_{TO DETERMINE THE TOTAL RESISTANCE IN A PARALLEL CIRCUIT WHEN THE TOTAL CURRENT AND TOTAL VOLTAGE ARE UNKNOWN USE EITHER OF THE FOLLOWING FORMULAS:}

_{POWER IN SINGLE PHASE RESISTIVE CIRCUITS
WHERE POWER FACTOR IS 100 PERCENT
(THESE FORMULAS ARE COMMONLY USED TO SOLVE MOST CIRCUIT POWER PROBLEMS ON TESTS)}

_{TO DETERMINE THE POWER CONSUMED BY AN INDIVIDUAL RESISTOR IN A SERIES CIRCUIT USE THIS FORMULA:}
 

_{TO DETERMINE THE POWER CONSUMED BY AN INDIVIDUAL RESISTOR IN A PARALLEL CIRCUIT USE THIS FORMULA:}
 

_{TO DETERMINE THE TOTAL POWER CONSUMED BY AN INDIVIDUAL CIRCUIT USE THIS FORMULA:}

POWER = E (TOTAL VOLTAGE)  x  I (TOTAL CURRENT)
 
 
 

_{RULES OF THUMB:}

• _{THE TOTAL RESISTANCE OF RESISTORS IN PARALLEL IS ALWAYS LESS THAN THE VALUE OF ANY ONE RESISTOR.}
• _{THE TOTAL RESISTANCE OF PARALLEL RESISTORS THAT ARE ALL THE SAME VALUE IS THAT VALUE DIVIDED BY THE NUMBER OF RESISTORS.}
• _{ALWAYS USE THE PRODUCT OVER SUM RULE TO BREAK DOWN TWO PARALLEL RESISTORS INTO ONE RESISTOR.  THIS IS MUCH EASIER THAN TRYING TO SOLVE LARGE ALGEBRAIC EXPRESSIONS.}
• _{746 WATTS IS EQUAL TO ONE HORSEPOWER}
• _{EFFICIENCY IS EQUAL TO OUTPUT DIVIDED BY INPUT}
• _{IN INDUCTIVE CIRCUITS CURRENT LAGS VOLTAGE.}
• _{IN CAPACITIVE CIRCUITS CURRENT LEADS VOLTAGE.}
• _{POWER FACTOR IS A MEASURE OF HOW FAR CURRENT LEADS OR LAGS VOLTAGE.}

_{POWER IN ALTERNATING CURRENT CIRCUITS WHERE POWER FACTOR IS NOT 100 PERCENT}

_{POWER = E x I x POWER FACTOR (FOR SINGLE PHASE)}

_{POWER = E x I x 1.732 X POWER FACTOR (FOR THREE PHASE)}

_{THIS POWER IS ALSO CALLED TRUE POWER OR REAL POWER AS OPPOSED TO APPARENT POWER FOUND BY CALCULATING VOLT-AMPERES.}

_{VOLT-AMPERES = E x I (FOR SINGLE PHASE)}

_{VOLT-AMPERES = E x I x 1.732 (FOR THREE PHASE)}

_{IT CAN READILY BE DETERMINED BY ALGEBRA THAT}

_{MOTOR APPLICATION FORMULAS}

_{MOTOR MARKINGS AND CONNECTIONS
CONNECTIONS FOR NINE LEAD
THREE PHASE MOTORS}

_{THREE PHASE STAR OR Y}

_{THREE PHASE DELTA}
 
 

_{DELTA WYE HOOKUP FOR TRANSFORMER}

_{MOTOR CONTROLLER WITH THREE
START STOP STATIONS
(HOLDING CONTACTS NOT SHOWN)}

_{TRANSFORMER TURNS RATIO}

     Ep   ₌  Tp
     Es        Ts

Where
Ep is primary voltage
Es is secondary voltage
Tp is number of turns in primary
Ts is number of turns in secondary

Typical 3-Phase Wiring Diagrams and Equations for Resistive Heaters Definitions For Both Wye and Delta (Balanced Loads) VP = Phase Voltage VL = Line Voltage IP = Phase Current IL = Line Current R = R1 = R2 = R3 = Resistance of each branch W = Wattage Wye and Delta Equivalent WDELTA = 3 WWYE Open 3-Phase Circuit Formulas: Open Delta Watts = 2/3 WDELTA Open Wye Watts = 1/2 WWYE Open 4-wire Wye Watts = 2/3 WWYE

