Electrical Contractors’ Association
KEY FACTSHEET
VOLTAGE DROP IN CONSUMER INSTALLATIONS
Voltage drop in a consumer’s installation can be a contentious issue. Nevertheless, it is an
important aspect of installation design because, if it is too high, certain equipment will either
not function correctly or not function at all.
BS7671 Requirements:
525.1 In the absence of any other
consideration, under normal service conditions
the voltage at the terminals of any fixed
current-using equipment shall be greater than
the lower limit corresponding in the product
standard relevant to the equipment.
525.100 Where fixed current-using equipment
is not the subject of a product standard the
voltage at the terminals shall be such as not to
impair the safe functioning of that equipment.
achieved. Circuit cable conductor sizes are then
calculated and selected to ensure that the total
voltage drop from the origin of the installation is such
that, under full load conditions, the lower voltage
limits recommended by the equipment manufacturers
are maintained. In the event that the minimum
voltage cannot be achieved it may be necessary to
provide protection against under-voltage or voltage
fluctuations.
The following does not take account of any spare
capacity that may be required within the total voltage
drop assessment process. The designer should
discuss such requirements with the client before the
assessment is undertaken.
The Origin of the Installation:
525.101 The above requirements are deemed
to be satisfied if the voltage drop between the
origin of the installation (usually the supply
terminals) and a socket-outlet or the terminals
of fixed current-using equipment, does not
exceed that stated in Appendix 4 Section 6.4.
It is important to note that the main criteria in
525.100 is the safe functioning of the equipment
which means that, providing the equipment can
operate safely and function correctly at its supply
voltage, there is no limit on the voltage drop in the
system. This is also important where voltage
optimisation equipment is utilised.
The designer should be aware that Appendix 4
(referred to in Regulation 525.101) provides one
method of complying with BS 7671 requirements.
However, other methods that take into account
permissible system tolerances are equally valid.
It should also be noted that BS 7671 appendices
provide guidance and are non-regulatory.
It is important when designing an installation, to
assess the characteristics of the equipment being
installed. In particular, the designer should identify
the equipment manufacturers’ recommended
operating voltages and ensure that they can be
For installations supplied from the Distribution
Network Operator (DNO) low-voltage Public Network,
the origin is normally the point at which electricity is
supplied to the premises; e.g. the service cable at the
intake cut out and metering point.
Installations that are supplied at HV to a dedicated
on-site transformer or a private generator are usually
deemed to be a Private Network. In such
installations the origin is the supply transformer or
generator output terminals.
Calculating Voltage Drop
When calculating voltage drop due consideration
should be given to the following: motor starting
currents; in-rush currents; control voltages (particularly
those associated with computerised systems).
Notes:
(i) Motor control contactors and relays can ‘drop
out’ if the coil voltages fall towards 80% of the
operating voltage.
(ii) The effects of harmonic currents may also need
to be considered and included in the calculation.
(iii) Voltage transients and voltage variations due to
abnormal operation can be ignored.
VOLTAGE DROP IN CONSUMER INSTALLATIONS - CONTINUED
Control equipment circuits can be protected against
voltage drop or voltage fluctuations by using an
Uninterruptable Power Supply (UPS).
The nominal voltage from a DNO supply for an
installation is 230V single phase and 400V three
phase, with a permitted tolerance of +10% / -6%.
Electricity Safety, Quality and Continuity Regulations
(ESQCR) 2002 - requirements in premises supplied
from a Public Network are fixed and cannot be
altered by the consumer. On a Private Network there
is more flexibility, as the consumer is able to adjust
the transformer tappings and thus vary the open
circuit voltage.
BS 7671:2008 Section 6.4, Table 4Ab gives
guidance on voltage drop percentages that are
deemed to satisfy the regulations. It also shows the
means of calculating voltage drop that may be used
by the designer. The Table shows voltage drop
percentage limits for lighting and other circuits in
both low voltage Public and Private Networks. The
limits apply to the nominal voltage of 230 V single
phase and 400V three phase.
The maximum voltage drop values taken from Table
4Ab are shown below:
230 VOLTS
Network Type
Lighting
Other Circuits
(i) Public
Networks
3% (6.9V)
5% (11.5V)
(ii) Private
Networks*
6% (13.8V)
8% (18.4V)
*The voltage drop within each final circuit on Private
Networks, should not exceed the values given in (i) above
for Public Networks
400 VOLTS
Network Type
Lighting
Other Circuits
(i) Public
Networks
3% (12.0V)
5% (20.0V)
(ii) Private
Networks
6% (24.0V)
8% (32.0V)
*The voltage drop within each final circuit on Private
Networks, should not exceed the values given in (i) above
for Public Networks
When calculating the voltage drop in a circuit,
the design current can be taken as being either
the equipment rated current or, where there
are a number of loads, the total connected
load multiplied by a diversity factor.
