Building Electrical Design Principles--Device and Equipment Requirements

Requirements for Switches and Receptacles

Switches must be selected to match the load they control. Large lighting installations that require many switches may have the switches contained within a panelboard-like enclosure called I _ P>E _ 5000 VA>240 V _ 20.8 A _ 21 A

...lighting control panel. Receptacles must be selected to match the appliance or equipment they serve.

Ordinary convenience receptacles and switches are generally wall mounted. There are no specific mounting height requirements for wall switches and receptacles. Switches are normally mounted approximately 48 in (1.2 m) above finished floor (AFF), unless otherwise specified. Convenience receptacles are normally mounted approximately 16 in AFF (400 mm), unless otherwise specified. Convenience receptacles in bathrooms and restrooms are normally mounted approximately 44 in AFF (1.1 m).

Switches are typically oriented so they trip off in a down ward orientation (e.g., switch down). Receptacles are customarily installed with the grounding slot oriented downward, unless the receptacle is controlled by a switch, in which case the receptacle is frequently aligned with the grounding slot upward. Preference of grounding slot orientation (e.g., grounding slot up or down) appears to be a cosmetic decision because there are no regulations that mandate orientation of the grounding slot.

It’s recommended that receptacles be installed with the grounding slot oriented upward in contrast to historical practice. This recommendation is made to improve safety. With the grounding pin of a plug installed so it’s oriented downward as is common practice, if a metal object (e.g., a tool or kitchen utensil) falls and strikes a grounded plug, the first blades the object could contact are the ungrounded (hot) and grounded conductors. A contacting condition would result in a short-circuit and arcing. By orienting the grounding slot upward, the grounding pin would be the first blade to contact the object, which would not create a short-circuit.

Overcurrent Protection (Circuit Breakers and Fuses) Requirements

An overcurrent protection (OCP) device, a fuse, or circuit breaker serves to limit current levels in a conductor by interrupting power when current limitations are exceeded. It pre vents excessive heat from damaging conductors and related equipment. Therefore, the overcurrent device must be matched to the conductor and equipment so that the current-carrying capacity of the conductor and equipment are not exceeded.

The current carried (amperage) by the electrical circuit or system protected by an OCP device must not exceed the maxi mum current rating of the circuit breaker. Additionally, conductors must be protected in accordance with their ampacity. For example, a circuit with a 20 A rating should have a 20 A fuse or circuit breaker protecting it and the ampacity of the conductors in the circuit must have an ampacity (after corrections) of at least 20 A.

The voltage rating of a fuse or circuit breaker must be equal to or greater than the voltage of the circuit in which the fuse is applied. For power systems of 600 V or less, fuses of a higher volt age rating can be applied on circuits of a lower system voltage.

For example, a fuse rated at 600 V can be used on a 480 V system.

Additionally, the amperes interruption current (AIC) rating for circuit breakers should be at least 5000 A and 10 000 A for fuses.

The fuse or circuit breaker must be installed at a location in the circuit where the conductors receive power-that is, generally at the panelboard where the circuit originates. The OCP device must protect the ungrounded conductors in a circuit to ensure that power to the circuit is interrupted by the OCP de vice where the circuit originates (generally the panelboard).

The neutral (grounded) and grounding conductors are not protected by overcurrent protection.

Feeder Requirements

A feeder is a set of conductors that carry a comparatively large amount of power from the service equipment to a second panel board, called a sub-panelboard, where branch circuits further distribute the power. For example, a feeder may originate at the main panelboard and feed a lighting sub-panelboard that further divides power to branch circuiting for lighting. In an apartment building electrical system, several individual feeders will run from an individual meters in at a central meter bank to individual apartment panelboards where current is ultimately distributed to the apartment unit by branch circuits.

Feeders must be designed to provide sufficient power to the branch circuits they supply so feeder conductor size is based on the maximum load to be supplied by the feeder. Feeders should be capable of carrying the amount of current required by the load, plus any current that may be required in the future.

It’s not likely that all connected loads on a feeder will be in operation at a specific time. Thus, feeder conductors don’t need to be sized to carry the total connected load served by the feeder. A variety of demand factors reduce the connected load to the computed load. These demand factors are too numerous to mention here. The reader is referred to local Code requirements. Additional capacity may be warranted for future expansion.

Switchboard and Panelboard Requirements

Switchboards and panelboards can be used as distribution equipment, at a point downstream from the service entrance equipment. By definition, panelboards feeding lighting and convenience receptacles and having at least 10% of the circuits rated at 30 A or less are identified as lighting and appliance panel boards. Power distribution panelboards feed other panelboards (called subpanelboards), motors, and transformers, but not circuits powering lights and convenience receptacles. In a single panelboard, not more than 42 overcurrent protection devices may be used for protecting lighting and appliance branch circuits.

