Limited to 7.5kw shower?

danrv

Hi
I’m looking to fit a new electric shower as part of a DIY bathroom project. The old one was a Triton Ivory 4 8.5kw and has been running fine on 6mm cable and a 32amp MCB. Ideally would like the same power for the new shower.
The cable run (8m) is mostly across the attic and insulation but also in an upstairs stud wall so not easily changed.
I’ve contacted Mira regarding installing a Mira Jump and they said the fuse would needed to be 40amp. Haven’t heard back regarding the cable.
I think it should be 10mm for 8.5kw but not sure why the Triton only has 6mm.
Any help appreciated.

rob7475

As long as the cable isn't covered in insulation, you should just about be OK to run a 8.5kw shower on a 6mm cable. Just don't run the shower for hours on end.

danrv

rob7475 said:

As long as the cable isn't covered in insulation, you should just about be OK to run a 8.5kw shower on a 6mm cable. Just don't run the shower for hours on end.

Thanks for the reply.

jamesd

Plumbworld has a list of requirements which includes showers above 7kW up to 9kW on 6mm cable with runs up to 18m and a 40 amp circuit breaker instead of the common 32 amp. On the 40 amp breaker there are many reports of 32 amps working fine for years or decades.

If you want to understand why 32A can do the job, have a look at the trip curves for class C MCB. That'll trip instantaneously somewhere between five and ten times the rated 32 amps but much more slowly for slight overloads, which only cause a trip if the bimetalic strip inside gets too hot. You can see from the trip curve that this can mean thousands of seconds of run time for 25% overloads.

So why 40? Because that is above the shower rating so there's no chance of a nuisance trip. For any new job an electrician would be likely to insist on 10mm cable to ensure that the cable can't be overloaded. Both good things.

Risteard

jamesd said:

Plumbworld has a list of requirements which includes showers above 7kW up to 9kW on 6mm cable with runs up to 18m and a 40 amp circuit breaker instead of the common 32 amp. On the 40 amp breaker there are many reports of 32 amps working fine for years or decades.

If you want to understand why 32A can do the job, have a look at the trip curves for class C MCB. That'll trip instantaneously somewhere between five and ten times the rated 32 amps but much more slowly for slight overloads, which only cause a trip if the bimetalic strip inside gets too hot. You can see from the trip curve that this can mean thousands of seconds of run time for 25% overloads.

So why 40? Because that is above the shower rating so there's no chance of a nuisance trip. For any new job an electrician would be likely to insist on 10mm cable to ensure that the cable can't be overloaded. Both good things.

A 32A protective device should not be used. It is incorrect design to have a protective device which is of a lower rating than the load. This will lead to excess heating of the device.

As for a cable being overloaded, a shower is a fixed load and therefore cannot cause an overload. Therefore the protective device is simply providing fault protection. As such there is no issue having a 6mm^2 cable protected by a 40A protective device in this instance, assuming of course that the load does not exceed the current-carrying capacity of the cable taking account of the installation method and relevant derating factors.

jamesd

Yes, it's incorrect to use a 32 amp breaker. And yes, the expected result is that the breaker will heat up. It might even eventually start to trip even though it doesn't at first. That's why 40 is recommended. Also why it's what I'd want to be using so long a the cable can handle that current in its environment.

Not so sure that an 8.5kW shower drawing 35.4 amps at 240 volts can't cause an overload given that it exceeds the 6mm cable rating for methods A, 100, 101, 102 and 103. Risteard will know this and far, far more but for anyone who's curious a description of when each rating applies and what the limits are can be found here, among other places.

In this case at least the attic looks most likely to be method C with a 47 amp (22.3kW) capability but not so sure about the stud wall and its insulation level and cable placement within it. danrv, you might want to read the method descriptions to see if you can sort out which one applies to the stud wall part of the route.

