Warning: To complete electrical works you must comply with Electrical Regulations – Click here for more information.
Safety is paramount when dealing with any form of DIY, but never more so than when dealing with electricity. If you are unsure about anything, or not confident you can complete the job safely, always consult a Professional Electrician, to not do so could be risking your life and that of your family.
Regulations now insist that all domestic DIY electrical work is checked by a qualified electrician and a minor works certificate issued. Failure to do this could render your house insurance invalid and make selling your home very difficult.
Please see our project here PART P Building Regulations
All the advice contained here is in accordance with BS7671: 2002 The IEE Wiring Regulations, and if followed will ensure a safe installation that will last many years.
Before you even decide you want an electric shower, you should check your consumer unit (fuse box) and the size of the supply coming in to your property.
Large showers use a lot of power (load), and this can cause issues on older installations that may have consumer units that are not capable of handling this extra load.
As a rule of thumb, if you have a 60A or 63A main switch, then your supply fuse will likely be 40A or 60A, if it is 80A or 100A the supply fuse will likely be 60A, 80A or 100A, quite often a phone call to your supply company can resolve this.
NEVER break the seal on your incoming fuse holder to look at the fuse, this is illegal and could lead to prosecution.
Common Shower Ratings and Protective Devices
A common question that’s asked is: What size cable to use for an electric shower? This will depend on the shower rating in kW and how much it draws, but a very rough guide is from 6kW to 7kW 6mm2 cable should be used, from 7.5kW to 10kW 10mm2 cable should be used and from 10.5kW to 11kW 16mm2 should be used.
Ths is a very rough guide however and table 1 below gives full information and figures for common shower ratings and the protective device and cable required to install them safely.
|Standard Shower Rating Watts (kW)||Maximum load from the supply Amps||Maximum size of Protective device||Recommended Minimum Cable Size to be Used||Recommended Protective Device|
All data in the table above has been compiled with reference to BS7671: 2001. The IEE Wiring Regulations. Cable Data taken from Table 4D2A. Page 222.
Note on Table 1
The above table gives worst case scenarios, no account has been taken for volt-drop or diversity. Applying diversity to a shower supply is not recommended unless the shower to be installed is fully thermostatic in operation.
What Size Cable and Fuse to use for an Electric Shower
If the shower only has two power settings, then no diversity should be factored into the load calculation. This is because the shower will work on 50% or 100% of full load, the temperature of the water being regulated by the flow rate through the shower heater unit.
When doing a calculation for the shower circuit it is wise to consider volt-drop and other factors when considering the size of cable to be used. If the shower is a fully thermostatically controlled unit, then a diversity factor may be assumed of not less that 80%
Wherever possible showers should always be protected by an RCBO, this is a device that does the job of both an MCB, to provide overload protection and short circuit protection, and an RCD to give Earth Fault protection.
Showers should not, ideally, be protected by Semi-Enclosed Rewirable fuses, if this means fitting an additional enclosure beside your main consumer unit, then this is the best way to proceed. Fuses should only be used as an absolute last resort.
Fuses take higher fault currents to cause them to “blow” than either MCB’s or RCBO’s, up to 1.8 times the rating of the fusewire in the holder in some cases. Water and Electricity are a dangerous mix, the current drawn by even a small shower will likely prove fatal in the event of something going wrong or it being installed incorrectly.
Electric showers should always be installed using an RCD or an RCBO. The RCBO is the better option as this gives overload, short circuit and Earth fault protection.
Please note that some shower manufacturers demand the installation of an RCD/RCBO, if this is the case, failure to do so could invalidate your warranty so it is advisable to follow this demand.
What Size Electric Shower can I Install?
When you know the size of the supply you have from your supplier, you can use the above table (table 1) as a reference guide to see the maximum size of shower your installation can support.
To determine this you will need to take into account all of the electrical appliances that may be running when the shower is in use. Common heavy load items in modern homes are Washing Machines, Tumble Dryers, Dishwashers, Combined Cookers, Electric Hobs, Electric Ovens, kettles and the good old Immersion heater.
One thing to bear in mind at this juncture is that the main isolator of a consumer unit is also a limiting factor in the size of shower you can install, regardless of the fuse in your supply. The Table below is equally accurate as a guide for this thorny problem as it is for the size of the main fuse.
