
Title |
| NFPA 70 |
National Electrical Code (Articles 110, 230, 240, 250, 404, 408, 430, 440) |
| NFPA 70E |
Standard for Electrical Safety in the Workplace |
| UL 98 |
Standard for Enclosed and Dead-Front Switches |
| UL 50 |
Standard for Enclosures for Electrical Equipment, Non-Environmental Considerations |
| UL 50E |
Standard for Enclosures for Electrical Equipment, Environmental Considerations |
| UL 248-1 |
Low-Voltage Fuses, General Requirements |
| NEMA KS 1 |
Enclosed and Miscellaneous Distribution Equipment Switches (600V Maximum) |
| NEMA 250 |
Enclosures for Electrical Equipment (1000 Volts Maximum) |
| OSHA 29 CFR 1910.147 |
The Control of Hazardous Energy (Lockout/Tagout) |
| NEMA FU 1 |
Low Voltage Cartridge Fuses |
| NETA ATS |
Acceptance Testing Specifications for Electrical Power Equipment and Systems |
2.2Where the contract documents, the adopted building code, or a referenced standard conflict, the more stringent requirement shall govern unless the Engineer of Record directs otherwise in writing.
NOTE UL 98 is the listing standard that establishes the switch's voltage, ampere, horsepower, and short-circuit ratings, while NFPA 70 governs where and how the switch is applied as a disconnecting means. (2.3)
NOTE NEMA KS 1 defines the heavy-duty and general-duty construction classes referenced throughout this standard and the corresponding endurance and performance requirements. (2.4)
3 Submittals
3.1 Action Submittals
3.1.1The Contractor shall submit the following for the Engineer's review and approval prior to procurement:
- Product data for each switch identifying the duty class (heavy-duty or general-duty), fusible or non-fusible construction, ampere rating, voltage rating, horsepower rating, number of poles, and neutral arrangement
- The UL 98 listing and the marked short-circuit current rating, including the fuse class and maximum fuse ampere rating on which a fusible switch's SCCR depends
- Catalog data confirming the enclosure NEMA 250 type and the enclosure listing to UL 50 or UL 50E for the installation environment
- For service-entrance-rated switches, documentation of the UL 98 service-entrance listing and the neutral bonding and equipment grounding provisions
- A schedule identifying every switch on the project, its tag, the equipment it serves, its location, and its ratings
- Where a series rating is relied upon, documentation of the tested series combination identifying the upstream device type and rating
☐ Product data (duty class, fusible/non-fusible, ratings, poles, neutral)
☑ UL 98 listing and marked SCCR with fuse class basis
☐ Enclosure NEMA type and UL 50 / UL 50E listing
☐ Service-entrance listing and bonding/grounding provisions (where applicable)
☐ Project switch schedule
☐ Series rating documentation (where applicable)
3.1.2Switch procurement shall not proceed until the corresponding submittals are reviewed and returned.
NOTE The marked SCCR submitted for a fusible switch shall state the specific fuse class and maximum fuse ampere rating, because the assembly SCCR is established only with those fuses installed. (3.1.3)
3.2 Closeout Submittals
3.2.1The Contractor shall provide the following at substantial completion before the switch work is accepted:
- As-built switch schedule reflecting installed locations, tags, ratings, and the equipment served
- Operation and maintenance data covering fuse type and rating for each fusible switch, interlock defeat procedure, and recommended maintenance
- Field test and torque records for line and load terminations
- Warranty documentation
☑ As-built switch schedule
☑ Operation and maintenance data (fuse data, interlock, maintenance)
☑ Field test and termination torque records
☑ Warranty documentation
4 Quality Assurance
4.1 Listing and Labeling
4.1.1Every enclosed switch shall be listed and labeled to UL 98 by a Nationally Recognized Testing Laboratory.
4.1.2Each switch shall be marked with its voltage rating, ampere rating, horsepower rating, number of poles, and short-circuit current rating.
4.1.3A switch that is not listed to UL 98, or whose marking does not display the ratings required by this standard, shall not be installed.
NOTE A horsepower rating is required for any switch applied as a motor disconnecting means, because NFPA 70 430.109 requires the disconnect to be horsepower-rated, not merely ampere-rated. (4.1.4)
4.2 Manufacturer Qualifications
4.2.1Switches shall be the product of a manufacturer regularly engaged in producing UL 98 listed enclosed switches.
4.2.2All switches of a given duty class on the project should be furnished by a single manufacturer so that fuse provisions, handle and interlock operation, and spare parts are consistent across the facility.
4.3 Installer Qualifications
4.3.1Switch installation shall be performed by a licensed electrical contractor and journeyman-level electricians experienced in the installation of low-voltage disconnecting means.
