LOW VOLTAGE SYSTEMS AND APPLICATION

THE BASIC ELECTRIC SYSTEM

An “electric system” is composed of various interdependent components which either supplies, conveys, transforms and/or uses electric energy.

A “system” may also be defined as a group of customers served by a certain transformer, or a group of transformers served by a feeder, or a group of feeders served by a generating station, or the generating plant itself is considered as a system.

ELEMENTS OF AN ELECTRIC SYSTEM

The Generation System

Production of electric power is generally done in a power plant where bulk power is generated through a system of prime movers attached to generators producing electricity at voltages varying from 4,000 to 24,000 volts - depending on the size of the generating units. The prime movers may be in the form of internal combustion engines (ICE’s) or turbines (hydraulic, steam or gas).

A number of industrial plants in the country are generating their own power without the support of the utility companies - in this case, they are considered as “Island Generation”. Or an industrial plant maybe self-generating with the power utility company in parallel with its own system. In any case, the power plant becomes the heart of the power system.

The Transmission System

Generated electricity is then fed to transformers where by transformation action the voltage is raised to approximately 1,000 volts for every mile of transmission. Transmission of energy over the feeders may take place at potentials as high as 400,000 volts. With sufficiently large load justifying the transmission, the farther the distance, the higher is the line voltage employed. Extra-High Voltage (EHV) reaching up to 1,200,000 volts is no longer uncommon in the United States and Sweden.

The transmission line can be considered as bridging between one remote area and another remote area. Its purpose is to deliver bulk power through long spans of distances. This is accomplished by a system of towers carrying conductors at high potentials. The towers are usually of steel construction and are now a familiar sight across the countryside. High Voltage DC transmission systems are now gaining popularity because of the tremendous economic advantage. The Philippines is not an exception.


The Distribution System

When the transmission line reaches the proximate area of distribution, electricity is once more fed into transformers that reverse the process by lowering the voltage to some degree. Downstream of the system, another voltage transformation may still be necessary and be routed to various places for local distribution and utilization.

The distribution system is designed to cover a specific areas such as a town, city or industrial plants. The method of distribution is usually by the use of 60 footer wooden or concrete poles at 69 KV. These voltages may feed directly to large industrial plants while for towns or cities, another voltage transformation may be necessary to produce 13.8 or 23 or 34.5 KV. These feeders are usually called laterals.

In industrial plants, scenarios like self-generating power at 2.4 KV, 4.16 KV or 13.8 KV and distribute it directly to power centers which in turn transform the voltage to 480 V or 240 V are common sights. Underground distribution or through cable trays are likewise employed as common feeder management methods in industries.


BRANCH CIRCUIT DESIGN

TERM DEFINITION

A branch circuit is that part of a wiring system extending beyond the last or final protective device to the load it specifically serves. In its simplest form, a branch circuit consists of two wires (single phase) or three wires (three-phase) which carry currents at a particular voltage from the protective device to the utilization device.

The branch circuit represents the last step in the transfer of power from the service or source of energy to utilization devices. A branch circuit may have several lighting or power outlets connected to it as a circuit or may serve a single load as in motor or heavy appliance. A branch circuit will qualify as such when it has a protective device from the point of tapping.


BRANCH CIRCUITS – WIRING FUNDAMENTALS
RELEVANT CODE REQUIREMENTS

The ampacity of branch circuit conductors must not be less than the maximum load to be served. (NEC Section 210-19a).

The maximum load to be served by the branch circuit conductors must not be more than 80% of the ampacity of the conductors. (NEC Section 210-19a).

The ampacity of branch circuit conductors must not be less than the rating of the branch circuit. (NEC Section 210-19a).

The rating of a branch circuit is established or defined by the rating or setting of its protective device. (NEC Section 210-3).

The total load on any overcurrent device located in a panelboard must not exceed 80% of the rating of the overcurrent device. (NEC Section 384-16c).



BRANCH CIRCUITS – WIRING FUNDAMENTALS
RELEVANT CODE REQUIREMENTS

Circuit conductors shall be protected against over-current in accordance to their ampacities, but where the ampacity of the conductor does not correspond with the standard ampere rating of a fuse or a circuit breaker, the next higher rating shall be permitted only if this rating does not exceed 800 amperes. (NEC Section 240-3).

The normal maximum ampacities of conductors in cables or raceways are given in Tables 310-16 (copper) and Table 310-18 (aluminum) based on a 30 deg C ambient temperature. For ambient temperatures under or over 30 deg C, Correction Factors must be considered.

These normal ampacities may have to be reduced or derated where there are more than three conductors in a cable or raceway (Note 8 to Tables 310-16 through 310-19). This means a change in ampacities of circuit conductors


The current permitted to be carried by the branch circuit conductors may have to be reduced if the load is continuous. This does not mean a change in the ampacities of the conductors but the rule refers to a limit of the load to be carried by the conductors. The change of ampacity of conductors because more than 3 conductors are installed in a cable or raceway is distinctly different from limiting the load. (NEC Section 210-22c).

