138 Electric Wiring: Domestic
Power This is the product of current and voltage, hence P I V.
Relationship between voltage, current and resistance Voltage Current Resistance V I R, Current Voltage/Resistance I V/R or Resistance Voltage/Current R V/I
Common multiples of units Current I amperes
kA Kilo-amperes 1000 amperes
mA Milli-amperes 1/1000 of an ampere
Voltage V volts
kV Kilovolts 1000 volts
mV Millivolts 1/1000 of a volt
Resistance R ohms
M立 Megohms 1 000 000 ohms
m立 Milli-ohms 1/1000 of an ohm
Power P watts
MW Megawatt 1 000 000 watts
kW Kilowatt 1000 watts
Resistance in series These are resistances joined end-to-end in the form of a chain. The total resistance increases as more resistances are added (Figure A1.1). Hence, if a cable length is increased, its resistance will increase in proportion. For example, a 100 m length of conductor has twice the resistance of a 50 m length of the same diameter.
Resistance in parallel These are resistances joined like the rungs of a ladder. Here the total resistance decreases the more there are (Figure A1.2).