5 Environmental and Service Conditions
NOTE A switch and its enclosure shall be suitable for the electrical system, ambient conditions, and physical environment at its point of installation. (5.1)
5.2 Installation Environment
5.2.1The installation environment shall determine the required enclosure NEMA 250 type.
Indoor, dry, climate-controlled
Indoor, damp or uncontrolled (unconditioned mechanical room)
Outdoor, exposed (rain, sleet, snow)
Wet location or washdown (hose-directed water)
Corrosive (coastal salt air, chemical exposure)
Industrial, dust and falling dirt (indoor, dust-tight)
5.2.2Indoor dry installations may use a NEMA Type 1 enclosure.
5.2.3Outdoor and damp locations shall use a NEMA Type 3R enclosure as a minimum.
5.2.4Wet and washdown locations shall use a NEMA Type 4 or Type 4X enclosure.
5.2.5Corrosive locations shall use a NEMA Type 4X enclosure of stainless steel or nonmetallic construction.
5.2.6Industrial dust-tight indoor locations may use a NEMA Type 12 enclosure.
NOTE The detailed enclosure requirements for each environment are addressed in the Enclosure section. (5.2.7)
5.3 Ambient Temperature
40°C (standard rating)
50°C (elevated indoor or mechanical room)
55°C (rooftop or process area, consult manufacturer)
5.3.1Switches installed in locations with sustained ambient temperatures above 40°C shall be derated or an oversized switch shall be specified per the manufacturer's published derating data.
NOTE Rooftop-mounted switches serving air-conditioning equipment routinely see elevated ambient temperatures from solar gain and shall be rated accordingly. (5.3.2)
6 Duty Class
NOTE NEMA KS 1 defines two construction classes for enclosed switches, heavy-duty (HD) and general-duty (GD), differing in endurance, voltage rating, fault withstand, and available enclosure types. (6.1)
NOTE Heavy-duty switches are rated to 600V and up to 1200A, accept current-limiting fuses for a high short-circuit current rating, and are available in the full range of enclosure types; general-duty switches are limited to 240V and 600A and to NEMA Type 1 and Type 3R enclosures. (6.2)
● Heavy-duty (HD) — to 600V, to 1200A
○ General-duty (GD) — to 240V, to 600A
6.3Switches applied at 480V or 600V shall be heavy-duty, because general-duty switches are rated only to 240V.
6.4Switches applied as feeder or service disconnecting means shall be heavy-duty.
6.5Switches subject to frequent operation, motor switching, or higher fault exposure shall be heavy-duty.
NOTE General-duty switches may be used only on 240V and below circuits with light operating duty and low available fault current, typically small commercial and residential service or branch applications. (6.6)
NOTE Applying a general-duty switch where a heavy-duty switch is required is a frequent and consequential error, because the general-duty switch lacks the voltage rating, fault withstand, and endurance the application demands. (6.7)
7 Switch Function
7.1 Fusible or Non-Fusible
○ Fusible (switch with integral fuse provisions)
● Non-fusible (disconnect only, no overcurrent device)
7.1.1A fusible switch shall be provided where the switch is required to provide branch or feeder overcurrent protection in addition to disconnection.
7.1.2A non-fusible switch may be provided where overcurrent protection is supplied by an upstream device and the switch is required only as a disconnecting or isolating means.
NOTE A non-fusible switch shall be applied only where the upstream overcurrent device protects the conductors and equipment served by the switch. (7.1.3)
7.2 Fuse Provisions
Class R (rejection, RK1 or RK5)
Class J
Class T
Class L (above 600A)
Class H / K (basic, low SCCR)
7.2.1Fusible switches shall be furnished with fuse clips and rejection features matched to the specified fuse class.
7.2.2Fusible switches shall use current-limiting fuses of Class R, J, T, or L where a short-circuit current rating above 10kA is required.
7.2.3Class H and Class K fuses shall not be used where the available fault current exceeds 10kA, because a switch fused with non-current-limiting fuses is rated only 10kA SCCR.
NOTE Rejection-style fuse clips shall be provided so that a current-limiting fuse cannot be replaced with a non-current-limiting fuse of a lower class, which would void the marked short-circuit current rating. (7.2.4)
NOTE Class R, J, and T fuses are current-limiting and establish a switch SCCR of up to 200kA, while Class H and K fuses are not current-limiting and hold the switch to a 10kA SCCR; the fuse class is therefore a short-circuit decision, not only an overcurrent decision. (7.2.5)
8 Electrical Ratings
8.1 Voltage Rating
240V (1Φ or 3Φ)
480V, 3Φ
600V, 3Φ
208Y/120V, 3Φ 4-wire
480Y/277V, 3Φ 4-wire
8.1.1The switch voltage rating shall equal or exceed the nominal system voltage of the circuit it disconnects.
