Installing an electric shower

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Introduction

This article describes the electrical aspects of installing an instantaneous electric shower. This is the type of unit that takes a single mains cold water feed, and heats the water as it passes through the shower unit "on demand". For the sake of clarity, this does not include shower units that simply boost the pressure of existing hot and cold water supplies.

This is a deceptively complex task, and should not be attempted unless you fully understand the implications of the design decisions you will need to make, and are confident that you can produce a sufficiently high standard of workmanship.

Selecting the Power

The performance of an electric shower will be dictated by its power output, and since there is very little heat wasted in the shower itself, this is largely the same as the electrical power input. The advantage of the more powerful showers is they are able to deliver a greater volume of hot water in a given time. All showers ought to be able to provide a shower that is "hot enough" - however the less power you have available, the less water can be heated to an adequate temperature in a given time. Delivery rates are usually measured in Litres Per Minute (LPM). The table below gives you some idea of what to expect from the commonly available shower powers.

Note that even the most powerful shower will still only deliver a relatively modest flow of warm water - especially in the depths of winter when the incoming mains water is particularly cold.

Assuming a shower temperature of 40°C, you will get the following best case flow rates:

Shower input Power (W) Flow rate (LPM)
at 5°C inlet temperature
Flow rate (LPM)
at 15°C inlet temperature
7500 3.06 4.29
8000 3.27 4.57
8500 3.47 4.86
9000 3.67 5.14
9500 3.88 5.43
10000 4.08 5.71
10500 4.29 6.00
11000 4.49 6.29
For comparison, a 24kW
Gas fired Combi boiler
9.80 13.71

Design and Selection

Electric showers are one of the highest power loads to be found in most domestic properties, and they may also be run for an extended time. This places a high demand on the electrical installation, and hence it must be designed and implemented correctly or there is a serious danger of undesirable consequences such as cables overheating and the danger of fires. That is before you add water to the mix!

Where to take the power from

Electric showers must be supplied from their own dedicated circuit. Under no circumstances can the power be taken from a normal socket circuit, or shared with any other device. This may pose a problem if the existing consumer unit does not have a spare way to accommodate a new circuit. In these circumstance consider changing the CU, or installing an additional CU in the way described here.

Choosing the components

Step one: Find out the design current Most showers will include this in their technical details. If not, take the input power and divide by 230. So a 9500W shower will draw 9500 / 230 = 41.3A (note many showers actually quote their full output power at 240V, and will deliver somewhat less at 230V, but this is the nominal value used for design purposes).

Step two: Select a circuit breaker The shower will need a circuit or fuse of the appropriate rating equal or greater than the design current. This will normally mean a 40, 45, or 50A breaker. (keep in mind that the more powerful showers will take a significant chunk of the available supply capacity to your house - especially of you only have a 60A supply. Some properties may have a supply even smaller than this. That can impose a practical limit on the size of shower to the very smallest.

Step three: Select cable You will need to chose a cable of sufficient size to carry the design current. All but the lowest powered showers will require at least 6mm² T&E. 10mm² will commonly be required for the higher powered ones. See the article on calculating cables sizes for the full details of how to do this correctly.

Step four: Select other parts The only other component in a typical shower circuit is the isolation switch. This will typically be a ceiling mounted, pull cord operated, high current switch designed for the purpose. The switch needs to have a mechanical flag or some form to indicate when it in is the on position (most use a protruding tab, or colour patch to indicate the on state), and optionally, an illuminated indication.

RCD Protection

The 17th edition of the wiring regulations now mandate the use of RCD protection for electric shower circuits. Note however that prior to the 17th edition it was possible to design a shower circuit to offer adequate protection even without a RCD in many cases. Refer to the calculating cables sizes article for full details, however don't assume that not having an RCD on a shower makes it in any way unsafe.

Supplementary equipotential bonding

Under the 17th edition of the wiring regs, it is permissible to avoid the need for supplementary equipotential bonding so long as the main equipotential bonding is present and to current standards, and all circuits that enter the shower room and protected by RCD(s) with a trip threshold not exceeding 30mA.


Installation

Electric showers will need the mains water and electricity taken to the same physical location in the shower enclosure. It is vital that only electrical equipment designed specifically for installation into this location (i.e. Zone 1) be used here. Most shower units obviously will be, although there are a few more exotic models where the actual water heater unit is separate from the control panel. With these follow the manufacturers instructions for safe location of the heater unit and its high current connections.

