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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.

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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

POWER FACTOR = TRUE POWER
APPARENT POWER

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MOTOR APPLICATION FORMULAS

HORSEPOWER =
(for three phase motors) 1.732 x VOLTS x AMPERES x EFFICIENCY x power factor
746

THREE PHASE AMPERES =
(for three phase motors) 746 x HORSEPOWER
1.732 x VOLTS x EFFICIENCY x POWER FACTOR

SYNCHRONOUS RPM = HERTZ x 120
NUMBER OF POLES

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MOTOR MARKINGS AND CONNECTIONS
CONNECTIONS FOR NINE LEAD
THREE PHASE MOTORS

THREE PHASE STAR OR Y

STAR CONNECTED

Voltage Line 1 Line 2 Line 3 Together
Low 1 & 7 2 & 8 3 & 9 4 & 5 & 6
High 1 2 3 4 & 7, 5 & 8, 6 & 9

THREE PHASE DELTA

DELTA CONNECTED

Voltage Line 1 Line 2 Line 3 Together
Low 1 & 6 & 7 2 & 4 & 8 3 & 5 & 9 NONE
High 1 2 3 4 & 7, 5 & 8, 6 & 9

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

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:

Prevent accidental contact with live parts.
Protect the control from harmful environmental conditions.
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 - DUST TIGHT, RAIN TIGHT

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 - OIL TIGHT, DUST TIGHT

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.