Continuous load (other than motor loads) refers to a load that operates for three (3) hours or more, such as store lighting, office lighting and similar lighting loads. This rule limits the load on the circuit conductors; it does not change the ampacity of the circuit conductors or the rating or setting of the circuit over-current protective device. (NEC Section 210-22c).

Overcurrent protection for any single non-motor operated appliance with ratings of 10 amperes or more must not be more than 150% of its ampere rating. (NEC Section 422-27e).

General-purpose receptacle outlet in other than dwelling occupancies shall be taken as a load of 180 volt-amperes. (or 1.5 Amps) (NEC Section 220-2c). In the Philippines, 360 Volt-Amperes, 1.5 A @ 240 V.

DERATING CONDUCTOR AMPACITIES

Ampacities Derating Factor Due to “ More than three (3) curent carrying conductors in a conduit or cable”.

Correction to the conductor ampacities when installed or operated at temperatures over or under 30 deg C ambient.

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ILLUSTRATIONS 1

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ILLUSTRATIONS 2

Rate the Branch Circuit Protection applicable for the ff. conditions.Consider a 6 # 12 THW conductors in every conduit. Load is Continuous. Ambient Temp : 39 deg Celsius (wirings are in between the ceiling and roofing)
1) More than 3 conductors :: Derating Factor = 80%
2) Continuous Load :: Load Limit = 80%
3.) Ambient Temp = 31 – 40 deg Celsius :: C.F. = 88%
Effective Ampacity of # 12 THW = 20A x 088 x 0.80 = 14.08 A
Note: Limitation of load to 80% of wire Ampacity due to continuous load does not reduce the Ampacity of the wire
Hence, use a 15 A Fuse or 15 AT CB

L.O. AND C.O. IN #12 THW

Max no. of Office Fluorescent L.O. in # 12 THW, 15 A Circuit
= (240V x 20A x 0.80 x 0.88 x 0.80) / 240VA
= 11.2 say 12 LO’s
Max no. of Convenience Outlets in a #12THW 15A Circuit
= (240V x 20A x 0.80 x 0.88 x 0.80) / 360 VA
= 7.5 say 6 CO’s


CONCLUSION

FOR GENERAL LIGHTING

1) Number of Office Lighting Outlets in a Single 15 Amp # 12 THW Circuit = 12 LO’s

NOTE: The # 12 THW circuit is fitted with 15AT CB, not 20 A

2) Number of Office Lighting Outlets in a Single 20 Amp # 10 THW Circuit = 16 LO’s

NOTE: The # 10 THW circuit is fitted with 20AT CB, not 30 A


FOR GENERAL –PURPOSE OUTLET
(Industrial or Commercial Offices)

1) Max Number of Duplex Outlets in a # 12 THW 15 Amp Rated Circuit = 8 (Single or Duplex or Triplex CO’s)

2) Max Number of Duplex Outlets in a # 10 THW 20 Amp Rated Circuit = 12 (Single or Duplex, or Triplex CO’s)


PANELBOARDS

The NE Code defines that a lighting and appliance branch circuit panelboard is “one having more than 10% of its overcurrent devices rated 30 Amps or less, (for which neutral connections are provided)”.

Note that a single-pole circuit breaker is counted as one. A three-pole circuit breaker is counted as three and a two-pole overcurrent device is considered as two. The code limits the number of overcurrent devices in the panelboard at 42 maximum, other than those provided in the mains may be installed in any one cabinet.

If the mains is a three-pole overcurrent device, then that makes the maximum number of circuit breakers in the panelboard to be 45.


IMPORTANT!!!!!!

In real-life designing work, we don’t know what loads the circuit may be subjected to after 5, 10 or 15 years in deviation to the original design. In the Philippines, here are some common practices. These practices may or may not conform to code rules:

- lighting & general purpose receptacle circuits are generally floating 240 V not 120V grounded as in the US

- 20 amp #12THW circuit is generally employed in smallest lighting & general-purpose receptacle circuits. Drops are commonly # 14 THW for switches

- for office building lighting, E27 base lamp-holders are used for incandescent lamps

- for office building lighting, fluorescent lamps are connected thru general-purpose receptacles or spliced at a junction box,


IMPORTANT!!!!!!


general purpose receptacle outlets are generally duplex

possible incandescent lamps that can be placed in an E27 base lamp-holder could be 25 W to 300 W, or even greater because of the recent availability of mogul adapters.

the maximum possible fluorescent fixture in a office lighting could be 4 x 40 watts or 240 VA (power factor & ballast loss considered)

the maximum possible mogul based lamp could be 250-1000 VA

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