Additionally, where the total circuit length
exceeds 100 metres, the limits given in Table
4Ab may be increased by 0.005% per metre
up to a maximum of 0.5%.
The voltage drop can be apportioned throughout the
system circuits as the designer wishes, but the final
circuit voltage drop is limited to the values given for
Public Networks, regardless of whether it is a Public
Network or a Private Network. For example: in a
Private Network a lighting final circuit has a voltage
drop limit of 3%, which allows 3% for the
distribution circuit(s). ‘Other Circuits’ in a Private
Network have a final circuit voltage drop limit of 5%,
leaving 3% for the distribution circuit(s) installed from
the origin to the final circuit distribution board(s).
Therefore, in order to apply a higher level of voltage
drop on the distribution circuit(s), it is necessary to
reduce the voltage drop on final circuits further to
compensate for the gain in voltage drop on the
distribution circuit(s).
Where a dwelling is supplied from the Public Network,
the above is not normally necessary as final circuit
distribution board(s) are usually close to the supply
origin. Normally calculations are based on a nominal
supply voltage of 230/400V at the origin. If the supply
voltage is known to be permanently in excess of
230/400V the designer has scope for increasing the
voltage drop percentages. In a Private Network these
percentages could be increased by selecting and
setting a higher transformer output terminal voltage.
Where the supply voltage at the origin is lower than
the nominal 230/400V, the designer needs to
consider the effect of the minimum permissible
supply voltage. This is a maximum of 6% below the
nominal supply voltage, which equates to 216.2V
and 376V respectively.
This tolerance may be used when calculating the
overall voltage drop in a Private Network, which
means there can be 12% allowable voltage drop for
lighting circuits and 14% for other circuits, whilst still
remaining compliant with BS 7671. The designer can
take advantage of this and apportion it throughout
the installation to a cost advantage, but with the two
caveats outlined above for the final circuit voltage
drop limitation and the necessity for the operating
circuit voltage to be at the level required by the
connected equipment.
VOLTAGE DROP IN CONSUMER INSTALLATIONS - CONTINUED
Example for a Private Network
Supply:
A 10KW single phase load requires a minimum of
220 volts to operate correctly. The final circuit is a
63 amp protected circuit supplying the load via a 2
core 10mm2 PVC/PVC XLPE SWA armoured cable.
The final circuit length is 30 metres and the constant
load current is 52.17 amps. The Vd/A/m figure is 4.7
(Table 4E2B of BS 7671:2008).
Maximum voltage drop for the final circuit is 5%
(from (i) of the Table above). The note below the
table says you must use Public Network figures on
Private Network final circuits.
Voltage drop on final circuit:
4.7 x 52.17 x 30/1000 = 7.36 volts. This
equates to 3.2% of the nominal voltage, which is
below the maximum permitted 11.5 volts (5%).
The load only requires 220 volts to operate, so the
minimum voltage we require at the distribution
board is 220 + 7.36 = 227.36 volts.
If the Private Network transformer has a single
phase open circuit voltage of 245 volts, we have
available 17.64 volts for use on the distribution
circuit(s) design. This equates to 7.6% of the
nominal voltage (230v), which makes the total
voltage drop 10.9%. This is below the 14%
figure given above, which takes into account
the permissible tolerances on the DNO supply.
It can be seen from this, the lower the open
circuit transformer voltage, the less the
designer has available to him for calculating
circuit voltage drop in his design.
Extending from an Existing
Distribution Board
The foregoing applies to new installations where the
designer has control over the distribution as well as
final circuit(s). Difficulties arise when adding circuits
to an existing distribution board(s). The designer
needs to ensure that the new circuit(s) complies with
the current BS 7671 requirements, particularly with
respect to voltage drop.
In an ideal situation the designer of the added
circuit(s) will have the original design information
available to use. Such information would include:
final circuit load currents; submain and distribution
circuit load currents; diversity factors that have been
applied in the primary design; conductor sizes and
voltage drop. This information will enable the
designer to assess the effect the additional load will
have on the supply voltage to existing loads, thus
preventing power supply problems on both new and
existing equipment within the installation.