You need to ensure that the load drawn by the shower when in use will not cause your supply to be overloaded. For reference see table 2 below.
|Maximum Supply Fuse and/or CU Isolator rating||Common Circuits Example only – Not definitive||Recommended Maximum Shower Rating|
|40A||1 Ring Main, 1 or 2 Lights, 1 Boiler||Not Recommended|
|60A||2 Ring Mains. 2 Lights. 1 Boiler||7kW|
|80A||1 Cooker. 2 Ring Mains. 2 Lights. 1 Boiler||9.5kW|
|100A||1 Cooker. 2 Ring Mains. 2 Lights. 1 Boiler||11kW|
The above should only be used as a guide, individual installations vary considerably, and to list all possible scenarios would take more space than is available or practical.
Installing an Electric Shower
OK, so you know the size of shower you want, and you know the size you can have, the problem now is in installing it safely.
Remember, all large loads should be placed as near to the main isolator of a consumer unit as is possible, therefore you are advised to move all circuit breakers along one place to ensure that the circuit breaker for the shower is either in place adjacent to similar rated breakers, or if it is the largest, nearest to the isolator.
Should you find yourself in a position where you must use fuses to protect the shower, the same advice still applies.
One important thing to remember is that some manufacturers state that certain consumer units they produce will only support the use of one MCB rated higher than 32A, this will be true for any consumer unit with either a 63A or 80A main Isolator or RCD.
It may be possible to upgrade this Isolator to 100A, however you should seek advice from a professional electrician on this as your supply may not support this higher load You will need to have your supply disconnected temporarily in order to change the Isolator or RCD, this is not a DIY job undertake this disconnection as it is Illegal to break the seals on Meters and Main Fuses, so can only be done by a professional.
Further, your existing consumer unit may not be suitable for a higher rated Isolator or RCD.
Running an Electrical Supply Cable for an Electric Shower
When deciding the route your cable will take to reach your shower, ensure that you take the shortest route wherever possible and practical. This is because the current drawn through the cable will cause a loss in voltage (volt drop) over extended lengths of cable.
The higher the current load, the greater the volt drop. The regulations say this must not exceed 4% (=9.2V (AC) ) of the nominal supply voltage of 230V (AC) over the length of the circuit. Normally in domestic situations this is not an issue but must be bourne in mind.
Never route cables near sources of heat, such as central heating pipes or hot water pipes, the extra heat could damage the cable but also impede its ability to carry the full load of the shower without suffering damage.
Higher temperatures reduce the current carrying capabilities of cables. Try not to route cables through insulation, and NEVER if the insulation is Polystyrene, this will damage PVC insulated cable.
You may find that you need to drill holes through joists to get from the consumer unit to the shower, if this is the case, ensure that the holes you drill are in the middle of the joist and are large enough to safely route the cable through. See our project here on notching joists for more information.
20mm diameter holes are adequate for 6mm2 cable, but for 10mm2 you will need 25mm diameter holes, and for 16mm2 cables, a whopping 32mm. Be careful when drilling holes of this size, ensure there is at least 50% of the joist still intact (25% above and below the hole) as you could damage the structural abilities of the joist.
Never drill a hole in a joist within 70mm of the end of the joist, you may damage the structural abilities of the Joist to do its’ job. See diagram below.
Try not to twist the cable when installing it, this will not reduce its electrical properties, but it will make it harder to install and more likely to be damaged.
If the cable is to be buried in a wall at any point of the circuit, most likely in the shower room, always cover it over with METAL capping.
This metal capping should be carefully, but firmly secured to the wall to ensure the cable is protected before you plaster over it and then install the shower cubicle etc.
It is not recommended that you use plastic capping for this type of supply. When fixing the capping, be VERY careful not to damage the cable underneath. Lack of care here could be dangerous, but most certainly costly.
Connecting and Wiring an Electric Shower
When you terminate the cable into the shower, ensure you use the cable clamp provided in the shower to firmly secure it, but ensure it is the outer sheath you secure, not the inner cores. These plastic clamps are reversible depending upon which size cable you are using. It may be a problem if you have to use 16mm2 cable, as the clamp may not be large enough to support this size of cable.
When exposing the inner cores you should only expose the minimum necessary for safe and secure connections. No copper should be visible outside of the connection block.
The connections should be tight, not so tight as to damage the cable cores, but tight enough to ensure electrical arcing does not occur.
The bare conductor in the cable is designed for use as the circuit protective conductor (CPC) or Earth, this must be sleeved using Green/Yellow sleeving to ensure it is clearly identified.
Using this same terminal in the shower, a short length of 4mm2 Green/Yellow PVC insulated cable should be connected to the water pipe supplying the shower. This connection should only be made using a proprietary EC14 Earth clamp manufactured to BS951. The cable should be terminated to the clamp using an insulated crimp lug.
