# Replacement Cable for Electric Shower

Discussion in 'Electrics UK' started by imroberts, 6 Oct 2020.

1. ### imroberts

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I'm just planning various electrical jobs around the house and I'm contemplating installing a replacement shower cable to my downstairs bathroom while I have all the floors etc up.

My current shower is only 8.5kW but if I'm replacing the cable I'd like to be able to accommodate 9.5kW or 10.5kW when I come to replace the shower.

I can get a cable from the consumer unit to the shower with around 17.5m of cable, but let's say 20m to allow a bit of leeway. Some of the route will be through the garage loft space which is uninsulated, part under a bedroom floor upstairs, and the last 2-3m will be through the loft space in a single storey extension. This loft space is insulated however the cable could easily be run on top of the insulation or even clipped to a brick wall around two sides of the loft space.

While I'm at it I'm planning to install a 10mm cable as the cost difference over 6mm is minimal and it makes sense to be over-rated rather than borderline, however from an academic point of view I'd like to confirm my understanding and thought process.

1) Am I right in assuming the cable could be sized using reference method C so long as I'm not running any length of it under insulation? Is it acceptable for the last few metres to be simply placed loosely over insulation or would it have to be clipped to the wall to satisfy this method? Assuming reference method C that would give me a current carrying capacity of 47A for 6mm?

2) 10.5kW at 230V is 45.6A so while close to the limit, would be within the above limit of 47A and so considered acceptable?

3) Voltage drop for this setup would be 7.3mV x 45.6A x 20m = 6.66V = 2.9% so within the limit of 5% allowable voltage drop?

4) Assuming all the above is acceptable from a mathematical perspective, I'm guessing in reality I couldn't install a 10.5kW shower on a 6mm cable anyway. I'm assuming it would be unacceptable to run a 45.6A load on a 45A MCB even though the MCB wouldn't trip at that current (?) and increasing the MCB to 50A would mean the cable was insufficiently protected?

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So... why not future proof & install 10mm2 ?

4. ### EFLImpudence

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The 10.5kW is probably quoted at 240V so is actually 43.75A.

At 230V it will be 9.6kW and 42A - not reaching 45A until 247V when 11.1kW

5. ### JohnW2

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IF the load were 45.6A that would, strictly speaking, be correct (although .... ). However ....
If you look at a 10.5kW shower, you will almost certainly find that the quoted 10.5kW relates to use at 240V (makes it sound 'bigger and better' ), but the required calculations are done on the basis of 230V.

If a shower is rated at 10.5kW at 240V, at 230V it will represent a load of about 41.9A at 230V - hence well within the capabilities of a 45A MCB.

Kind Regards, John
Edit: typed too slowly again!
Edit2: It wasn't slow typing this time - I somehow missed seeing EFLI's post, 2 or 3 hours ago!!

6. ### Taylortwocities

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Not your fault, he lives in a different time zone!

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7. ### winston1

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Out of interest what size is the existing shower cable?

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8. ### SUNRAY

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heaaaaaay maaaaaan.

9. ### imroberts

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That's certainly interesting and not something I'd considered!

11. ### imroberts

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The existing cable is a 6mm.

12. ### EFLImpudence

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It will be fine as long as it is not run under or through any thermal insulation.

Incidentally, the volt drop 5% is not a mandatory regulation; just a consideration.
It is calculated for the conductors at 70°C so will, at least for a while, be slightly less than your figure of 2.9%.

13. ### winston1

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In that case you don’t need to replace it then.

14. ### JohnW2

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Ah, I forgot that

Kind Regards, John

15. ### EFLImpudence

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'He' doesn't actually - but I did post it a couple of hours before I wrote it.

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16. ### imroberts

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At one end of the run the cable will have to pass vertically through the loft insulation - so for a length of 20cm of so - would that class as "through" insulation or is it insignificant enough to ignore?

Given I need to re-route the cable, wouldn't it make more sense to replace it with a 10mm anyway, or is it completely unnecessary and simply makes it harder to terminate etc?

17. ### EFLImpudence

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Well, through means through but it only affects the cable if it is surrounded by touching insulation.
Can you make an opening for it.

It's up to you but it's not necessary.

You could replace all your cables with the next size up.

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