Shower circuit design supplies

[Buzz1980]

Hi all
Question regarding shower circuit designs. How so many times I see 6mm t&e cables supplying shower circuits on a 40amp MCB ? So for example if you were carrying out a EICR and you noticed a 6mm t&e cable on a 40amp MCB, 8.5kw/9kw shower would you put this down as a C3 or would you not put anything at all ? Giving that you don’t know how the cable is installed ? I have came across a few that when inspecting the cable has came from consumer unit up through floor floor boards and up inside external wall/pipe duct. Run approx 18 to 20meters. Now with not knowing the installation method if tangled in insulation in wall or bunched under the floor boards. Going with current carrying capacity 6mm good for 47amps but add in the factors could bring this down. Do you quote customer for changing MCB or do you make note of it on test sheet, or just leave as is ? Would like to hear other opinions ??

 

[Ian1981]

How can you change the mcb to a 32 amp mcb if the shower is either 8.5 or 9.5kw?
No evidence of thermal damage, I leave it depending on what I believe the ccc of the cable is and it is not subject to overload conditions.
Remember overload protection can be omitted for fixed loads like showers but the ccc of the cable must be suitable for the load and you must have adequate fault protection in accordance with section 434.
There are certain limitations to carrying out an EICR in which you cannot overcome.
Inspect the cable where you can, if it’s been in service for quite a number of years and has no obvious damage to it , insulation resistance test results are good, I see no reason to code it anything.

 

[Buzz1980]

Hi Ian1981
I was thinking that the shower ratings will prob not get up to current levels where it would trip a 32amp MCB so was thinking of installing a 32amp. As regards to install, the cables run in the attic space is only where I can see any exposed run there is insulation in the attic space but these cables have been run along side the insulation and at points under but only for short run of say 600mm or so. They have also extended cables with through crimps must have been when they dropped the ceiling. The reason for doubting is I don’t know if coming up external wall there in long runs of insulation, then if they totally insulate the loft at any point this then brings the current rating down. Mostly I would only ever put a 6mm on a 32amp RCBO/MCB but if your having to state to client that this has change and pay extra for MCBs ?

 

[telectrix]

Hi Ian1981
I was thinking that the shower ratings will prob not get up to current levels where it would trip a 32amp MCB so was thinking of installing a 32amp. As regards to install, the cables run in the attic space is only where I can see any exposed run there is insulation in the attic space but these cables have been run along side the insulation and at points under but only for short run of say 600mm or so. They have also extended cables with through crimps must have been when they dropped the ceiling. The reason for doubting is I don’t know if coming up external wall there in long runs of insulation, then if they totally insulate the loft at any point this then brings the current rating down. Mostly I would only ever put a 6mm on a 32amp RCBO/MCB but if your having to state to client that this has change and pay extra for MCBs ?

you can only code (or not) the installation as it is at the time of inspection. future possibleaddition of insulation is irrelevant as it may never happen. you are inspecting the installlation as it is, not as what it might be.

 

[Buzz1980]

Yeah, good point. Just doesn’t seem right leaving 6mm cables on 40amp MCBs have never installed them like that so find it hard to leave them like that but you see them all over installed like this. Even seen new installed circuits to showers in 6mm when you wouldn’t think to install like this.

 

[Ian1981]

If the shower is either 8.5 or 9.5 kw as you said, then the load current will always exceed 32 amps unless there’s an eco setting on the shower and it stays on that setting.
All the temperature dial does is increase the flow of cold water, the current demand remains the same, depending on what the resistance of the heating element is.

A lot of electricians seam to have an issue with a 6mm2 cable being protected by a 40 amp ocpd,why?
The question is , For a shower circuit is the cable adequate for the load of the shower?
The load of the shower could be 36 amps, the ccc of the 6mm could be 38 amps, the ocpd needs to be at least 40 amps.
This set up is acceptable as the load is fixed and the ccc of the cable is acceptable for the loading of the shower.

Cables going through insulation need correction factors as per table 52.2, as an example a cable passing through insulation covered more than 400 mm has a derating factor of 0.51, so the ccc is just about halved so this presents an issue.
Are the cables/conductors that are crimped and extended covered by a suitable enclosure?

 

[Buzz1980]

Hi yeah the crimp connections are in a suitable enclosure inside a pattress box with 1gang front plate. Im prob getting more on the side I’ll change them for. 32amp MCB that way the cable is protected ?

 

[Ian1981]

Hi yeah the crimp connections are in a suitable enclosure inside a pattress box with 1gang front plate. Im prob getting more on the side I’ll change them for. 32amp MCB that way the cable is protected ?

Can you clear the insulation away from the cable(s)?
A 32 amp mcb will be subject to constant overload conditions whenever the shower is in use.
Can you clamp the Line or neutral conductor to see what the shower is pulling?

 

[Buzz1980]

I could head back and get this cleared away once this lockdown has passed. As they are occupied top flats. I think if I go and do the calculation with the section of 600mm of insulation then Current carrying capacity will be below 40amps. Might be best to run shower full blast and clamp cable to see how much it is pulling ?

 

[Ian1981]

I could head back and get this cleared away once this lockdown has passed. As they are occupied top flats. I think if I go and do the calculation with the section of 600mm of insulation then Current carrying capacity will be below 40amps. Might be best to run shower full blast and clamp cable to see how much it is pulling ?