Switchboards and panelboards used as service equipment should have a rating not less than the minimum allowable service capacity of the computed load. Panelboards used as subpanelboards should have a rating not less than the minimum feeder capacity of the computed load. A variety of demand factors reduce the connected load to the computed load. These demand factors are too numerous to mention here. The reader is referred to local Code requirements. Additional capacity may be war ranted for future expansion.

When locating overcurrent protection in a panelboard, it’s important to balance the anticipated load so that both bus bars are carrying a similar load. The loads should be balanced on phase bus, as discussed earlier. Similarly loaded circuits should be shared between bars so one bus bar is not overloaded. Two- and three pole beakers (e.g., for a kitchen range or three-phase motor) automatically share the load on each phase. When possible, single-phase circuits should be distributed such that there anticipated loads served by that circuit are equal on each phase bus.

In service equipment panelboards, the neutral and equipment grounding conductors are bonded (connected) together. In subpanelboards, the neutral is isolated from ground; that is, there are separate neutral/grounded and grounding buses. Neutral and grounded conductors serving branch circuits originating at a subpanelboard originate at the neutral bus and grounding conductors originate at the grounding bus.

The minimum headroom requirement of working spaces about service equipment is 6.5 ft (2 m). In general, a fuse or circuit breaker must be installed at a location in the circuit where the conductors receive power. Generally, this location is in the panelboard or load center before the conductors leave to convey current to the outlets in the circuit.

Ex. **17

A feeder on a 120/240 V, three-wire circuit consisting of aluminum conductors with THHN insulation will serve a continuous load on a panelboard with 75°C/167°F terminals. The continuous load is computed to be 115 A.

a. Size the panelboard served by this feeder, based on single phase panels in Section 18, Tbl. 4.

The feeder must be sized not less than 125% of 115 A:

The panelboard served by this feeder must be sized no less than 144 A. From Tbl. 4, a 150 A panelboard and disconnect are selected.

b. Select a feeder conductor (THHN, 75°C/167°F).

A feeder conductor must be sized no less than 125% of the continuous load. Initially, it appears that a feeder conductor should be selected such that it’s sized no less than 125% of the continuous load (115 A _ 125% _ 144 A).

However, because a 150 A overcurrent protection device (disconnect) must protect the conductor, the ampacity of the conductor must be sized no less than the rating of this device. Therefore, conductor size must be increased to 350 kcmil, which has an ampacity of 250 A.

Service Entrance Equipment Requirements

Service equipment must be large enough to supply the computed load of the building or area of the building being served.

It’s calculated using code requirements and utility regulations.

In most commercial and industrial installations, several disconnects may be used. Commonly, a maximum of six service disconnects per service are allowed. So, in large installations where more than six switches or circuit breakers could be used as disconnects, a single main service disconnect must be pro vided to disconnect power to the building. In very large buildings, there can be several service entrances.

The most common sizes of residential service equipment are 100, 125, 150, 175, and 200 A. RHW, THWN, THHN, XHHW, and USE aluminum conductors are commonly used.

Minimum conductor sizes for service entrance conductors are provided in Tbl.18. Larger service equipment is used in commercial and industrial applications.

In single-family residences and multifamily dwellings, the main service panelboard can be mounted either outside or inside the dwelling as near as possible to the point of entrance of the service conductors to the building. All service equipment and electrical panels shall have a clear area 30 in (0.75 m) wide and 36 in (0.9 m) deep in front. This clear area must extend from floor to ceiling with no intrusions from other obstructions (e.g., equipment, cabinets, counters, and appliances). Panels are not allowed in clothes closets or bathrooms.

Generally, all electrical service entrances rated at 250 V and over or exceeding 250 A must be located in a separate room used for no other purpose, except that telecommunications equipment may terminate in that room. Where a separate electrical room is required, it should have a 1 HR fire rating unless the building and room is sprinklered. Boiler and furnace rooms are not acceptable for service entrance or main distribution equipment. No pipes containing a liquid should enter that room unless the pipe terminates in that room and is intended for use in that room (i.e., sprinkler system).

It’s necessary to maintain safe working clearances from overhead electrical conductors to reduce risk of accidental con tact. The required clearance can be found in the Code. Typically, minimum vertical clearances of 18 ft (5.5 m) above roadways, 12 ft above driveways, and 10 ft above sidewalks are the required minimum. An 8 ft (2.4 m) clearance is required above low-sloped (less than 4 in 12 slope) rooflines. A 3 ft (0.9 m) clearance is required for steep-sloped roofs.

