3. The total load on any overcurrent device, located in a panelboard, will not exceed 80 percent of its rating. If in normal operation, the load will be continuous (3 hours or more) unless the assembly including the overcurrent device is approved for continuous duty at 100 percent of its rating.
Power-distribution panels are similar to the feeder-distribution type. They have bus bars normally rated up to 1,200 amperes at 600 volts or less and contain control and overcurrent devices sized to match connected motor or other power circuit loads. Generally, the devices are three-phase.
Special panelboards. containing relays and contactors, can be obtained and installed when remote control of specific equipment is specified. A thorough knowledge of all the available types of panelboards aids in the selection and installation of the proper unit.
Service-equipment panelboards, for loads up to 800 amperes, containing six or fewer main fused switches, fused pullouts, or circuit breakers are available. These panels constitute service equipment and frequently contain split buses that supply branch circuit or feeder overcurrent devices installed in the same enclosure (figs. 5-4 and 5-6).
Feeder distribution panels generally contain circuit overcurrent devices rated at more than 30 amperes to protect subfeeders that extend to smaller branch-circuit panelboards.
The uses and limitations of the common types of branch circuits are outlined in the Summary of Branch- Circuit Requirements (NEC
® table 210-24). Lighting branch circuits may carry loads as high as 50 amperes, although florescent lighting is limited to use on circuits of 15-ampere or 20-ampere rating. Such circuits are extensively used in commercial and industrial occupancies. Branch circuits, supplying convenience outlets for general use in other than manufacturing areas, are usually limited to a maximum of 20 amperes. The type of outlet required for heavier capacity circuits usually will not accommodate the connection plug found on portable cords, lamps, motor-driven office machinery, and so forth.
Any individual piece of equipment (except motors) also may be connected to a branch circuit meeting the following requirements: Conductors must be large enough for the individual load supplied. Overcurrent protection must not exceed the capacity of the conductors or 150 percent of the rating of the individual load if the single-load device is a nonmotor- operated appliance rated at 10 amperes or more. Only a single outlet or piece of equipment may be supplied.
Because of the peculiar conditions obtained during the starting period of a motor and because it may be subjected to severe overloads at frequent intervals, motors, except for very small sizes, are connected to branch circuits of a somewhat different design from that previously discussed.
The Code covers general requirements for conductors and their type designations, insulations? markings, mechanical strengths, ampacity ratings, and uses. These requirements do not apply to conductors that form an integral part of the equipment, such as motors, motor controllers, and similar equipment, or to conductors specifically provided for elsewhere in the Code.
Conductors must be insulated except where covered or bare conductors are specifically permitted by the NEC
® . The Code covers the insulation of neutral conductors of a solidly grounded high-voltage system. When stranded conductors are installed, the Code states that stranded conductors installed in raceways must be a size No. 8 or larger with the following exceptions:
Exception No. 1: When used as bus bars or in mineral-insulated, metal-sheathed cable
Exception No. 2: When bonding conductors are required
Aluminum, copper-clad aluminum, or copper conductors of size 1/0 and larger, in each phase of the current; neutral; and grounded circuit conductors may be connected in parallel (electrically joined at both ends to form a single conductor).
Exception No. 1: Conductors in sizes smaller than No. l/O AWG will be permitted to run in parallel to supply control power to indicating instruments, contactors, relays, solenoids, and similar control devices provided:
1. They are contained within the same raceway or cableContinue Reading