DAILY FORMULA

KUJO ETC...

Electrical/Safety Training & Consulting Subcontractor

International Falls, MN

218-360-1432


KURT'S LIST of NEED to KNOW FORMULAS for the NEC EXAM Revised: January, 2011

OHMS LAW AND POWER LAW: NOTE: P = Power in Watts

E = I x R	R = E/I	I = E/R
DC OR SINGLE PHASE : P = E x I = I² x R THREE PHASE: P = E x I x √3 x P.F.
IMPEDANCE & REACTANCE:
X_L = 2πFL X_C = 1 ⁄ (2πFC) Z = √[R² + (X_L – X_C)²] In Ohms ( Ω )
VOLTAGE DROP:
SINGLE PHASE:	THREE PHASE:	% OF VOLTAGE DROP:
VOLTS DROP = 2 x L x I x R 1000	VOLTS DROP = L x I x R x √3 1000	%VD = VOLTS DROP CIRCUIT VOLTAGE

L = LENGTH OF CIRCUIT ONE WAY IN FEET I = CURRENT IN AMPS √3 = 2 x .866

R = RESISTANCE IN OHMS PER 1000 FEET (see NEC Table 8) Sin 60⁰ = .866

	SYNCHRONOUS SPEED OF MOTOR:
		RPM = 120 x FREQUENCY # OF POLES	RPM = 60 x FREQUENCY # OF POLES PAIRS

THREE PHASE VOLT-AMPS: VA = VOLTS X AMPS X √3

	POWER FACTOR (P.F.):	SINGLE PHASE:	THREE PHASE:
	P.F. = TRUE POWER (WATTS) APPARENT POWER (VA)	P.F. = WATTS VOLTS x AMPS	P.F. = WATTS VOLTS x AMPS x √3

EFFICIENCY: EFF % = OUTPUT (WATTS) NOTE: EFFICIENCY IS ALWAYS LESS THAN 100%

INPUT (WATTS)

	HORSEPOWER: NOTE: 746 WATTS = 1 HORSEPOWER
	SINGLE PHASE:	THREE PHASE:
	MOTOR HP = E x I x EFFICIENCY x P.F. 746	= E x I x EFFICIENCY x P.F. x √3 746

NEUTRAL LOAD CALCULATION FOR WYE SYSTEM:

Take the SQUARE ROOT of : {[ (A²) + (B²) + (C²) ] - [ (A x B) + (B x C) + (A x C) ]}

NOTE: A, B & C ARE THE CURRENTS IN EACH OF THE THREE PHASES

TRANSFORMER RATIO:

PRIMARY VOLTS x AMPS = SECONDARY VOLTS x AMPS

SECONDARY VOLTS = PRIMARY VOLTS x (SECONDARY TURNS ⁄ PRIMARY TURNS)

	BUSS BAR AMPACITY: COPPER = 1000 AMPS PER SQ. INCH ALUMINUM = 700 AMPS PER SQ. INCH
	TRANSFORMER FAULT AMPS (I_SC): I_SC = I_FL ⁄ %Z
	I_FL = Transformer Full Load Current Base Rating %Z = Percent Impedance (as a decimal)