And definitely pay attention to Risteard, who is clearly way more knowledgeable about this than I am. I just happened to have recently done this sort of checking for myself.

grumbler

jamesd said:

Plumbworld has a list of requirements which includes showers above 7kW up to 9kW on 6mm cable with runs up to 18m

IMO, up to 18m (and 35m)  restriction on the distance from the fuseboard is absolutely nonsensical. The bigger the distance, the better as the current gets only smaller.

This undermines credibility of all their other requirements.

Section62

grumbler said:

jamesd said:

Plumbworld has a list of requirements which includes showers above 7kW up to 9kW on 6mm cable with runs up to 18m

IMO, up to 18m (and 35m)  restriction on the distance from the fuseboard is absolutely nonsensical. The bigger the distance, the better as the current gets only smaller.

This undermines credibility of all their other requirements.

The longer the cable the greater the voltage drop.

The length of cable of a given CSA passing a given current has to be limited to ensure the total voltage drop isn't excessive.  If it is, then a larger CSA cable has to be used.

GDB2222

Section62 said:

grumbler said:

jamesd said:

Plumbworld has a list of requirements which includes showers above 7kW up to 9kW on 6mm cable with runs up to 18m

IMO, up to 18m (and 35m)  restriction on the distance from the fuseboard is absolutely nonsensical. The bigger the distance, the better as the current gets only smaller.

This undermines credibility of all their other requirements.

The longer the cable the greater the voltage drop.

The length of cable of a given CSA passing a given current has to be limited to ensure the total voltage drop isn't excessive.  If it is, then a larger CSA cable has to be used.

I guess Grumbler is saying that, if the voltage drop is too big, the expected output of the shower would not be achieved. That would not be dangerous, as long as the cable is not getting too hot?

danrv

jamesd said:

In this case at least the attic looks most likely to be method C with a 47 amp (22.3kW) capability but not so sure about the stud wall and its insulation level and cable placement within it. danrv, you might want to read the method descriptions to see if you can sort out which one applies to the stud wall part of the route.

Thanks.
Around half of the cable is laying on loft insulation, some is covered but loft isn’t boarded.
It then drops down 2.4m into a hollow bedroom wall and probably not clipped in place.
Then under the floor for another 2m to the CU.

grumbler

Section62 said:

grumbler said:

jamesd said:

Plumbworld has a list of requirements which includes showers above 7kW up to 9kW on 6mm cable with runs up to 18m

IMO, up to 18m (and 35m)  restriction on the distance from the fuseboard is absolutely nonsensical. The bigger the distance, the better as the current gets only smaller.

This undermines credibility of all their other requirements.

The longer the cable the greater the voltage drop.

The length of cable of a given CSA passing a given current has to be limited to ensure the total voltage drop isn't excessive.  If it is, then a larger CSA cable has to be used.

GDB2222 said:

Section62 said:

grumbler said:

jamesd said:

Plumbworld has a list of requirements which includes showers above 7kW up to 9kW on 6mm cable with runs up to 18m

IMO, up to 18m (and 35m)  restriction on the distance from the fuseboard is absolutely nonsensical. The bigger the distance, the better as the current gets only smaller.

This undermines credibility of all their other requirements.

The longer the cable the greater the voltage drop.

The length of cable of a given CSA passing a given current has to be limited to ensure the total voltage drop isn't excessive.  If it is, then a larger CSA cable has to be used.

I guess Grumbler is saying that, if the voltage drop is too big, the expected output of the shower would not be achieved. That would not be dangerous, as long as the cable is not getting too hot?

Absolutely. We and they are talking about the cable cross-section area.

"Showers up to 7kw may carry a 6mm cable as long as the shower unit is within 18 metres of the fuseboard. Fuses and switches should be rated at 32 amps."

Yes, if the length is bigger than 18m, the voltage drop is bigger and the shower power will get smaller. So will be the water flow for the required temperature.

However, this isn't a safety issue and in terms of the cable (over)heating this is perfectly fine and even better.