8.1.2A general-duty switch shall not be applied above 240V.
8.2 Ampere Rating
30 A
60 A
100 A
200 A
400 A
600 A
800 A
1200 A
Per drawings — one-line diagram (deferred by default)
8.2.1The switch ampere rating shall equal or exceed the continuous load it carries and shall not be less than the rating of the overcurrent device protecting the circuit.
NOTE Enclosed switches are manufactured in the standard ampere frames of 30, 60, 100, 200, 400, 600, 800, and 1200A; the selected switch shall be the standard frame that equals or exceeds the required ampacity. (8.2.2)
8.3 Horsepower Rating
8.3.1A switch applied as a motor disconnecting means shall be horsepower-rated for the motor it serves, in accordance with NFPA 70 430.109.
8.3.2The switch horsepower rating at the applied voltage shall equal or exceed the rated horsepower of the motor.
NOTE A horsepower-rated switch is tested to interrupt the locked-rotor current of a motor of that rating; an ampere-only rating does not establish that the switch can safely break motor starting and stalled-rotor currents. (8.3.3)
8.4 Motor Disconnect Sizing
8.4.1The ampere rating of a switch serving a single motor shall be not less than 115 percent of the motor full-load current, in accordance with NFPA 70 430.110(A).
8.4.2The full-load current used for disconnect sizing shall be the value from the applicable NFPA 70 motor full-load current table, not the motor nameplate current.
NOTE Undersizing the motor disconnect to the nameplate current rather than to 115 percent of the table full-load current is a common code violation that an inspector will reject. (8.4.3)
8.5 Short-Circuit Current Rating
8.5.1The short-circuit current rating (SCCR) of the switch, as marked and with the specified fuses installed for a fusible switch, shall equal or exceed the available fault current at its line terminals.
10 kAIC (non-current-limiting fuses or non-fusible, default duty)
100 kAIC (Class R, J, or T fuses)
200 kAIC (Class R, J, T, or L current-limiting fuses)
Per drawings — one-line diagram (deferred by default)
8.5.2The available fault current shall be determined by the project short-circuit study and shall be confirmed for each switch location per Arc Flash Study. 8.5.3A non-fusible switch carries the SCCR established by its tested combination with a specific upstream overcurrent device, and that combination shall be verified for the available fault current at the switch.
NOTE A fusible switch with current-limiting Class R, J, or T fuses achieves an SCCR of up to 200kA, which is the principal reason a fusible switch is selected at locations with high available fault current. (8.5.4)
NOTE An SCCR below the available fault current allows the switch to fail destructively during a fault rather than safely interrupting or withstanding it, and is both a code violation and a safety hazard. (8.5.5)
8.6 Series Rating
8.6.1Where a non-fusible or fusible switch relies on a tested series combination with an upstream overcurrent device to achieve its SCCR, the combination shall be a laboratory-tested and listed series combination per NFPA 70 240.86.
8.6.2Where a series combination is relied upon, the switch enclosure shall be field-marked to identify the series-rated combination and the required upstream device, in accordance with NFPA 70 110.22.
NOTE Substituting an upstream device that is not part of the tested series combination breaks the series rating and renders the installation noncompliant. (8.6.3)
9 Poles and Neutral
9.1 Number of Poles
○ 2-pole (1Φ, or 2-wire)
● 3-pole (3Φ, 3-wire)
○ 4-pole (3Φ with switched neutral)
○ 6-pole (double-throw or special)
9.1.1The number of switched poles shall match the number of ungrounded conductors to be disconnected.
9.1.2A switch used as a disconnecting means shall simultaneously open all ungrounded conductors of the circuit, in accordance with NFPA 70 404.
9.2 Neutral Arrangement
● Solid neutral (bonded-through, not switched)
○ Switched neutral (4th switched pole)
○ No neutral (3-wire system)
9.2.1A solid neutral assembly shall be provided where the system includes a grounded neutral conductor that is carried through the switch but not switched.
9.2.2A switched neutral (fourth switched pole) shall be provided where the neutral must be opened together with the ungrounded conductors, such as where the switch isolates a separately derived system or where required by the equipment served.
