Advice on extending shower cable

arrow said:
I would run a 10mm cable from the new switch position to the shower. If in the future you put in a bigger shower you will only need to rewire to the switch.
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I can see your point. However, there are other considerations required. A guesstimate doesn't always work.

For instance, 10500 watts @ 230 volts = 45.6 amps. Using Ref Method C, a 6mm T&E cable is big enough.

or

10500 watts @ 230 volts = 45.6 amps. Using Ref Method 103#, a 16mm T&E is not big enough.

All this information comes from Appendix 4 which is informative only, not a regulation. A nice mix of engineering judgement (ie is the shower going to be continually in use) with a dash of Appendix 4 will give the correct answer.

With this in mind, a 6mm cable will be big enough for a lot of installations concerning 10 kw or even 10.5 kw showers with a 45 amp breaker. Obviously, this will depend on the supply voltage.
6mm is also okay, volt drop wise, for a 25+ metre cable run @ 45 amps.
 
I did exactly this a few month ago. Bathroom fitter put in a small electric shower because it’s all we could do. I installed 10mm to the switch but then connected to existing 6mm from switch to CU. Customer said he might upgrade later when decorating different room on the path back to the CU so I future proofed it. Over specified cable isn’t always a bad thing.
 
WH55 said:
Over specified cable isn’t always a bad thing
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I agree. But only when realistically needed at a later date. As long as a 6mm cable is buried in plaster and/or clipped, it is fine to supply a 10.5 kw shower. Although the cable run should be less than 30 metres in length to satisfy voltage drop and max allowed Zs.

That's not an opinion, it's just counting (thank you James O'brien -LBC presenter- for that little quote)
 
malbec4 said:
I can see your point. However, there are other considerations required. A guesstimate doesn't always work.

For instance, 10500 watts @ 230 volts = 45.6 amps. Using Ref Method C, a 6mm T&E cable is big enough.

or

10500 watts @ 230 volts = 45.6 amps. Using Ref Method 103#, a 16mm T&E is not big enough.

All this information comes from Appendix 4 which is informative only, not a regulation. A nice mix of engineering judgement (ie is the shower going to be continually in use) with a dash of Appendix 4 will give the correct answer.

With this in mind, a 6mm cable will be big enough for a lot of installations concerning 10 kw or even 10.5 kw showers with a 45 amp breaker. Obviously, this will depend on the supply voltage.
6mm is also okay, volt drop wise, for a 25+ metre cable run @ 45 amps.
Click to expand...

10.5kW will be te rating at 240v - thus 43.75A Showers normally have a headline power based on 240V

Restistance of heater - 240*240/10500 = 5.4857 ohms

At 230v 230*230/5.4857 = 9640W and 41.9 A


But if voltage strays up to 250v current will also push up to 46A
 
malbec4 said:
With this in mind, a 6mm cable will be big enough for a lot of installations concerning 10 kw or even 10.5 kw showers with a 45 amp breaker. Obviously, this will depend on the supply voltage.
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As you say 6mm will be big enough for some installations, however not all. Put it in some trunking and the capacity drops down. I prefer to run 10mm and I am covered.
 
adgjl said:
I’ve been waiting for the suggestion that a shower only needs 4mm cable for ages! What a load of “carp”!
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Who suggested that? I said a 32a MCB only needs 4mm cable. The purpose of the MCB is to protect the cable.

But having said that a 6kW shower would be OK on 4mm cable.
 
Tom Bartonn said:
Hi,

I recently moved my electric shower unit to another wall during a bathroom renovation. I initially extended the cable with 6mm T+E cable, connected with a 32A push-fit connector.

I want to improve the quality here, as we will need an electrical certificate.

I'm planning on crimping the connection with 6mm butt joints, and putting that in a waterproof box within the wall behind the shower. This was on recommendation from our electrician doing the certification. I had a couple of specific questions on this, to make sure I do a decent job:

- What IP rating would be best? I see a few boxes with IP55.
- Would you use cable glands on the sides for the two wires entering the box, or is this more for outdoor applications? I could silicon the holes instead.
- It might be a silly question, but some boxes I see don't have screw holes that are separated from the waterproof compartment. I would be compromising the waterproofing if I screw through it, so how would you affix the box to the wall?

Would appreciate any opinions on this Teka HBB 435 User Guide online. This won't be my last bathroom renovation, so I want to make sure I learn all I can this time.
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I have shower cabling running through my garage but need to extend the cable. Can the cable be extended using junction boxes or must the whole cable be replaced with a longer length from fuse box to shower unit
 
michaelwilliams said:
I have shower cabling running through my garage but need to extend the cable. Can the cable be extended using junction boxes or must the whole cable be replaced with a longer length from fuse box to shower unit
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It’s always better to replace a full length of cable if possible. That said, you can get junction boxes that are built for 60amp etc. If the join is in the garage and, therefore, visible and easy to check, a junction box may be suitable.

EDIT - doesn’t look like our hosts sell them but I know other suppliers do.
 
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malbec4 said:
I never knew that! I thought everything was based on 230 volts.

Thanks for the insight. Does this only apply to showers?
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Just had a look at the link @The Happy Builder provider in post #27 and on page 2.

SPECIFICATIONS
Electrical
Nominal power Nominal power
rating at 240V rating at 230V
8.5kW – (40A MCB rating) 7.8kW – (40A MCB rating)
9.5kW – (40A MCB rating) 8.7kW – (40A MCB rating)
10.5kW – (45A MCB rating) 9.6kW – (45A MCB rating

Don't know about other appliances, but certainly a lot of showers use the headline power based on 240v.
 
FlyByNight said:
Just had a look at the link @The Happy Builder provider in post #27 and on page 2.

SPECIFICATIONS
Electrical
Nominal power Nominal power
rating at 240V rating at 230V
8.5kW – (40A MCB rating) 7.8kW – (40A MCB rating)
9.5kW – (40A MCB rating) 8.7kW – (40A MCB rating)
10.5kW – (45A MCB rating) 9.6kW – (45A MCB rating

Don't know about other appliances, but certainly a lot of showers use the headline power based on 240v.
Click to expand...
Makes sense. That is the voltage most people get in mainland UK. Also the numbers look bigger so more attractive.
 
FlyByNight said:
Just had a look at the link @The Happy Builder provider in post #27 and on page 2.

SPECIFICATIONS
Electrical
Nominal power Nominal power
rating at 240V rating at 230V
8.5kW – (40A MCB rating) 7.8kW – (40A MCB rating)
9.5kW – (40A MCB rating) 8.7kW – (40A MCB rating)
10.5kW – (45A MCB rating) 9.6kW – (45A MCB rating

Don't know about other appliances, but certainly a lot of showers use the headline power based on 240v.
Click to expand...
Exactly. The manufacturers instructions tell you that for your 7.5kW shower, the minimum cable size is 6mm, that a 30mA RCD is required in the circuit, and that a double pole isolator is required. The solution to extending the cable seems to be the suggested one of placing an isolating switch just outside the bathroom door.
 
As the regs don’t require an isolator the MI are wrong to say one is required. That’s not the only thing wrong either. Thet talk about IEE wiring regs. Now how long is it the IEE ceased to exist?

If the MI are wrong we take account of that and don’t follow them.
 
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