Installing a Shower – Electrically !

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Wiring a Shower.

Perhaps one of the easiest circuits to install is the one that feeds a shower. Although it’s just a simple radial circuit there are a few pit holes you’ll need to avoid. We are going to assume that we are dealing with a domestic installation and as such Volt Drop will not be an issue, unless the shower is on the 3rd floor over on the east wing.

Size of cable required?

More than likely it will be a 10kW shower these days and it will use 10000/230 = 44A. So, we’ll need a cable that can carry 44A or more.

Looking at Table 4D5 BS 7671 we see that if we have the cable clipped direct (ref method C) we could use a 6mm sq twin and earth as it will carry up to 47A

However, it’s not a simple as that and it’s also highly unlikely that it’s going to be clipped directly throughout the building and we must allow for that.

Taking the scenic route and avoiding too much thermal insulation we’ll settle for reference method 100 and this will give us 45A current carrying capacity for a 10mm sq twin and earth.

Size of circuit breaker?

Have a look at Table 41.3 in BS761, not for the maximum Zs values for these circuits but for the size of circuit breakers commonly available.

You’ll notice that there is a 40A circuit breaker but, that would be too small as every time the shower was switched on it would trip. The next one shown is a 50A circuit breaker, but again this is no good as it is higher than the current carrying capacity of the 10mm sq cable (45A).

We can do one of two things.

Put a bigger cable in with a 50A circuit breaker – 16mm will carry up to 57A ref method 100. Good luck getting that in the pull switch though. Or…

Use a 45A circuit breaker! This will protect the cable against overload and they are available just not listed in BS7671. They are listed in the On-Site Guide, Table B6 along with the maximum Zs (80%) value.

So, it’s 10 mm sq cable and a 45A Type B BSEN 60898 circuit breaker.

But there’s more. According to Part 7 Section 701 BS7671 (701.411.3.3) we are also going to need Additional Protection using a 30mA RCD.

Other things to consider…

  • Is the supply big enough for the additional load?
  • Will you need to put a separate consumer unit (shower unit) at the distribution board or will it fit into the existing one?
  • Is all of the earthing and bonding satisfactory?
  • Position of the pull switch – at least 600mm away from the edge of the bath or shower cubical if the ceiling is less than 2.25m.

Installing a shower can easily escalate into a ‘big job’ especially if you need to add an additional consumer unit and upgrade all of the earthing and bonding.

Plus don’t forget to inspect and test and it’ll need a Electrical Installation Certificate and also ‘notifying ‘ under Part P as well as it’s a new circuit and also because it’s in the bathroom.

See my other blog posts for max Zs value, types of circuit breakers, additional protection, earthing and bonding etc and also BS7671 for more information if in doubt.

There’s also more information regarding training Part P of the Building Regulations over at our new City & Guilds Part P course website.

 

 

 

 

 

 

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