UK Maximum Demand Calculator — IET OSG
Maximum demand is the largest current an installation actually
draws at any one moment. It is almost never the simple arithmetic
sum of every connected load — diversity reduces the figure,
because not every appliance runs at full rated current at the same
time. WireWrite's UK maximum-demand calculator applies the
diversity assessment method in the IET On-Site Guide Appendix A
and recommends a main switch and consumer-tails size from the
result.
Why diversity matters
A typical UK home might have a 9 kW shower, an 11 kW electric
hob, an immersion heater, ring final circuits, lighting, and a
7.4 kW EV charge point. The arithmetic sum of those rated loads
is well over the standard 100 A single-phase service capacity.
In practice the shower is short-duration, the hob cycles, the
immersion is on a timer, lighting is at a fraction of rated, and
the EV charges overnight. Diversity factors capture that reality.
The two assessment methods
BS 7671 Regulation 311.1 requires the maximum demand to be
determined; the IET On-Site Guide Appendix A describes how:
- Assessment by calculation — applies
per-load-group diversity allowances from the OSG Appendix A
tables for domestic and commercial installations. This is the
method the calculator implements.
- Assessment by measurement — for an
existing similar installation, a recording ammeter is fitted
for a representative period and the maximum recorded current
is taken as the demand. Used only when an equivalent
installation is available.
Why "load schedule" is the wrong shortcut
Adding up every connected load in a consumer-unit schedule
(the protective-device "load" column) and treating that as
maximum demand will always oversize the consumer tails. The IET
OSG explicitly recommends the diversity assessment for any
installation beyond a single final sub-circuit. The calculator
lets you enter each load group and applies the published
allowances per appliance type.
What the calculator outputs
You'll get the assessed maximum demand in amperes per phase,
the recommended main-switch and consumer-tails size from standard
UK ranges, and a phase-balance suggestion if the installation is
three-phase. The result is suitable as the calculation reference
on your design records and EIC.
Calculate maximum demand
for your installation →
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BS 7671 Calculators
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