The IEE Wiring Regulations Explained and Illustrated, 2nd Edition

Circuit Design

8 Check for thermal constraints.

Let us now examine each stage in detail.

Design current

In many instances the design current I

h

is quoted by the manu-

facturer, but there are times when it has to be calculated. In that

case there are two formulae involved, one for single phase and one

for three phase:

Single phase:

I

h

=

y (V usually

240

V)

Three phase:

/

b

= -7=^ (VL usually

415

V)

Current is in amperes, and power P in watts.

If an item of equipment has a power factor (PF) and/or has

moving parts, efficiency (eff) will have to be taken into account.

Hence:

Single phase:

PxlOO

b

~ VxPFxeff

Three phase:

PxlOO

b

^3*V

L

xPFxeff

Nomina ...