NOTE Switching the neutral of a system that is bonded both upstream and downstream of the switch creates objectionable neutral current paths; the neutral shall be switched only where the application requires it. (9.2.3)
9.3 Service-Entrance Rating
● Not service entrance (feeder or equipment disconnect)
○ Service-entrance rated (with neutral bonding and grounding kit)
9.3.1Where the switch serves as service disconnecting means, it shall be listed and labeled for use as service-entrance equipment per UL 98 and shall comply with NFPA 70 Article 230.
9.3.2A service-entrance-rated switch shall include a means to bond the grounded service conductor to the enclosure and an equipment grounding terminal, in accordance with NFPA 70 250.
NOTE The neutral bonding jumper of a service-entrance switch establishes the main bonding connection for the service; its omission leaves the service ungrounded and is a serious safety defect. (9.3.3)
NOTE The Contractor shall coordinate the service neutral bond and grounding electrode connection at a service-entrance switch with
Grounding And Bonding.
(9.3.4) 10 Construction and Components
10.1 Enclosure
NEMA Type 1 (indoor, general purpose)
NEMA Type 3R (outdoor, rainproof)
NEMA Type 4 (watertight, indoor or outdoor)
NEMA Type 4X stainless steel (watertight, corrosion-resistant)
NEMA Type 4X nonmetallic (watertight, corrosion-resistant)
NEMA Type 12 (indoor, dust-tight)
10.1.1The enclosure shall be listed to UL 50 and, for environmental types, UL 50E, and shall match the NEMA 250 type required by the installation environment.
10.1.2The enclosure NEMA type shall be consistent with the installation environment selected in the Environmental and Service Conditions section.
NOTE A NEMA Type 3R enclosure protects against falling rain, sleet, and ice but is not watertight; washdown and hose-directed-water locations require Type 4 or Type 4X. (10.1.3)
NOTE A NEMA Type 4X enclosure of Type 304 or Type 316 stainless steel, or of nonmetallic construction, shall be provided in coastal, food-processing, and chemical-exposure environments where a painted steel enclosure would corrode. (10.1.4)
10.1.5Where the switch is mounted outdoors, conduit shall enter from the bottom or be provided with listed rain-tight hubs where it enters from the top, to prevent water migration into the enclosure.
10.2 Operating Mechanism and Visible Blade
10.2.1The switch shall be operated by an external handle that indicates the ON and OFF position of the switch blades.
10.2.2The switch shall provide visible blade construction so that the open position of the blades can be confirmed by direct observation, where available for the ampere rating.
NOTE Visible-blade construction is commonly available through 400A and provides positive visual confirmation that the circuit is open before work begins; above 400A the open position is confirmed by the handle position indicator. (10.2.3)
10.2.4The operating handle shall be lockable in the OFF position to accept at least one padlock for lockout/tagout.
10.3 Door Interlock
● Defeatable (interlock can be bypassed by qualified person)
○ Non-defeatable (door cannot open with switch ON)
10.3.1The switch shall include a door interlock that prevents the enclosure door from opening while the switch is in the ON position.
10.3.2A defeatable interlock shall require a deliberate action by a qualified person to bypass and shall re-engage automatically when the door is closed.
NOTE A defeatable interlock allows a qualified person to open the door for inspection with the switch energized, while a non-defeatable interlock enforces de-energization before access and is preferred where unqualified persons may operate the equipment. (10.3.3)
10.4 Lockout/Tagout Provisions
10.4.1The switch shall provide provisions for lockout/tagout in the OFF position in accordance with OSHA 29 CFR 1910.147.
10.4.2The lockout provision shall accept the number of padlocks required by the facility's energy-control program, with multiple-lock hasps provided where more than one worker must lock out the switch.
NOTE A line-side barrier or shield shall be provided where required so that the line terminals remain guarded when the door is open and the switch is OFF. (10.4.3)
11 HVAC and Equipment Disconnects
NOTE A disconnecting means shall be installed within sight from, and readily accessible from, air-conditioning and refrigerating equipment, in accordance with NFPA 70 440.14. (11.1)
11.2The disconnect serving air-conditioning or refrigerating equipment shall not be mounted on a panel of the equipment that must be removed to service the equipment, per NFPA 70 440.14.
11.3The disconnect serving air-conditioning or refrigerating equipment shall meet the working space requirements of NFPA 70 110.26(A).
● Non-fusible (overcurrent protection at upstream panel)
○ Fusible (overcurrent protection at the disconnect)
11.4The disconnect type for air-conditioning and refrigerating equipment shall be selected according to the equipment nameplate, which states whether fuses are required at the disconnect.
NOTE A rooftop equipment disconnect shall be provided in a NEMA Type 3R enclosure as a minimum, and in a Type 4X enclosure in coastal or corrosive environments. (11.5)
12 Installation
12.1 Location and Mounting
12.1.2Switches shall be mounted plumb, level, and secured to structural backing or to the equipment they serve.
