6.2.1 -  Maximum demand

Maximum demand (often referred to as MD) is the largest current normally carried by circuits, switches and protective devices; it does not include the levels of current flowing under overload or short circuit conditions, Assessment of maximum demand is sometimes straightforward. For example, the maximum demand of a 240 V single-phase 8 kW shower heater can be calculated by dividing the power (8 kW) by the voltage (240 V) to give a current of 33.3 A. This calculation assumes a power factor of unity, which is a reasonable assumption for such a purely resistive load.

There are times, however, when assessment of maximum demand is less obvious. For example, if a ring circuit feeds fifteen 13 A sockets, the maximum demand clearly should not be 15 x 13 = 195 A, if only because the circuit protection will not be rated at more than 32 A. Some 13 A sockets may feed table lamps with 60 W lamps fitted, whilst others may feed 3 kW washing machines; others again may not be loaded at all. Guidance is given in {Table 6.1}.

Lighting circuits pose a special problem when determining MD. Each lamp-holder must be assumed to carry the current required by the connected load, subject to a minimum loading of 100 W per lampholder (a demand of 0.42 A per lampholder at 240 V). Discharge lamps are particularly difficult to assess, and current cannot be calculated simply by dividing lamp power by supply voltage. The reasons for this are:

1. - control gear losses result in additional current,

2. - the power factor is usually less than unity so current is greater, and

3. - chokes and other control gear usually distort the waveform of the current so that it contains harmonics which are additional to the fundamental supply current.

So long as the power factor of a discharge lighting circuit is not less than 0.85, the current demand for the circuit can be calculated from:

current (A) =	lamp power (W) x 1.8
	supply voltage (V)

For example, the steady state current demand of a 240 V circuit supplying ten 65 W fluorescent lamps would be:

I =	10 x 65 x 1.8 A	= 4.88A
	240

Switches for circuits feeding discharge lamps must be rated at twice the current they are required to carry, unless they have been specially constructed to withstand the severe arcing resulting from the switching of such inductive and capacitive loads.

Table 6.1 - Current demand of outlets
Type of outlet	Assumed current demand
2 A socket outlet	At least 0.5A
Other socket outlets	Rated current
Lighting point	Connected load, with minimum of 100 W
Shaver outlet, bell transformer or any equipment of 5 W or less	May be neglected
Household cooker	10A + 30% of remainder + 5A for socket in cooker unit

When assessing maximum demand, account must he taken of the possible growth in demand during the life of the installation. Apart from indicating that maximum demand must be assessed, the Regulations themselves give little help. Suggestions for the assumed current demand of various types of outlet are shown in {Table 6.1}.