Page 130 - E24 Electrical Notebook

126 - Useful Electrical Formulae

Distribution SINGLE PHASE THREE PHASE

TO CALCULATE = I x V x p.f. = I x V x √3 x p.f.
1000 1000
Kilowatts
= IxV = I x V x √3
kVA 1000 1000

Motor HP = I x V x % Eff. x p.f. = I x V x √3 x % Eff. x p.f.
746 746
Current
= HP x 746 = HP x 746
Current V x % Eff. x p.f. √3 x V x % Eff. x p.f.

Transformer = kW x 1000 = kW x 1000
Secondary Current V x p.f. √3 x V x p.f.

Approximate prospective 3 phase transformer = kVA x 1000 = kVA x 1000
bolted short circuit secondary current V √3 x V
I sc
Maximum prospective 3 phase transformer = kVA x 105
H.V. switchgear MVA rating kVAsc V x √3 x % Z

Time for which a copper / PVC conductor will accept t = MVA x 106
a known fault current V x √3
%Z =
V= Volts = 115² S² seconds
I² sc

Transformer percentage impedance

I = Amperes kVA = Transformer Rating

% Eff. = Percentage Efficiency HP = Horsepower = 746 Watts

p.f. = Power Factor kVAsc = Prospective fault level

I sc = Short Circuit current ( RMS ) S = Conductor C.S.A. in mm²

Lighting F = The Lumen output of the lamp(s) or tube(s)
N = Number of Luminaires
Average Illumination (Lumen Method) M = Maintenance Factor

EAV = F x N x M x C or N = Eav x A A = Area (M²)
A CxMxF

EAV = Average illumance (lux)

C = Coefficient of utilisation (ascertained by Room Index & Manufacturers data)

Room Index L = Length of room
N = Width of room (metres)
RI = LxW Hm = Mounting height of luminaire above working plane (metres)
E = Point height of luminaire above working plane (metres)
Hm (L + W) I = Source intensity (candelas)
Ø = Angle of beam from vertical plane
Point by Point Method H Height of source (metres)

E = I Cos³ Ø or E = I Cos Ø

H² D² + H³

D = Horizontal distance away from source (metres)