Lighting Maths


It is useful to be able to tell how many amps a combination of lanterns will pull when switched on. Simply add up the powers (in watts) and divide by 230 (the mains voltage).
Power = Current x Voltage  (Watts = Amps x Volts)
Current = Power ÷ Voltage

So a 15A socket can provide a total of 3450W and a 13A socket 2990W.

RMS (Root Mean Squared)

RMS is used to quote voltages and currents of AC supplies. A 10V AC supply is actually 10V RMS, not 10V peak to peak. A 1V RMS signal has the equivelent average voltage of a 1V DC supply. It may be useful to be able to convert between RMS and peak values. The 1.414 in the formulas comes from the square root of 2, I've just worked it out as a number).
Peak = RMS x 1.414
RMS = Peak
÷ 1.414

Voltage Drop Along a Cable

With professional equipment and cables this shouldn't be a problem, it is included here in case it is. It is possible that a weak signal put into a long cable will emerge too weak to use, this is because too much of the signal's voltage has been lost. This can be used with either a DC voltage or an RMS AC voltage.
Voltage Drop = Current x Length x Resistance per metre
(Volts = Amps x Metres x Resistance per metre)

Approximate values for the resistance per metre are (assumes copper wire):

  • 1mm = 0.0214
  • 1.5mm = 0.0107
  • 2mm = 0.00531
  • 2.5mm = 0.00377
  • 3mm = 0.00265
  • 5mm = 0.0010
  • 6mm = 0.000522

Unit Conversions

Millimetres = 25.4 x inches
Metres = 0.3048 x feet

Lux = 10.8 x foot candles

Kilograms = 0.454 x Pounds
Pounds = 2.205 x Kilograms