Electrical Contractors Association

KEY FACTSHEET

BS7671 Requirements:

It is important to note that the main criteria in525.100 is the safe functioning of the equipmentwhich means that, providing the equipment canoperate safely and function correctly at its supplyvoltage, there is no limit on the voltage drop in thesystem. This is also important where voltageoptimisation equipment is utilised.

The designer should be aware that Appendix 4(referred to in Regulation 525.101) provides onemethod of complying with BS 7671 requirements.However, other methods that take into accountpermissible system tolerances are equally valid.

It should also be noted that BS 7671 appendicesprovide guidance and are non-regulatory.

It is important when designing an installation, toassess the characteristics of the equipment beinginstalled. In particular, the designer should identifythe equipment manufacturers recommendedoperating voltages and ensure that they can be

VOLTAGE DROP IN CONSUMER INSTALLATIONS

achieved. Circuit cable conductor sizes are thencalculated and selected to ensure that the totalvoltage drop from the origin of the installation is suchthat, under full load conditions, the lower voltagelimits recommended by the equipment manufacturersare maintained. In the event that the minimumvoltage cannot be achieved it may be necessary toprovide protection against under-voltage or voltagefluctuations.

The following does not take account of any sparecapacity that may be required within the total voltagedrop assessment process. The designer shoulddiscuss such requirements with the client before theassessment is undertaken.

The Origin of the Installation: For installations supplied from the DistributionNetwork Operator (DNO) low-voltage Public Network,the origin is normally the point at which electricity issupplied to the premises; e.g. the service cable at theintake cut out and metering point.

Installations that are supplied at HV to a dedicatedon-site transformer or a private generator are usuallydeemed to be a Private Network. In suchinstallations the origin is the supply transformer orgenerator output terminals.

Calculating Voltage DropWhen calculating voltage drop due considerationshould be given to the following: motor startingcurrents; in-rush currents; control voltages (particularlythose associated with computerised systems).

Notes:

(i) Motor control contactors and relays can dropout if the coil voltages fall towards 80% of theoperating voltage.

(ii) The effects of harmonic currents may also needto be considered and included in the calculation.

(iii) Voltage transients and voltage variations due toabnormal operation can be ignored.

Voltage drop in a consumers installation can be a contentious issue. Nevertheless, it is animportant aspect of installation design because, if it is too high, certain equipment will eithernot function correctly or not function at all.

525.1 In the absence of any otherconsideration, under normal service conditionsthe voltage at the terminals of any fixedcurrent-using equipment shall be greater thanthe lower limit corresponding in the productstandard relevant to the equipment.

525.100 Where fixed current-using equipmentis not the subject of a product standard thevoltage at the terminals shall be such as not toimpair the safe functioning of that equipment.

525.101 The above requirements are deemedto be satisfied if the voltage drop between theorigin of the installation (usually the supplyterminals) and a socket-outlet or the terminalsof fixed current-using equipment, does notexceed that stated in Appendix 4 Section 6.4.

VOLTAGE DROP IN CONSUMER INSTALLATIONS - CONTINUED

Control equipment circuits can be protected againstvoltage drop or voltage fluctuations by using anUninterruptable Power Supply (UPS).

The nominal voltage from a DNO supply for aninstallation is 230V single phase and 400V threephase, with a permitted tolerance of +10% / -6%.Electricity Safety, Quality and Continuity Regulations(ESQCR) 2002 - requirements in premises suppliedfrom a Public Network are fixed and cannot bealtered by the consumer. On a Private Network thereis more flexibility, as the consumer is able to adjustthe transformer tappings and thus vary the opencircuit voltage.

BS 7671:2008 Section 6.4, Table 4Ab givesguidance on voltage drop percentages that aredeemed to satisfy the regulations. It also shows themeans of calculating voltage drop that may be usedby the designer. The Table shows voltage droppercentage limits for lighting and other circuits inboth low voltage Public and Private Networks. Thelimits apply to the nominal voltage of 230 V singlephase and 400V three phase.

