Circuit Design

Details

NOTE: This article is based on the 16th Edition Wiring Regulations but in principle this is a process that still has to be done using the current version of the Wiring Reguations. The current 17th Edition Amd 2 only requires supplementary bonding under certain circumstances where RCD's are not fitted. Manufacturers requirements should also still be followed.

 

For the purposes of this exercise, we will consider installing a new circuit for an Electric Shower. This one of the most adventurous jobs a DIYer will try to undertake so we will look at what an electrician does.

Considerations

When designing or planning a circuit, there are certain things that MUST be taken into consideration:

    1. Is there adequate earthing arrangements in the property?

    2. How much load, (Amps), does the accessory take? Usually the load is expressed in kilowatts (kW or Watts (W) which is actually power.

    3. Is there a spare position or WAY on the fuseboard to fit another fuse.

 

  1. a) If using the existing Consumer Unit, will the new circuit for the shower be
    protected by an existing RCD
    b) Is the existing RCD, if fitted, capable of supporting the extra load?
    c) Is the RCD rated at 30mA?

  2. Can the main switch take the additional load?

  3. What length will the supply cable be in the completed circuit? That is from the fuseboard to the accessory i.e. the Shower controls.

  4. What route will the cable take to get from the fuseboard to the accessory?

  5. How will the cable be run from the fuseboard to the accessory? I.e. clipped to the wall, chased in or in trunking etc. (this is known as Method of Installation)

  6. How will the supplementary earthing be routed in the bathroom.

  7. Is there an existing shower that is being replaced?

  8. If there is an existing shower, do you propose to use the existing cable?

  9. Is the existing cable in good order?

  10. Will the existing cable take the new load?

  11. If you propose to use the existing cable are any existing switches, fuses etc usable?

  12. Is it safe for the person carrying out the construction to carry out the work without causing danger to any other person in the property.

 

Planning

  1. Adequate earthing arrangements
    a) It needs to be established that the main earth is in place and of a suitable size. It should have, at the very minimum, a cross sectional area (c.s.a.) of 6mm². There would be a recommendation on the certificate to have the size increased to a c.s.a. of 10mm² or 16mm² depending on the type of supply coming into the property.
    b) The water service equipotential earth bonding should be checked and established that it is suitably located and of a suitable size. It should be within 600mm on the customers side of the stop cock, or as reasonably near as possible, but before any branching in the pipe work. . It should have, at the very minimum, a c.s.a. of 6mm². There would be a recommendation on the certificate to have the size increased to a c.s.a. of 10mm².
    c) The Gas service equipotential earth bonding should be checked and established that it is suitably located and of a suitable size. It should be within 600mm on the customers side of the meter, or as reasonably near as possible, but before any branching in the pipe work. . It should have, at the very minimum, a c.s.a. of 6mm². There would be a recommendation on the certificate to have the size increased to a c.s.a. of 10mm². Obviously if there is no Gas supply to the property there is no need to check for this.
    d) The same principal should be applied to any oil service coming into the property.
    e) Cross-bonding, usually located in the airing cupboard should be examined to see if it is in place.
    f) Is there equipotential bonding to the boiler. It should have, at the very minimum, a c.s.a. of 6mm². There would be a recommendation on the certificate to have the size increased to a c.s.a. of 10mm².
    If any of the points a, b, c, d, & e can not be satisfied, no electrical work should be carried out until they have been dealt with. The normal procedure would be for the owner of the property to be advised in writing.

  2. Accessory Load
    If it is not obvious as to how much current an accessory uses, there is a simply calculation:

    P=I x V
    where
    P= Power (expressed in watts)
    I = Current (expressed in Amps or Amperes)
    V = Volts

    In modern day equipment one may find, for example in showers, two sets of power expressions, one at 230volts and one at 240 volts which are quite different. This is because the UK is harmonising with Europe where we will all have the same voltage. As you will not know if the property you are at is 230 or 240 volts you must calculate on the worst scenario. We will take a an example where they have expressed the following:
    8.7kW at 240 volts & 8kW at 230 volts.

    Applying the above equation, P=I x V for 8.7kW @ 240v, you get the following:


     

    Therefore if the voltage is 240 the shower will draw a current of 36.25 Amps

    Applying the above equation, P=I x V for 8kW @ 230v, you get the following:


     

    Therefore if the voltage is 230 the shower will draw a current of 34.78 Amps

    We are going to take the worst case scenario of 36.25 Amps which is your DESIGN CURRENT for the circuit.

