Consumer unit change - is cable size corrct?

[Kido44]

Hi
My mother in law has just had a new fuse board fitted.
The electrician informed us that the 2 showers in the house (1 a 10.5kw & 1 a 8.5kw) are each fed by 6mm cable.
The 8.5kw was on a 40a cartridge fuse and the 10.5kw on a 45a cartridge fuse.
He said the cable is sufficient for the 8.5 but not suitable for the 10.5.
He has put both cables into the new fuse board but has downgraded the fuses. The 8.5kw is on a 32a mcb and the 10.5kw is now on a 40amp mcb.
Is this ok?
He said he has lowered the 10.5kw shower mcb to protect the cable.
Is that ok?
I appreciate any help, thanks.

 

[Aragorn84]

Not really, no.

10.5KW shower will draw somewhere in the region of 45A.

This means the breaker will always be operating in an overloaded state.

The cable will also likely not be rated for 45A and should be changed.

The 8.5 on a 32 is also iffy.

 

[Kido44]

Not really, no.

10.5KW shower will draw somewhere in the region of 45A.

This means the breaker will always be operating in an overloaded state.

The cable will also likely not be rated for 45A and should be changed.

The 8.5 on a 32 is also iffy.

Isn't that why he has put the 40amp breaker on the 10.5kw shower so the mcb will trip if the shower pulls to much to protect the cable from over heating?

Regards the 8.5kw shower how is the 32amp iffy? Please explain ? 8500 / 240 = 35amp so that's a maximum pull of only 3 amps more than the breaker if the shower was on maximum, therefore th 32 amp fuse will trip. It's only iffy if a higher rated mcb was put in surely?

Both these showers have been fitted for Over ten years before the board has been changed and have been ok.
Own it safer to have a mcb slightly lower rather than slightly higher?
The mcb is fitted to protect the cable isn't it? From over heating?

 

[EFLImpudence]

No, he need not have altered anything as long as the 10.5 shower cable does not run through any conduit and neither runs through any thermal insulation.

The MCBs would have protected the cable satisfactorily as they were.
The showers will not cause an overload.

8.5 @ 240V = 35.4A
10.5 @ 240V = 43.75A
both will reduce when hot.

6mm² can carry 47A with the above provisos.

As it is now the MCBs will slightly overheat which could lead to long term failure

 

[EFLImpudence]

Regards the 8.5kw shower how is the 32amp iffy? Please explain ? 8500 / 240 = 35amp so that's a maximum pull of only 3 amps more than the breaker if the shower was on maximum, therefore th 32 amp fuse will trip. It's only iffy if a higher rated mcb was put in surely?

Is 35 more than 32?

Both these showers have been fitted for Over ten years before the board has been changed and have been ok.

There you go.

Own it safer to have a mcb slightly lower rather than slightly higher?
The mcb is fitted to protect the cable isn't it?

Yes, but the cables won't; now the MCBs will.

 

[JohnW2]

Isn't that why he has put the 40amp breaker on the 10.5kw shower so the mcb will trip if the shower pulls to much to protect the cable from over heating?

Yes, but it is bad practice, and non-compliant with regulations, to have a circuit in which one knows that the load ('the design current') will be greater than the rating of the MCB (and possibly the rated current-carrying capacity of the cable). Also, don't forget that a Type B 40A MCB will allow about 45A to flow indefinitely, and about 58A for around an hour. The 40A MCB will therefore probably never trip, but that doesn't alter the fact that the set-up is bad practice, and non-compliant with regs.

Regards the 8.5kw shower how is the 32amp iffy? Please explain ? 8500 / 240 = 35amp so that's a maximum pull of only 3 amps more than the breaker if the shower was on maximum, therefore th 32 amp fuse will trip. It's only iffy if a higher rated mcb was put in surely?

See above - same issues.

Both these showers have been fitted for Over ten years before the board has been changed and have been ok.

As above, both will almost certainly always be 'OK' in the sense that the breaker won't trip (because of the operating characteristics of MCB), but they are nevertheless 'not OK' in the sense that they are non-compliant with the regs. I guess it depends on what version of 'OK' you want - but the electrician really shouldn't have given you the one you have!

Kind Regards, John

 

[Kido44]

Regards the 8.5kw shower how is the 32amp iffy? Please explain ? 8500 / 240 = 35amp so that's a maximum pull of only 3 amps more than the breaker if the shower was on maximum, therefore th 32 amp fuse will trip. It's only iffy if a higher rated mcb was put in surely?

Is 35 more than 32?

Both these showers have been fitted for Over ten years before the board has been changed and have been ok.