The maximum voltage drop values taken from Table4Ab are shown below:

230 VOLTS

*The voltage drop within each final circuit on PrivateNetworks, should not exceed the values given in (i) abovefor Public Networks

400 VOLTS

*The voltage drop within each final circuit on PrivateNetworks, should not exceed the values given in (i) abovefor Public Networks

The voltage drop can be apportioned throughout thesystem circuits as the designer wishes, but the finalcircuit voltage drop is limited to the values given forPublic Networks, regardless of whether it is a PublicNetwork or a Private Network. For example: in aPrivate Network a lighting final circuit has a voltagedrop limit of 3%, which allows 3% for thedistribution circuit(s). Other Circuits in a PrivateNetwork have a final circuit voltage drop limit of 5%,leaving 3% for the distribution circuit(s) installed fromthe origin to the final circuit distribution board(s).Therefore, in order to apply a higher level of voltagedrop on the distribution circuit(s), it is necessary toreduce the voltage drop on final circuits further tocompensate for the gain in voltage drop on thedistribution circuit(s).

Where a dwelling is supplied from the Public Network,the above is not normally necessary as final circuitdistribution board(s) are usually close to the supplyorigin. Normally calculations are based on a nominalsupply voltage of 230/400V at the origin. If the supplyvoltage is known to be permanently in excess of230/400V the designer has scope for increasing thevoltage drop percentages. In a Private Network thesepercentages could be increased by selecting andsetting a higher transformer output terminal voltage.

Where the supply voltage at the origin is lower thanthe nominal 230/400V, the designer needs toconsider the effect of the minimum permissiblesupply voltage. This is a maximum of 6% below thenominal supply voltage, which equates to 216.2Vand 376V respectively.

This tolerance may be used when calculating theoverall voltage drop in a Private Network, whichmeans there can be 12% allowable voltage drop forlighting circuits and 14% for other circuits, whilst stillremaining compliant with BS 7671. The designer cantake advantage of this and apportion it throughoutthe installation to a cost advantage, but with the twocaveats outlined above for the final circuit voltagedrop limitation and the necessity for the operatingcircuit voltage to be at the level required by theconnected equipment.

Network Type Lighting Other Circuits

(i) Public Networks 3% (6.9V) 5% (11.5V)

(ii) PrivateNetworks* 6% (13.8V) 8% (18.4V)

Network Type Lighting Other Circuits

(i) Public Networks 3% (12.0V) 5% (20.0V)

(ii) PrivateNetworks 6% (24.0V) 8% (32.0V)

When calculating the voltage drop in a circuit,the design current can be taken as being eitherthe equipment rated current or, where thereare a number of loads, the total connectedload multiplied by a diversity factor.

Additionally, where the total circuit lengthexceeds 100 metres, the limits given in Table4Ab may be increased by 0.005% per metreup to a maximum of 0.5%.

VOLTAGE DROP IN CONSUMER INSTALLATIONS - CONTINUED

Example for a Private NetworkSupply:A 10KW single phase load requires a minimum of220 volts to operate correctly. The final circuit is a63 amp protected circuit supplying the load via a 2core 10mm2 PVC/PVC XLPE SWA armoured cable.The final circuit length is 30 metres and the constantload 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 thetable says you must use Public Network figures onPrivate Network final circuits.

Extending from an ExistingDistribution BoardThe foregoing applies to new installations where thedesigner has control over the distribution as well asfinal circuit(s). Difficulties arise when adding circuitsto an existing distribution board(s). The designerneeds to ensure that the new circuit(s) complies withthe current BS 7671 requirements, particularly withrespect to voltage drop.

In an ideal situation the designer of the addedcircuit(s) will have the original design informationavailable to use. Such information would include:final circuit load currents; submain and distributioncircuit load currents; diversity factors that have beenapplied in the primary design; conductor sizes and

voltage drop. This information will enable thedesigner to assess the effect the additional load willhave on the supply voltage to existing loads, thuspreventing power supply problems on both new andexisting equipment within the installation.

In many cases the information is not going to bereadily available, if at all. Never-the-less, the designermust still consider all the existing circuits listed abovefor their loading and voltage drop.

In which case, clearly, some form of survey of theexisting installation needs to take place.

Voltage drop on final circuit:

4.7 x 52.17 x 30/1000 = 7.36 volts. Thisequates to 3.2% of the nominal voltage, which isbelow the maximum permitted 11.5 volts (5%).

The load only requires 220 volts to operate, so theminimum voltage we require at the distributionboard is 220 + 7.36 = 227.36 volts.