  3. The next thing to look at is if there is a spare place in the fuse board to put a fuse or miniature circuit breaker (mcb). Most modern day boards are quite obvious but the older ones can be more difficult.

  4. Whilst there is no requirement in the Electrical Installation regulations for an RCD to provide protection for a shower circuit, there is a requirement by the shower manufacturers to fit one for supplementary protection.

    a) you may find RCD's installed in one of the following three ways.
    (i) As a Main Switch protecting the whole consumer unit.
    (ii) In a split load consumer unit only protecting some of the circuits.
    (iii) As a totally separate unit either protecting the whole installation or only a dedicated part.
    b) Which ever way has been used above, it must be determined that the RCD is capable of carrying the additional load as described in (5) Main Switch below.
    c) It should be checked what tripping current is required to activate the RCD. The most common ones used in a domestic premises is 30mA or 100mA. For showers it must be rated at 30mA.

  5. Main Switch
    Very rarely there will be a switch on its own that isolates(switches off) all the fuseboards, if there are more than one. If there is only one fuseboard the main switch is integral to the board and isolates all the circuits. It depends on the age of the fuseboard as to how easy it is to determine the rating of the switch.

    It is important that you get this right so that time and money can be saved when having to correct this problem. Once you have determined the size, you need to ascertain whether the additional load, in this case the shower, can be managed by the switch without causing damage. If there is more than one fuseboard and there is a spare way in one of them, regardless as to whether there is a separate main switch, you must also satisfy yourself that the switch on the fuseboard itself can also take the additional load.

    Be careful selecting which fuseboard, in the case of more than one being present, as there are from time to time, one could be on a time switch, more commonly for storage heaters, and it will not have power to it 24/7.

    Whilst there is guidance in the regulations for determining the potential load at a fuseboard applying diversity, it can quite often be a case of judgement. There follows a couple of examples:

    In a flat one could find the following fuseboard arrangement
    32A mcb Cooker + integral socket
    32A mcb Ring
    16A mcb Immersion Heater
    6A mcb Lighting (6 lighting points)
    Spare Spare
    60A Main switch

    By adding all the fuses together 32+32+15+6= 86 Amps which obviously has the potential to overload the main switch. By applying DIVERSITY in accordance with the regulations you will get the following using the same mcb's

    32A mcb Cooker + integral socket Allowing diversity 24.6A
    32A mcb Ring Allowing diversity 32A
    16A mcb Immersion Heater Allowing diversity 16A
    6A mcb Lighting (6 lighting points) Allowing diversity 2.5A
    Spare Spare
    60A Main switch

    We can now re-calculate as follows: 24.6+32+16+2.5= 75.1 Amps. So by applying DIVERSITY we have managed to reduce the load by nearly 11 amps. We can further reduce the potential load by applying electricians judgement. This is where the electrician will look around the property to see what sort of electrical items are being utilised to form a judgement. In most cases the main switch and configuration is quite adequate however to add a shower to this, which based on our previous calculation would require a 40 Amp mcb, would not be satisfactory because the main switch is only rated at 60A. We would therefore have to consider installing a new fuseboard, to cater for existing circuits and the new load or installing a separate fuseboard. By installing a separate fuseboard a main switch to isolate both the new board and the existing board would need to be installed.

  6. The length of cable is very relevant as there is a factor to consider called Volt Drop. Again there is a formula that can be applied, with reference to the regulations, to determine the maximum length of the cable. Other considerations that affect the maximum length permitted by the regulations are voltage and maximum likely current.
  7. When routing the cable from the fuseboard, spur or any other source, there are specific locations and specific ways that cables can be installed. It is extremely important that these are adhered to in order that the installation complies with the regulations. If the cable is being routed on the surface either by being clipped or in trunking, you can pretty much put it where you wish. However consideration should be given to the appearance, ambient temperatures, likelihood of mechanical damage and surface material that the cable is touching. If the cable is chased in the wall there are designated areas where cables should be placed.
  8. The method of installation is relevant to determine the size of cable. The two methods most commonly used in a domestic property are Method 1, Clipped direct to the wall
    or chased into the wall or laying on a surface and Method 3, Enclosed in Plastic Trunking.
  9. When introducing new fixed electrical equipment in the bathroom the Supplementary Earth Bonding within the bathroom must comply with the regulations. All electrical items within the bathroom must be linked together within the bathroom with a 4mm² earth cable along with pipe work and any exposed metal materials that could become live if there is a fault. With regard to exposed metal materials don't go mad e.g. Grab rails are not likely to become live. As a guide any metal item that could be reasonably be believed to have continuity to earth should be connected to the supplementary earthing.
  10. If there is an existing shower that is going to be replaced or moved then it will probably be found that the circuit is already live. It must therefore be established where the main fuse for it is located in order the circuit can be safely isolated. It is NOT sufficient to just switch it off at the pull cord switch. (Refer to Safe Isolation)
  11. If consideration is being given to using the existing cable to an existing shower, then it must be established that:
    a) The cable is of a suitable size that it can carry the proposed new load.
    b) The cable is in good order and not damaged either by mechanical damage or by over heating.
    c) There are no joints or junction boxes in the cable that are not capable of carrying the new load and that any joints comply with the regulations and are in an enclosure.
    d) Any accessories, switches etc are correctly rated for the proposed modification to the circuit and they are in good working order.
    e) Any switches are suitable located especially within the bathroom taking into consideration the Zoning of the bathroom.

