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E L E CT R I C AL CONNE CT I ON
AUTUMN 2 01 5
METHOD IN THE MADNESS
An acceptable drop
T
he voltage in a circuit will
drop due to the resistance and
reactance of cables.
Designers and installers must consider
voltage drop to ensure safety and proper
operation of the equipment, i.e.: adequate
voltage must be supplied.
If there is too much voltage drop the
equipment won’t work, or safety will
be compromised – such as a motor
overheating and catching fire.
We therefore need to consider voltage
drop before running cables and connecting
loads. A larger cable than intended may
be needed to ensure compliance with
voltage drop requirements. Considering the
voltage drop issue at the design stage of an
installation will help avoid problems.
The Wiring Rules – AS/NZS 3000, Section
2 at Clause 3.6 – provides the relevant
detail for low-voltage installations. Clause
7.3 covers stand-alone systems and Clause
7.5.7 covers extra-low-voltage installations.
All sparkies should be familiar with these
requirements.
LOW VOLTAGE
Clause 3.6.1 discusses having the
voltage for a piece of equipment higher
than the lower limit of the Standard for
that equipment.
The clause does allow for equipment not
covered by a Standard to function safely
at lower voltage, but it does not say this
negates other sub-clauses in Section 3.6.
Conductors shall be sized to ensure that
the voltage at any point in an installation
does not drop more than 5% from the
nominal (refer Clause 3.6.2). This equates
to 11.5V for a 230V system and 20V for a
400V system.
The clause does not consider transient
currents such as motor starting, solenoid
closing and other similar applications.
The permissible voltage drop can be 7%
if the supply comes from the low-voltage
terminals of a substation on the premises
and is dedicated to the installation (refer
exceptions for Clause 3.6.2).
The voltage drop limits just mentioned
are the total voltage drops for an installation.
Voltage drops in the consumer mains, sub-
mains and final sub-circuits must be tallied
to ensure compliance with the limits.
Most of the voltage drop could be
allocated to the final sub-circuit if there are
no sub-mains and very short consumer
mains. Likewise, long consumer mains and
sub-mains will severely limit the available
voltage drop in the final sub-circuit.
The old Australian Standards handbook
HB301, for designing to the Wiring
Rules, states that 2% to 3% is considered
reasonable for the final sub-circuits, but it
also discusses rearranging percentages for
a satisfactory outcome.
Prudent placement of main switchboards
and sub-boards will help to ensure that
voltage drop is not an issue. This is a design
matter and cannot be left to chance.
STAND-ALONE SYSTEMS
Stand-alone systems, in accordance with
Clause 7.3, are required to have a voltage
drop of no less than 11% from the nominal.
This must take account of the output
voltage of the source and the drop within
the installation under normal operating
conditions (refer exceptions for Clause 3.6.2).
It is really the same voltage-drop limit
as for a grid-connected system, as the
network company can have a drop of 6%
and the installation a further 5%.
EXTRA-LOW VOLTAGE
A maximum voltage drop of 10% for
extra-low-voltage installations, when
conductors are carrying the circuit-
operating current, is applied by Clause 7.5.7.
That is, unless the extra-low-voltage
equipment (not exceeding 50V AC or 120V
ripple-free DC) is specially designed to
operate over a wider voltage range.
The 10% limit excludes transient currents
caused by such things as motor starting,
solenoid closing and other similar events.
CALCULATING VOLTAGE DROP
Two methods are generally used for
calculating voltage drop, as provided by
AS/NZS 3008.1.1, and both use a value of
current in the calculation.
Clause 3.6.2 of AS/NZS 3000 provides
guidance on what value of current
should be used in calculations and
should not exceed:
>
total circuit current;
>
circuit maximum demand; or
>
circuit protective device rating.
For circuits such as lighting and socket
outlets, where the load is distributed over
the circuit length, half the current rating
of the protective device may be used for
calculations (refer exceptions for Clause 3.6.2).
The first method uses conductor
impedance, length and current for the
calculation. This is slightly complex, as the
resistance and reactance of the cable must
be known to allow calculation of the cable
impedance (refer Clause 4.3 of AS/NZS
3008.1.1).
The resistance and reactance values for
various cable types are provided by Tables
30-39 of AS/NZ 3008.1.1. For smaller cables
It might have been some
time since you learned about
voltage drop, so what are
the requirements and the
calculations?
Chris Halliday
explains.