12.1.3A disconnecting means shall be located so that it is readily accessible and within sight of the equipment it controls where the applicable NFPA 70 article requires it.
12.1.4The operating handle, at its highest position, shall be not more than 6 ft 7 in. above the floor or working platform, in accordance with NFPA 70 404.8(A).
12.2 Working Space
12.2.1Minimum working space shall be maintained in front of each switch in accordance with NFPA 70 110.26.
12.2.2Working clearance shall be measured from the front of the enclosure or the front of any live parts, whichever is greater, to the nearest obstruction.
NOTE Working space in front of a disconnecting means shall not be used for storage. (12.2.3)
12.3 Conductor Terminations
12.3.1Line and load conductors shall be terminated in the switch lugs in accordance with Conductors And Cables and the switch manufacturer's instructions. 12.3.2Line conductors shall be connected to the line terminals and load conductors to the load terminals, so that the line side is de-energized only by the upstream device and the load side is de-energized by the switch.
12.3.3All conductor terminations shall be torqued to the manufacturer's published torque values using a calibrated torque wrench.
12.3.4Termination torque values shall be recorded as part of the field test documentation.
12.3.5Aluminum conductors shall be terminated only in lugs listed for aluminum and prepared per the lug manufacturer's instructions.
12.4 Fuse Installation
12.4.1Fuses shall be installed in fusible switches only after the switch is verified de-energized and the conductors terminated.
12.4.2The installed fuse class and ampere rating shall match the fuse class on which the switch SCCR and the circuit overcurrent protection were based.
NOTE A label identifying the required fuse class and maximum ampere rating shall be applied to or within each fusible switch. (12.4.3)
13 Testing
13.1 Factory Tests
13.1.1Each switch shall be production-tested by the manufacturer in accordance with UL 98 prior to shipment.
13.1.2The Contractor shall furnish the manufacturer's certification that each switch passed its production tests on request.
13.2 Field Tests
13.2.1After installation and before energization, the Contractor shall perform a visual and mechanical inspection of each switch, including verification of ratings, enclosure type, interlock operation, and termination torque.
13.2.2The Contractor shall operate each switch through at least one open-close cycle to verify free mechanical operation and correct handle indication.
13.2.3The Contractor shall perform an insulation-resistance test on each pole of switches 400A and larger, phase-to-phase and phase-to-ground, in accordance with NETA ATS.
☑ Visual and mechanical inspection (ratings, enclosure, interlock)
☑ Operational test (open-close cycle, handle indication)
☑ Termination torque verification
☐ Insulation-resistance test (400A and larger)
13.2.4The Contractor shall correct any switch that fails inspection or test and shall re-verify.
14 Identification and Labeling
14.1Each switch shall be labeled to identify its tag, the equipment or circuit it serves, and the source of its supply, in accordance with Equipment Labeling. 14.2Each switch shall receive an arc-flash warning label in accordance with NFPA 70E prior to energization.
NOTE Where a fusible switch's fuse class is critical to its short-circuit rating, the required fuse class and maximum ampere rating shall be marked at the switch. (14.3)
14.4Where a series-rated combination is relied upon, the switch shall be field-marked to identify the series combination and the required upstream device per NFPA 70 110.22.
15 Delivery, Storage, and Handling
15.1Switches shall be delivered in the manufacturer's original packaging with listing marks and ratings intact.
15.2Switches shall be stored indoors in a clean, dry location and protected from moisture, corrosive atmospheres, and physical damage until installed.
15.3Switches shall be inspected for damage on delivery, and any damage shall be documented and reported before installation.
NOTE A switch whose enclosure, operating mechanism, or interlock shows evidence of impact or moisture intrusion shall not be installed. (15.4)
16 Warranty
1 year from substantial completion
2 years from substantial completion
5 years from substantial completion
16.1Switches shall be warranted by the manufacturer against defects in materials and workmanship for the specified period.
16.2The warranty period shall begin at the date of substantial completion, not the date of shipment or installation.
16.3The Contractor shall warrant the installation, including terminations, interlock operation, and correct fuse selection, for the project warranty period.
17 Spare Parts
○ None
● Three spare fuses of each class and rating installed
○ One spare set per fusible switch
17.1Where fusible switches are installed, the Contractor shall provide spare fuses of the same class and ampere rating as those installed.
17.2Spare fuses shall be stored in the electrical room in their original packaging and identified with the switch tag and rating they serve.
NOTE Stocking spare fuses allows a cleared fuse to be replaced promptly, restoring the disconnect to service without waiting on procurement. (17.3)