Install and fix the shower unit as per instructions. Make sure the plumbing is water tight before going any further. Note that until power is applied, it may not be possible to actually turn the shower on - even to run cold water. Power will typically need to run straight up or down the wall concealed in a chase or a stud wall, and emerge in the correct place for the shower. Not all showers take their power feed in the same place, so it is worth buying the shower and studying the instructions before running the cable! Obviously care is needed if selecting a replacement shower.

Install the appropriate circuit breaker at the CU, and run the cable to the isolation switch position. Observe the bathroom zones and make sure the switch is located in Zone 2 or outside the zones. Note also it should not be possible to reach the switch while standing in shower enclosure / bath.

Continue the cabling from the switch position to the shower. Because of the very high currents carried by electric showers, it is important that terminations are made well. Take care when stripping insulation to not damage the conductors in the cable, you must retain all strands of each wire. Screw terminal connections need to be very tight. This means using an adequate size of screwdriver that fits the terminal well, and allows enough purchase to create a nice low resistance and gas tight termination. Careful preparation of cable lengths, and bending of cables into appropriate positions is also important to enable the switch etc to be reassembled easily. This is particularly important if using 10mm² which is particularly difficult to work with. Take great care if using pliers etc to bend and shape wires to not damage or nick the insulation while doing it.

Testing

Once everything is wired up it is important to test it before use. Start with a visual inspection, and make sure all terminations are to the correct place. Check each screw connection is tight. With the MCB for the shower "off", and the shower isolation switch "on", carry out an insulation resistance check between Line and Earth, and between Neutral and Earth. (Note if your shower switch has a neon indicator, this will cause you to get an extraneous reading if you attempt to conduct a N to L insulation resistance test). The minimum required insulation resistance is 2 MOhm, however on a new installation of such a simple circuit, one would expect significantly more than this (and typically outside the range of most test meters to measure).

Next conduct a low ohms continuity test. Again with the MCB off, create a temporary bridge from the output of the MCB in the CU to the earth bus bar, and measure the resistance between L & E at the shower. Check the actual reading is close to that predicted in the design stage by careful reference to the expected cable resistance calculations. Next move the bridge in the CU to connect the output of the MCB to the neutral bus bar, and now measure the N to L resistance at the shower. This should be similar to, and a little lower than the L to E reading done previously. Now remove the bridge from the CU.

Finally conduct an RCD test, using a RCD tester at the shower position verify that the RCD operates at the expected trip thresholds, and also within a acceptable time.

Replace all terminal covers, and fit the shower enclosure. Pay particular attention to any sealing rings, or washers etc that are supplied.

Commissioning

Follow the manufacturers instructions for commissioning and first use of the shower. If none are supplied then proceed as follows: You can now turn the shower on at its coldest setting and check that water flows correctly from it. Make sure all air is expelled from the shower at this stage and the water runs smoothly. Turn on the shower heat (some have discrete heat settings - some are just on / off). And check the water is heater correctly. You should be able to vary the temperature by adjusting the flow rate (where less flow gives hotter water). Try each of the heat settings. Let the shower run for a couple of minutes and then shut off the shower. Finally check that the switch enclosure does not feel excessively hot (it may be warm), and you can't smell any melting plastics or other smells that might indicate overheating of connections.

Checking power consumption

If you wish to verify the actual power consumption of the shower, this can be done with a clamp meter clamped around either the circuit wire where it exists the MCB in the CU, or on a meter tail prior to entry to the CU (in the latter case you will need to look at the difference between current drawn with and without the shower). Depending on your actual mains voltage at the time of the reading, you may see a figure somewhat different from that expected - makers usually quote the power at 240V, so a swing of only 10V will show a 400W change in actual power consumption.

Living with an electric shower

There are some features of electric showers that can take some getting used to. These include:

Lights dimming

Due to the high load imposed by a shower, it is not unexpected to see a dip in the brightness of the house lights when it is turned on. The further you are from the substation, the more noticeable this might be.

Low flow rates

The actual flow rate of the shower can be less that one expects. Particularly if you are used to traditional showers fed from pre heated hot water suppliers, or powerful combi boilers. This will be more noticeable during the winter when the incoming water is colder and hence needs more heating. It is also important to understand that the temperature control of most showers is achieved by varying the water flow rate, and not controlling the electrical power (other than in large steps on some showers with multiple heat settings). The implication of this is that you may not be able to have the full flow rate available, at the temperature you would like.

See Also