It’s also necessary to bury underground conductors sufficiently below grade to reduce the hazard of unintentional contact.

The required depth of burial can be found in the Code. Minimum earth cover varies from 6 to 24 in (150 to 600 mm), depending on whether the conductors are in a cable or protected by a conduit, or covered with soil or below a concrete walk or street.

The maximum single-span distance that utilities will run overhead service drop conductors to the point of service en trance varies, but typically it’s 100 to 125 ft (30.5 to 38.1 m).

Building heights, large conductors, or the necessity for street, driveway, or sidewalk crossings may reduce maximum permissible spans.

Tbl. **19 APPROXIMATE FULL LOADS (AMPERES) FOR SINGLE-PHASE AND THREE-PHASE TRANSFORMERS. VALUES WILL VARY WITH UNIT. CHECK WITH MANUFACTURER FOR ACTUAL SPECIFICATIONS.

Transformer Requirements

Transformers may be located in a building to step up or step down the building system voltage. Transformer combinations, such as wye-wye (Y-Y), delta-delta ( - ), delta-wye ( -Y), and wye-delta (Y- ) are available for use in buildings. The first symbol (Y or ) indicates the configuration of the primary windings and the second the configuration of the secondary windings. The delta-wye ( -Y) is the most commonly found transformer combination. The reasons for choosing a Y or configuration for transformer winding connections are the same as for any other three-phase application: Y connections provide the opportunity for multiple voltages, while connections enjoy a higher level of reliability (if one winding fails to open, the other two can still maintain full line voltages to the load).

For example, when there is no need for a neutral conductor in the secondary power system, - connection schemes are preferred because of the inherent reliability of the configuration.

A 480 V primary, 208VY/120 V secondary, three-phase transformer is a popular unit used in large commercial buildings and industrial facilities. From a four-wire 480/277 V sup ply, 277 V lighting and 480 V heavy equipment can be powered before being stepped down to a 208 Y/120 V, three-phase, four wire system for convenience receptacles and light-duty equipment and appliances. Standard voltages for common commercially available primary to secondary voltage transformers are pro vided in Tbl. 2 in Section 18. Approximate full loads (amperes) for single-phase and three-phase transformers are pro vided in Tbl.**

Equipment designed to operate from delta-connected power, such as air conditioners or motors, can also operate from wye-connected power, because the phase-to-phase voltages are available in both systems. However, equipment that requires wye-connected power cannot operate from a delta-connected source because the phase to neutral voltages are not available.

A special isolation transformer, designed to convert delta to wye, is needed in this case.

An overcurrent protection device, typically rated at 150 to 200% of the rated primary current of the transformer, must protect the primary circuit on a transformer, which ensures adequate short-circuit protection for primary conductors and the transformer but less opportunity for nuisance tripping. The secondary overcurrent device is set at 125% of full load.

Primary and secondary conductors are sized to carry 100% of the ampere rating of the overcurrent protection. The equipment-grounding conductor size is based on the ampacity of the phase conductors. System grounding of a transformer is required to remove static electricity and if a short-circuit develops, such as if the transformer windings come in contact with the enclosure. It’s based on the ampacity of the phase conductors on the secondary side of the transformer.

Transformers located within buildings generally must be located in transformer closets (rooms) or vaults that have adequate fire ratings. They must be readily accessible to qualified personnel for inspection and maintenance. Small, dry-type transformers can be located in the open on walls, columns, or structures, or above suspended ceilings or in hollow spaces of buildings if they are readily accessible. Generally, transformers rated at 112 1/2 kVA or less must have a separation from combustible materials of at least 12 in and those rated above 112 1/2 kVA must be located in an approved transformer room.

Transformer vaults and rooms must be constructed of walls and ceilings that are structurally adequate and that have at least a 3 hr fire rating (i.e., 6 in thick reinforced concrete), unless the vault or room has an automatic fire protection sys tem (e.g., sprinklers), in which case the rating must be 1 hr.

The vault door must be tight fitting with a minimum fire resistance rating of 3 hr [450.43(A)]. This minimum fire resistance (for the vault and the door) drops to 1 hr, where an automatic sprinkler system protects the vault. Vault doors must swing out, be equipped with panic bars or pressure plates so the door can open from inside under simple pressure, and be provided with locks that are accessible only to qualified per sons. The vault cannot be used as a storage room. The aim of a transformer vault is keeping the transformer(s) cool and away from building occupants. Nothing unrelated to the transformers can go in a vault-that is, it cannot be used as a telecommunications closet or house a water heater. All transformer installations must provide enough ventilation so the trans former does not overheat.

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