3-Phase Wye (Balanced Load) 3-Phase Open Wye (No Neutral) IP = I L VP = VL/1.73 WWYE = VL2 /R = 3 (VP2) /R WWYE = 1.73VLIL IPO = I LO VPO = VL/2 WOWYE = 1/2 ( VL /R) WOWYE = 2 (VPO2/R) WOWYE = VLILO   3-Phase Delta (Balanced Load) IP = I L/1.73 VP = VL WDELTA = 3(VL2)/R WDELTA = 1.73VLIL   3-Phase Open Delta .   VP = VL IPO1 = I PO3 = I LO2 ILO3 = 1.73 I PO1 WODELTA = 2(VL2/R )

NEMA RATING FOR ENCLOSURES

NEMA and other organizations have established standards of enclosure construction for control equipment. In general, equipment would be enclosed for one or more of the following reasons:

1. Prevent accidental contact with live parts.
2. Protect the control from harmful environmental conditions.
3. Prevent explosion or fires which might result from the electrical arc caused by the control.

Common types of enclosures per NEMA classification numbers are:

NEMA I – GENERAL PURPOSE

The general purpose enclosure is intended primarily to prevent accidental contact with the enclosed apparatus. It is suitable for general purpose applications indoors where it is not exposed to unusual service conditions. A NEMA I enclosure serves as protection against dust and light indirect splashing, but is not dusttight.

NEMA 3 – DUSTTIGHT, RAINTIGHT

This enclosure is intended to provide suitable protection against specified weather hazards. A NEMA 3 enclosure is suitable for application outdoors, on ship docks, canal and construction work, and for application in subways and tunnels. It is also sleet-resistant.

NEMA 3R – RAINPROOF, SLEET RESISTANT

This enclosure protects against interference in operation of the contained equipment due to rain, and resists damage from exposure to sleet. It is designed with conduit hubs and external mounting, as well as drainage provisions.

NEMA 4 – WATERTIGHT

A watertight enclosure is designed to meet the hose test described in the following note: "Enclosures shall be tested by subjection to a stream of water. A hose with a one inch nozzle shall be used and shall deliver at least 65 gallons per minute. The water shall be directed on the enclosure from a distance of not less than 10 feet and for a period of five minutes. During this period it may be directed in any one or more directions as desired. There shall be no leakage of water into the enclosure under these conditions."

A NEMA 4 enclosure is suitable for applications outdoors on ship docks and in dairies, breweries, etc.

NEMA 4X – WATERTIGHT, CORROSION-RESISTANT

These enclosures are generally constructed along the lines of NEMA 4 enclosures except they are made of a material that is highly resistant to corrosion. For this reason, they are ideal in applications such as paper mills, meat packing, fertilizer and chemical plants where contaminants would ordinarily destroy a steel enclosure over a period of time.

NEMA 7 – HAZARDOUS LOCATIONS – CLASS I

These enclosures are designed to meet the application requirements of the National Electrical Code for Class I hazardous locations. In this type of equipment, the circuit interruption occurs in air.

"Class I locations are those in which flammable gases or vapors are or may be present in the air in quantities sufficient to produce explosive or ignitable mixtures."

NEMA 9 HAZARDOUS LOCATIONS – CLASS II

These enclosures are designed to meet the application requirements of the National Electrical Code for Class II hazardous locations.

"Class II locations are those which are hazardous because of the presence of combustible dust."

The letter or letters following the type number indicates the particular group or groups of hazardous locations (as defined in the National Electrical Code) for which the enclosure is designed. The designation is incomplete without a suffix letter or letters.

NEMA 12 – INDUSTRIAL USE

The NEMA 12 enclosure is designed for use in those industries where it is desired to exclude such materials as dust, lint, fibers and flyings, oil see page or coolant see page. There are no conduit openings or knockouts in the enclosure, and mounting is by means of flanges or mounting feet.

NEMA 13 – OILTIGHT, DUSTTIGHT

NEMA 13 enclosures are generally of cast construction, gasketed to permit use in the same environments as NEMA 12 devices. The essential difference is that, due to its cast housing, a conduit entry is provided as an integral part of the NEMA 13 enclosure, and mounting is by means of blind holes, rather than mounting brackets.