Preventing danger to yourself and others around you is paramount. If you do not feel confident to carry out the work, THEN DON'T DO IT. Ensure that you practice safe isolation, other people understand where it is safe to move, barriers or obstacles are in place to protect live circuits.

 

Calculation

This is the most important bit as it verifies that what is going to be used is suitable. The circuit for the shower will consist an 8.7kW shower @ 240v. The run of cable will be 25m and most of the cable run will be in trunking grouped with 3 other cables touching, 6m will be touching loft insulation on one side. It will also be going through thermal insulation for 100mm. The ambient temperature in any part of the circuit will be 30ºC in the loft. The proposed cable is PVC/PVC or more commonly known as Twin & Earth. The shower manufacturer has said use 6mm² cable. Care should be taken in taking a manufacturers recommendation as it does not cover ALL installation methods.

First we must calculate the Design Current.

Applying equation, P=I x V for 8.7kW @ 240v, you get the following:

 

Therefore if the voltage is 240 the shower will draw a current of 36.25 Amps.
Therefore the Design Current is 36.25 A or Ib

We can now decide on the size of fuse. The fuse is referred to as In and the next statement must apply.

 

The next size to cater for the design current is 40A. As there is no heavy starting current a type B fuse is suitable.

The next think to look at the current capacity of the cable which is known as It. We must look at the worst part of the installation which is the fact the cable is touching the loft insulation. This makes the installation Method 4 applicable. The regulations table 4D2A tells us that a cable size of 6mm² touching insulation on one side is only capable of carrying a current of 34A. Therefore the manufacturers recommendation of 6mm² is not adequate. Looking at the next size up of 10mm² Twin & Earth table 4D2A tells us that enclosed in conduit the cable can carry 43A which appears to be adequate.

The regulations permit a maximum volt drop of 4% so we must check that our selected cable is still compliant. Checking table 4D2B in the regulations we find that for the selected cable there is a volt drop of 4.4 mV/A/m. The following calculation will now check the cable is still suitable.

 

Therefore the calculated volt drop of 3.9875 is less than the permitted maximum of 9.6 volts so the cable still complies.

You might think this is the end of it but there is more. We must now look at the grouping of cables, thermal insulation, ambient temperature and the type of fuse. This is done by the following formula:

 

Where

It = Current capacity of the cable = 43 Amps
In= The fuse or over current device rating = 40 Amps
Ca = The correction factor for ambient temperature (Table 4C1) = 1
Ci = The correction factor for thermal insulation (Table 52A) = 0.81
Cg= The correction factor for grouping (Table 4B1) = 0.65
Cr = The correction factor 0.725 for semi enclosed fuses (Re-wirable) to BS3036 we are using an MCB so it will = 1

 

This calculation has proved the we have chosen the wrong cable for the route that we are taking. We would need, with reference to 4D2A, a cable of 35mm².

We need to rethink how the cable is going to be run. Instead of allowing the cable to touch the loft insulation, but clip it out of the way in the loft, the installation method has changed to Method 3, we can increase the current carrying capacity to 52 A. We can also keep the circuit away from other cables. The re-calculation with reference to the relevant tables is as follows:

 

So by changing a few things a sensibly sized cable can now be used.

So to recap we have decided that the

Design Current is 36.25 Amps
Fuse size 40 Amps and of Type B
Cable size will be 10mm² Twin & Earth
Installation Method 3