In many cases the information is not going to be
readily available, if at all. Never-the-less, the designer
must still consider all the existing circuits listed above
for their loading and voltage drop.
In which case, clearly, some form of survey of the
existing installation needs to take place.
One method to adopt is to measure or ascertain
all the existing circuit(s) currents under load
conditions, together with the resultant voltage
levels at the origin and all distribution board(s),
then the additional circuit(s) can be designed,
taking into account existing voltage drop
limitations. Permitted voltage tolerances can
also be taken into account, as described for
Private Networks above.
The increase in voltage drop due to the
additional circuit load can be calculated as a
% of the nominal supply by using the data
from the voltage drop tables for the cable(s).
However, it may be impractical to measure or
ascertain the maximum load currents and
resultant voltages by this method. Therefore,
other methods may have to be considered in
order to obtain a realistic voltage drop % in an
installation.
One other method is to determine the
maximum load of each circuit by surveying
and recording the information from the data
plates attached to the connected equipment.
The designer then has to make a judgement
on the diversity that can be applied, so that
the maximum actual circuit loading(s) and
voltage drop can be assessed throughout the
installation, as detailed above.
Another method is firstly to ensure that any
additional final circuits have their voltage drop
calculated in accordance with the criteria given
above for both Public and Private Networks
(Table 4Ab values). Then note the percentage
voltage drop obtained in each case. Existing
final circuits and distribution circuits should
have their voltage drop verified to ensure they
meet the same criteria, so as to verify that the
new circuits do not have any adverse affect on
any of the existing circuits.
VOLTAGE DROP IN CONSUMER INSTALLATIONS - CONTINUED
Vd (FC) = In (ZL – ZL (DB))
Vd (DIST) = In (DIST) (ZL (DB) – ZL (ORG))
Do this for every distribution circuit that is supplying
additional circuits and ensure it is not below the
minimum permissible supply voltages given above.
Again, the two caveats previously given must be
given due consideration. There should be sufficient
supply voltage remaining, above this minimum level,
to accommodate the voltage drop on the existing final
circuits. If not, further calculation will be required to
reapportion the voltage drops in the installation. The
minimum supply voltage required (V (REQ)) can be
expressed as:
V (REQ) > 216.2V + Vd (ORG) + Vd (FC)
(for single phase supplies) and;
V (REQ) > 376V + Vd (ORG) + Vd (FC)
(for three phase supplies)
Using this method a realistic worse case voltage drop
can be assessed and the designer is able to
demonstrate that the requirements of BS 7671
section 525 are satisfied.
FURTHER INFORMATION
Contact: Electrical Contractors’ Association Tel: 020 7313 4804 or visit the website www.eca.co.uk
P18331407
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Next, ensure the distribution circuit(s) will not
become overloaded by the additional circuit load and
then assess the effect on the distribution circuit(s)
voltage drop by the additional circuits. Calculate the
voltage drop on the distribution circuit(s) by
measuring or ascertaining the circuit impedance
between line and neutral or line and line at the origin
(ZL(ORG)), as applicable; then measure or ascertain
the circuit impedance between line and neutral or
line and line at the distribution board(s) (ZL(DB));
deduct ZL(ORG) from ZL(DB) to obtain the impedance of
the distribution circuit(s) (ZL(DIST)); multiply ZL(DIST)
by the current rating of the distribution circuit final
circuit protective device (In), to obtain the voltage
drop on the distribution circuit (Vd (DIST)); subtract the
voltage drop figure from the voltage measured or
ascertained at the origin (V (ORG)) and record the
resultant supply voltage figure (V(s)). This is shown in
the equation below. As mentioned above, Ib may be
used in place of In, where an accurate maximum load
figure for the circuit can be ascertained.
Information presented is accurate at time of printing.
Existing final circuits can have their voltage drop verified
by: measuring or ascertaining the line to neutral or line
to line impedances measured at the furthest point on
each circuit (ZL), then deduct the line to neutral or line
to line impedances (ZL (DB)), as applicable, from the ZL
value, giving the resultant value ZL (FC); multiply ZL (FC)
by the current rating (In) of the final circuit protective
device, to give the final circuit voltage drop, as shown
in the equation below. Use of In gives the worse case
condition for the circuit. Therefore, Ib may be used
instead of In, where an accurate maximum load figure
for the circuit can be ascertained.