Depending on the layout of the shower “guts”, it may be advisable to sleeve the inner cores of the supply cable with heat proof over sleeving of the appropriate colour for that core.
Some showers may have the electrical connections close to the boiler of the shower, and the heat this produces may cause damage to the inner insulation of the cable over a period of time that could later result in an electrical fault.
Fitting a Local Isolation Switch
Remember, showers are required to have local switches/isolators. These must be manufactured to relevant British Standards and be located so as to prevent a person using the shower from operating the switch.
Similarly, it should be located so as to prevent it from being unduly exposed to moisture. The switch can be a ceiling mounted pull cord, of the correct current rating, or a wall mounted unit.
Regardless of type, it must incorporate a neon indicator that illuminates when the supply to the shower is switched on.
Wall mounted switches are easier to terminate larger cables in, but ceiling mounted are OK for cables up to 6mm2 easily, 10mm2 with care. For showers rated at 10.5kW and above a switch capable of handling loads up to 50A is required, such a switch is the MK 3164WHI, this is a pullcord switch however.
If you use wall mounted switches, the rocker type, then these should not be mounted inside the room containing the shower but on an exterior wall to the room, unless it is possible to mount the switch at least 2.5 meters from the shower.
Care should be taken when mounting these switches, they need to be secure so ensure that you can get a secure fixing. One trick is to cut a piece of 18mm plywood or MDF large enough to fit the space between your joists, whether in the loft or floor space, this should be about 300mm (12”) long.
In the middle of the switch drill a hole large enough to pass both cables through. Place the wood over the area you wish to fix the switch, making sure to line the holes in the wood and ceiling up. When the wood is in place, use four 50mm (2”) nails to hold the wood in place by hammering them in sideways to the joists tightly against the wood.
This will stop the wood moving when you screw the switch base to it from below. This will give you a firm and secure mounting for the switch.
Connecting to the Consumer Unit
When terminating the cable into the consumer unit, always turn the power off using the main switch, never work on a consumer unit with the power switched on.
The same care should be taken in the consumer unit, ensure that the cable is firmly secured to prevent it accidentally being pulled out of the consumer unit. Then strip the outer sheath back, but make sure about 25mm of the outer white or grey sheath projects into the consumer unit, this can be taped up using (ideally) white PVC insulation tape, although other colours of insulation tape are available and equally safe for this use. It is wise to ensure that the green/yellow sleeving is on the bare core before you tape the end of the outer insulation, this will prevent it moving and exposing bare copper.
Again, only expose enough to make a neat job of terminating the cores securely, and only expose enough copper to enter the terminal without any showing outside it.
Check to ensure these terminations are secure, and then replace the cover of the consumer unit and turn on the main switch to restore power to all other circuits.
Testing an Electric Shower
Before turning on the power to your shower, ensure that all electrical connection are secure, the cover has been replaced on the shower unit and the local switch/isolator.
At this point it is prudent to ensure that you have turned on the water to the shower and that the shower cylinder is full, if it is not and you turn the shower on, you may burn out the heating element or cause other damage to the shower.
If you are sure all of the above has been followed to the letter, turn the power on to test.
It would be a wise precaution to check all the pipework for cross-bonding when you install the shower. All copper pipes should be cross-linked using EC14 Earth clamps, as described above, and lengths of 4mm2 green/yellow earth cable using crimp lugs on the ends.
There should be a main Earth bond linking your consumer unit earth terminal or the main earth terminal of the supply to both your Gas and water supplies, these bonds should be within 600mm of the consumer side of meter or stopcock. The cable used should be no smaller than 10mm2 Green/Yellow earth cable, in some cases they may be 16mm2
Cable Ratings for Shower Installation and Reference Methods
Regular reader, Oliver made a great point which is worth mentioning:
One of the main issues with installing a shower is that the route to a shower isolator and shower is often through the loft which is commonly insulated up to 250mm thick. Any competent electrician would take this into account as this would de-rate the cable by 50%. So even 16mm2 cable should be de-rated to 8mm2 equivalent according to the regulations.
We cover this is more detail in our project all about cable sizes and amp ratings.
It is actually quite a complex issue that a qualified electrician will be experienced in dealing with, which is why it is only them that are allowed to this kind on notifiable work. Essentially as a DIY enthusiast you need to know that where the cable is routed will affect the rating that you need to use.
There are tables, which we summarise on our cable size page (linked to above), in the regulations which can help a qualified electrician ensure that they have the right cable for where is it routed – this is referred to as the Reference Method.
Remember, stay safe get your installation checked. This is a legal requirement.
For regulations governing heights of sockets etc, please see our Electrical Socket Height project