Disconnecting means, panelboards and enclosures

Every piece of electrical equipment must be able to be switched off safely, and every circuit must start somewhere that is protected, labelled and accessible. This lesson covers the equipment that does that: disconnecting means for buildings, motors and appliances; panelboards and their ratings and connections; the enclosure types that suit indoor, outdoor, wet and corrosive locations; and the installation practices, from torque values to unused openings, that inspectors check on every job.

In the Canadian Electrical Code, disconnecting means and control are covered mainly in Section 14, service equipment in Section 6, motors in Section 28 and the general installation of equipment, including panelboards, in Section 26. Enclosure types are defined by CSA C22.2 No. 94.

Disconnecting means

A disconnecting means is a device that disconnects the conductors of a circuit from their source: a switch, a circuit breaker, a pull-out, or for small cord-connected equipment, the attachment plug. Its purpose is to let people make equipment safe to work on and to cut power quickly in an emergency. The code sets three recurring requirements:

• Where: each building or structure needs a disconnecting means at the point where its supply enters; equipment such as motors, air conditioners and water heaters needs a disconnecting means within sight of it (visible and not more than about 15 m, 50 ft, away), or one capable of being locked in the open position.
• What: the disconnecting means must open all ungrounded conductors, be rated for the load (horsepower-rated for motors), and plainly indicate whether it is open or closed.
• How: it must be readily accessible and identified as to its purpose.

Panelboards

A panelboard is an assembly of buses and overcurrent devices in a cabinet, from which branch circuits and feeders are distributed. Its key ratings are:

• Bus rating, such as 100 A, 225 A or 400 A, which must be at least the rating of the overcurrent device protecting it.
• Voltage and configuration, such as 120/240 V single-phase or 208Y/120 V three-phase.
• Short-circuit current rating, which must equal or exceed the available fault current (Lesson 5).
• Number of spaces or circuits.

A main breaker panelboard includes its own protective device; a main lug only (MLO) panelboard has no main and must be protected by an upstream device not exceeding its bus rating.

Neutrals and grounds in panelboards

At the service, the neutral is bonded to the enclosure and the grounding electrode system through the main bonding jumper (Lesson 9). In every panelboard downstream of the service (a subpanel), the neutral bar must be isolated from the enclosure, and the bonding conductors land on a separate bonding bar that is bonded to the enclosure. Bonding the neutral in a subpanel creates parallel paths so that neutral current flows on raceways and bonding conductors.

Good practice inside panels

• One conductor per terminal unless the terminal is marked for more.
• Terminations torqued to the manufacturer's values with a calibrated torque tool; loose connections cause heating and fires, over-tight ones damage conductors.
• Conductors routed neatly, not crossing in front of bus or breakers, with adequate bending space.
• Unused openings closed with listed filler plates or knockout seals.
• Breakers of the type listed for the panelboard: "classified" or look-alike breakers must be listed for use in that panel.
• A complete, specific circuit directory (Lesson 3).

The CEC requires the panelboard's circuit directory, prohibits more than one conductor per terminal unless the terminal is approved for it, and requires equipment to be installed according to its certification markings and instructions.

Enclosures

Enclosures protect equipment from the environment and protect people from live parts. They are classified by type, defined by the standards above. Common types are:

Equipment installation in general

A few general rules apply to all electrical equipment:

• Use only certified (listed) equipment, and install and use it according to its markings and instructions (Lesson 1).
• Provide and maintain the required working space and headroom (Lesson 2).
• Mark equipment with its ratings, and where required with arc flash warnings and the available fault current.
• Protect equipment from physical damage, moisture and corrosion appropriate to its location.
• Keep the inside of equipment free of debris, paint overspray and drywall dust, which can cause tracking and failures.

Labels, directories and field markings

Equipment must tell the next person what it is and what it can do. Every panelboard needs a circuit directory that identifies each circuit by its specific purpose and location, such as "Kitchen counter receptacles, east wall" rather than "Plugs". Spare breakers and blank spaces are marked as such. Disconnects that are not obviously associated with their equipment must be labelled with what they control, and where a building has more than one source of supply, such as a generator or solar system, signs at the service must show where all the disconnects are.

Several markings are applied in the field because they depend on the installation rather than the product:

• Arc flash warning labels on switchboards, panelboards and other equipment likely to be examined or worked on while energized, warning qualified persons of the hazard.
• The available fault current at service equipment in non-dwelling occupancies, with the date it was calculated, so that the interrupting ratings of devices added later can be checked.
• Voltage and phase identification where more than one nominal voltage system exists in a building, so that conductors of a 600Y/347 V or 480Y/277 V system are not confused with those of a 208Y/120 V system.

In the CEC, Rule 2-306 requires equipment such as switchboards and panelboards in other than dwelling units to be marked to warn of the shock and arc flash hazards, and CSA Z462 sets out how electrical work is done safely.

Surge protection

Surge protective devices (SPDs) limit the brief high-voltage transients caused by lightning and utility switching, which can damage electronics. A Type 1 SPD may be installed on the line side of the service disconnect, a Type 2 on the load side (often as a breaker-mounted device in the panel), and Type 3 devices at the point of use, such as plug strips. An SPD works by diverting the surge to the bonding system, so it must be installed with short, straight leads; every 30 cm of lead adds voltage during the fast surge and reduces its effect.

The CEC permits surge protective devices and sets installation rules for them; whether one is required depends on the application, and many designers specify them for services feeding sensitive equipment.