Electrical question

[DrPhill]

Hi all, just trying to figure out fix for our shower.

We had an 8.5kw fitted, but the replacement 9.5kw engine is cheaper than the 8.5. Yeah, go figure. Anyway, the higher power shower may need a higher rating cable (says the lady on the help line).

The cable and a new separate consumer unit were put in specially for the shower by a qualified electrical company - the docs say they used 16 mm2 cable. It looks like it could run a cooker. Is this fine or do I need to investigate more? (I am going to get a professional to fit the engine, just want to minimise the number of visits....)

Any help welcome.....

 

[owen]

I'm not an electrician but 16mm twin and earth sounds overkill for even 9.5kw. I'm pretty sure a 6mm twin and earth would have been enough for the original shower, but they have probably put larger incase the shower was ever changed in the future.

 

[Rorschach]

A cooker uses a lot less than a shower.

That being said that certificate looks to indicate a maximum continuous load of 65A, a 9.5kW shower will draw around 42A so would be fine.

 

[DrPhill]

Thanks to you both. That reassures me that I can replace the 8.5 with the cheaper 9.5.

I have a plumber coming round to look tomorrow. I will be good to have a shower again (though not having a shower could help with social distancing....)

 

[porker]

You should be OK on a 6mm cable to the shower but it is a bit marginal according to the regs- 10mm cable would be better (*caveat - depends on some other factors to strictly be correct). Unlikely though that the shower will draw full power for any length of time.

I think the previous poster may be confused as your certificate relates to the supply to your CU not the shower specifically. The 16mm cable are the tails to the CU not to the shower.

What rating breaker do you have on the shower circuit? At full load you will be drawing close to 40A

(Note - I'm not an electrician but an electrical engineer)

 

[minilathe22]

Interested to see this question as I am also replacing an 8.5kw shower with a 9.5kw one. I wonder what is a reliable way of measuring my existing cables and being confident of their maximum current specification?

 

[Rorschach]

Interested to see this question as I am also replacing an 8.5kw shower with a 9.5kw one. I wonder what is a reliable way of measuring my existing cables and being confident of their maximum current specification?

The cable will be marked with the size.

 

[Sideways]

As Porker says, the 16mm copper are the tails through the meter to your consumer unit. The document you posted doesn't tell you what cable was run to the shower.
Your new heater element will draw essentially 40 Amps
Your old one would have taken 35 Amps.
If the original electrics were done properly, the cable will have been chosen to be safe in your specific installation until the circuit breaker trips out due to overload.
If your circuit breaker is rated 40 Amps you're home and dry (warm & wet
If your circuit breaker is 30 or 32 Amps, you were probably overloading it a little and the new heater element will overload it more. The circuit breaker will trip if it carries too much overload for too long but in any case before the wires melt.
Don't change out a 30 or 32A circuit breaker for a 40A one without having an electrician check the cable size. How much current they can carry depends on how the cable is run. Cables warm up carrying current. If a cable passes through a bunch of insulation in the loft space, it can't radiate the heat as well and it's safe capacity will be less. Sometimes a lot less. Just looking at the ends doesn't tell the whole story.

Another EE, but I've done the electricians exams too.

 

[DrPhill]

Do these help? One is the consumer unit - which I cannot 'read' ratings from, the other is the cable going into the bathroom for the shower/lights/fan.

 

[Rorschach]

Ah I think you are going to have a problem

Your breaker is 40A and your new shower will draw up to 42A (9500/230).

 

[minilathe22]

Would it only draw the full 42amps with the incoming water quite cold and the shower on the hottest setting? Most electrical showers I have used, the hottest setting appears to be designed for stripping paint off the walls :shock:

 

[DrPhill]

Boogger.

I will have to stick with an 8.5kw (which perversely is more expensive).

 

[DrPhill]

Would it only draw the full 42amps with the incoming water quite cold and the shower on the hottest setting? Most electrical showers I have used, the hottest setting appears to be designed for stripping paint off the walls :shock:

It certainly goes hotter than either myself or the missus likes.... by quite a way. I am wondering if the cable is good enough to update the circuit breaker.

 

[Rorschach]

Would it only draw the full 42amps with the incoming water quite cold and the shower on the hottest setting? Most electrical showers I have used, the hottest setting appears to be designed for stripping paint off the walls :shock:

No, that's not how electric showers work.

They control the temperature by controlling the flow of the water. They always run on full power (or half power if you have more than 1 setting) and then the dial restricts the water flow. Less water = higher temperature. That's why in the winter you get less flow than in the summer.

 

[lurker]

What actually does the shower rating say?
Both on the old and new.
Is the difference just a function of the wider voltage tolerance throughout the eu?
I think this is 220 - 240 plus or minus 10%

My thinking is there must be a reason why the new one is cheaper and it might be because it is sold to a bigger EU wide market . And the resistance of both is actually the same.

Even so, as others have alluded, you need a decent safety margin between current drawn and the rating of the system.

 

[DrPhill]

Nearest I can find is this:https://www.aqualisa.co.uk/media/amasty/amfile/attach/f9361d8c0c931589c8ee892d795f4b62.pdf

Our cable is in plastic trunking, less than 10m last meter or two in insulated false ceiling.

 

[Rorschach]

You might be ok as long as your incoming supply is 240V, lots of houses these days are getting 230ish, some as low as 220.

 

[DrPhill]

I never thought of the tolerance in the supply. I wonder what guarantees the power company give?

 

[Sideways]

The power rating of the shower heater will be given at a nominal voltage. You won't really know until you see the box or specifications. If your actual voltage is higher than that, you'll get more power from it.
Euro harmonisation means that goods for sale in Europe should be safe to work on a nominal 230V with a tolerance band wide enough to accommodate the UK's 240V, or the continental European 220V.
Actual mains can (and does) vary a lot. The UK statutory limits are 230v-6% and 230v+10% at the consumer unit. This translates as 216.2v to 253v. There will be some voltage lost in the cables to the shower so the actual voltage at the heating element will be a bit less.
Take your shower on a sunny mid day when all the neighbourhood solar is generating and no one else is drawing power. You may find your mains up above 250V and your shower will work a treat

I won't churn through the whole argument here but this scenario is not a new one. Look here for technical discussion of whether or not a 9.5kW shower is SAFE on a 32A breaker with 10mm square cable. Ignore the bits about type B and C breakers. They are irrelevant.
https://www2.theiet.org/forums/forum/me ... TMP=Linear
A 9.5kW shower on a 40A breaker if the cable was properly sized and installed is very likely safe according to these arguments. It comes down to the facts that the over current is likely to be small and of relatively short duration (even if someone takes half hour showers). While the breakers by specification should carry at least 13% overload without tripping and cables should be sized not to overheat before the breaker trips.
At the same time it demonstrates that the question is a little complicated and the internet contains the whole spectrum of more and less expert opinion.

 

[DrPhill]

And there was me, naively assuming that this was engineering with yes-no answers.

Ah well, all I glean from that was that there are valid arguments that a 9.5kw shower is fine on a 32 Amp breaker. Since mine is on a 40 amp breaker, and the sparks would have made sure that the breaker protected the wiring does that mean that I am good to go with the upgrade from 8. to 9.5kw? Saves me 50 gbp, but I would value safety over cost savings......

All very puzzling to me.......

Edit: Thanks for the replies - I have learned a little more about electrickery. 9.5kw may be feasible but I guess that the final answer will rest with the guy doing the job.