If the Private Network transformer has a singlephase open circuit voltage of 245 volts, we haveavailable 17.64 volts for use on the distributioncircuit(s) design. This equates to 7.6% of thenominal voltage (230v), which makes the totalvoltage drop 10.9%. This is below the 14%figure given above, which takes into accountthe permissible tolerances on the DNO supply.

It can be seen from this, the lower the opencircuit transformer voltage, the less thedesigner has available to him for calculatingcircuit voltage drop in his design.

One method to adopt is to measure or ascertainall the existing circuit(s) currents under loadconditions, together with the resultant voltagelevels at the origin and all distribution board(s),then the additional circuit(s) can be designed,taking into account existing voltage droplimitations. Permitted voltage tolerances canalso be taken into account, as described forPrivate Networks above.

The increase in voltage drop due to theadditional circuit load can be calculated as a% of the nominal supply by using the datafrom the voltage drop tables for the cable(s).

However, it may be impractical to measure orascertain the maximum load currents andresultant voltages by this method. Therefore,other methods may have to be considered inorder to obtain a realistic voltage drop % in aninstallation.

One other method is to determine themaximum load of each circuit by surveyingand recording the information from the dataplates attached to the connected equipment.The designer then has to make a judgementon the diversity that can be applied, so thatthe maximum actual circuit loading(s) andvoltage drop can be assessed throughout theinstallation, as detailed above.

Another method is firstly to ensure that anyadditional final circuits have their voltage dropcalculated in accordance with the criteria givenabove for both Public and Private Networks(Table 4Ab values). Then note the percentagevoltage drop obtained in each case. Existingfinal circuits and distribution circuits shouldhave their voltage drop verified to ensure theymeet the same criteria, so as to verify that thenew circuits do not have any adverse affect onany of the existing circuits.

VOLTAGE DROP IN CONSUMER INSTALLATIONS - CONTINUED

Existing final circuits can have their voltage drop verifiedby: measuring or ascertaining the line to neutral or lineto line impedances measured at the furthest point oneach circuit (ZL), then deduct the line to neutral or lineto line impedances (ZL (DB)), as applicable, from the ZLvalue, giving the resultant value ZL (FC); multiply ZL (FC)by the current rating (In) of the final circuit protectivedevice, to give the final circuit voltage drop, as shownin the equation below. Use of In gives the worse casecondition for the circuit. Therefore, Ib may be usedinstead of In, where an accurate maximum load figurefor the circuit can be ascertained.

Vd (FC) = In (ZL ZL (DB))

Next, ensure the distribution circuit(s) will notbecome overloaded by the additional circuit load andthen assess the effect on the distribution circuit(s)voltage drop by the additional circuits. Calculate thevoltage drop on the distribution circuit(s) bymeasuring or ascertaining the circuit impedancebetween line and neutral or line and line at the origin(ZL(ORG)), as applicable; then measure or ascertainthe circuit impedance between line and neutral orline and line at the distribution board(s) (ZL(DB));deduct ZL(ORG) from ZL(DB) to obtain the impedance ofthe distribution circuit(s) (ZL(DIST)); multiply ZL(DIST)by the current rating of the distribution circuit finalcircuit protective device (In), to obtain the voltagedrop on the distribution circuit (Vd (DIST)); subtract the

voltage drop figure from the voltage measured orascertained at the origin (V (ORG)) and record theresultant supply voltage figure (V(s)). This is shown inthe equation below. As mentioned above, Ib may beused in place of In, where an accurate maximum loadfigure for the circuit can be ascertained.

Vd (DIST) = In (DIST) (ZL (DB) ZL (ORG))

Do this for every distribution circuit that is supplyingadditional circuits and ensure it is not below theminimum permissible supply voltages given above.Again, the two caveats previously given must begiven due consideration. There should be sufficientsupply voltage remaining, above this minimum level,to accommodate the voltage drop on the existing finalcircuits. If not, further calculation will be required toreapportion the voltage drops in the installation. Theminimum supply voltage required (V (REQ)) can beexpressed 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 dropcan be assessed and the designer is able todemonstrate that the requirements of BS 7671section 525 are satisfied.

P18331407 The ECA Logo is a Registered Collective Mark. Information presented is accurate at time of printing.

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