I mean yeah by all means see what the shower is drawing.
Dont Worry if the ccc is below 40 amps , just make sure that the ccc doesn’t drop below the current demand for the shower.
Out of interest , is this a new (ish) installation?

 

[pc1966]

If it is drawing around 37A running (8.5kW @ 230V) and you had a 32A MCB then it would not trip in under 10-20 minutes anyway, and most folk shower for less than that.

If the issue is thermal insulation covering the cable in one short area could you simply cut the thermal stuff and pull the cable over it so it is not enclosed?

If there is no sign of thermal stress (or loss of insulation resistance on the cable) it is probably fine, after all the duty cycle on it is very low.

 

[Buzz1980]

It’s an old installation, has been in for years but I think the shower was upgraded higher wattage and customer said that the shower switch stopped working once before with the Black/neutral slightly damaged so they got someone up must have been when they through crimped the cable to get more length, don’t know if this was due to overheating or loose connection ?

 

[Ian1981]

If it is drawing around 37A running (8.5kW @ 230V) and you had a 32A MCB then it would not trip in under 10-20 minutes anyway, and most folk shower for less than that.

If the issue is thermal insulation covering the cable in one short area could you simply cut the thermal stuff and pull the cable over it so it is not enclosed?

If there is no sign of thermal stress (or loss of insulation resistance on the cable) it is probably fine, after all the duty cycle on it is very low.

A 32 amp may not Tripp however it will be constantly overloaded every time it’s used which could damage it beyond its normal working capacity , and is a non compliance with 433.1.1 (i) and is completely unacceptable. Regardless if you shower for 5 minutes or 20 minutes is besides the point

 

[pc1966]

A 32 amp may not trip however it will be constantly overloaded every time it’s used which could damage it beyond its normal working capacity , and is a non compliance with 433.1.1 (i) and is completely unacceptable

Sorry, I should have made it clear that I am not suggesting a 32A MCB is a good idea, just that that it is not going to make a difference to cable protection in this scenario (i.e. cable would see the same sort of heating cycle as shower use probably below trip point).

Only real solutions are:

• Make sure the existing 6mm cable is not too thermally insulated
• Replace cable with 10mm or similar
• Replace shower with lower power version

First option may be OK if it can be seen where it runs, third option probably unacceptable to the end user.

 

[telectrix]

and bear in mind that the BS7671 figures for ccc are pessimistic. some time ago, with a 63A MCB welded closed due to a load of 110A to a pub kitchen DB fed with 16mm T/E,, i clamped the 16mm @ 104A (+/-3A) for 30 minutes. temp of the 16mm T/E reached 40 degrees C. after 30 minutes of 100+A. yet it's only rated for 80A.

 

[pc1966]

It’s an old installation, has been in for years but I think the shower was upgraded higher wattage and customer said that the shower switch stopped working once before with the Black/neutral slightly damaged so they got someone up must have been when they through crimped the cable to get more length, don’t know if this was due to overheating or loose connection ?

Loose connections are usually the reason, not just inadequate installation torque but also thermal cycling or vibration over time. But damage can extend quite a bit back so rarely is there enough slack to fix it without some sort of replacement or extension.

Where is the joint? Is it where there might have been an isolation switch or a pull-switch for the system originally?

 

[Buzz1980]

Yeah the joint has been put in the loft, on the switch wire side my guessing is that the switch was short and tight going into connection which caused the neutral to come loose and get slightly damaged on insulation. I would have thought that other conductors would see damage if this was anything to do with current carrying capacity but they never mentioned the other conductors just the neutral became damaged? So if I was to get back into attic to clear away insulation then would be a better set up rather than changing MCB out for 32amp where it could be overloadeded all the time ?

 

[davesparks]

Yeah, good point. Just doesn’t seem right leaving 6mm cables on 40amp MCBs have never installed them like that so find it hard to leave them like that but you see them all over installed like this. Even seen new installed circuits to showers in 6mm when you wouldn’t think to install like this.

You aren't carrying out an EICR based on what doesn't seem right to you, or how you prefer to install things, you are carrying it out to BS7671.

Do you recommend that every MCB be downrated just in case the cables are in insulation? Why are you focusing on the shower circuit for this when any other circuit could be similarly affected?

You also seem to have missed the point about the CCC being permitted to be less than the MCB rating due to it being a fixed load an overload protection not being required.

 

[Buzz1980]

The reason I’ve focused on the shower is that I have never put 6mm t&e on 40amp MCB when I’m newly installing only on 32amp MCB.


You also seem to have missed the point about the CCC being permitted to be less than the MCB rating due to it being a fixed load an overload protection not being required.

yeah sorry have missed that point ?

 

[davesparks]

The reason I’ve focused on the shower is that I have never put 6mm t&e on 40amp MCB when I’m newly installing only on 32amp MCB.


You also seem to have missed the point about the CCC being permitted to be less than the MCB rating due to it being a fixed load an overload protection not being required.

yeah sorry have missed that point ?

As I said, you are inspecting to the regulations, not to your personal opinion. Just because you've personally not used 6mm for a 40A supply it doesn't mean that it is unsafe to do so.

Overload protection may be omitted for fixed loads which are not subject to overload,