There you go.

Own it safer to have a mcb slightly lower rather than slightly higher?
The mcb is fitted to protect the cable isn't it?

Yes, but the cables won't; now the MCBs will.

Hi EFLImpudence
Really appreciate your help.
So it's safe but means mcbs may need replacing sooner as can over heat more?
That isn't the end of the world.
I've just looked at the cable and it looks slightly bigger than the cooker cable (6mm) but it doesn't look 10mm. Was 8mm ever used 10/15 years ago?

 

[Kido44]

Isn't that why he has put the 40amp breaker on the 10.5kw shower so the mcb will trip if the shower pulls to much to protect the cable from over heating?

Yes, but it is bad practice, and non-compliant with regulations, to have a circuit in which one knows that the load ('the design current') will be greater than the rating of the MCB (and possibly the rated current-carrying capacity of the cable). Also, don't forget that a Type B 40A MCB will allow about 45A to flow indefinitely, and about 58A for around an hour. The 40A MCB will therefore probably never trip, but that doesn't alter the fact that the set-up is bad practice, and non-compliant with regs.

Regards the 8.5kw shower how is the 32amp iffy? Please explain ? 8500 / 240 = 35amp so that's a maximum pull of only 3 amps more than the breaker if the shower was on maximum, therefore th 32 amp fuse will trip. It's only iffy if a higher rated mcb was put in surely?

See above - same issues.

Both these showers have been fitted for Over ten years before the board has been changed and have been ok.

As above, both will almost certainly always be 'OK' in the sense that the breaker won't trip (because of the operating characteristics of MCB), but they are nevertheless 'not OK' in the sense that they are non-compliant with the regs. I guess it depends on what version of 'OK' you want - but the electrician really shouldn't have given you the one you have!

Kind Regards, John

So should he have replaced mcbs with 40 & 45 amp even tho cable is only 6mm?
Isn't it safe to also assume either shower won't be used at its maximum load ( 8.5kw = 35a & 10.5kw = 44amp) and are only used at about 80%.
There isn't a 35a mcb for the 8.5 kw so wouldn't 40amp mcb be to high?
And there isn't a 45a mcb for the 10.5 kw so wouldn't a 50a be too high?

Thanks

 

[JohnW2]

So should he have replaced mcbs with 40 & 45 amp even tho cable is only 6mm?

As EFLI said, yes - provided only that the routing/method of installation of the cable is such that it's current-carrying capacity is at least 45A (which is probably is - assuming that it really is 6 mm²).

Kind Regards, John

 

[Kido44]

So should he have replaced mcbs with 40 & 45 amp even tho cable is only 6mm?

As EFLI said, yes - provided only that the routing/method of installation of the cable is such that it's current-carrying capacity is at least 45A (which is probably is - assuming that it really is 6 mm²).

Kind Regards, John

Isn't that why he has put the 40amp breaker on the 10.5kw shower so the mcb will trip if the shower pulls to much to protect the cable from over heating?

Yes, but it is bad practice, and non-compliant with regulations, to have a circuit in which one knows that the load ('the design current') will be greater than the rating of the MCB (and possibly the rated current-carrying capacity of the cable). Also, don't forget that a Type B 40A MCB will allow about 45A to flow indefinitely, and about 58A for around an hour. The 40A MCB will therefore probably never trip, but that doesn't alter the fact that the set-up is bad practice, and non-compliant with regs.

Regards the 8.5kw shower how is the 32amp iffy? Please explain ? 8500 / 240 = 35amp so that's a maximum pull of only 3 amps more than the breaker if the shower was on maximum, therefore th 32 amp fuse will trip. It's only iffy if a higher rated mcb was put in surely?

See above - same issues.

Both these showers have been fitted for Over ten years before the board has been changed and have been ok.

As above, both will almost certainly always be 'OK' in the sense that the breaker won't trip (because of the operating characteristics of MCB), but they are nevertheless 'not OK' in the sense that they are non-compliant with the regs. I guess it depends on what version of 'OK' you want - but the electrician really shouldn't have given you the one you have!

Kind Regards, John

So should he have replaced mcbs with 40 & 45 amp even tho cable is only 6mm?
Isn't it safe to also assume either shower won't be used at its maximum load ( 8.5kw = 35a & 10.5kw = 44amp) and are only used at about 80%.
There isn't a 35a mcb for the 8.5 kw so wouldn't 40amp mcb be to high?
And there isn't a 45a mcb for the 10.5 kw so wouldn't a 50a be too high?

Installation method is clipped around garge wall into floor boards.

Thanks

 

[EFLImpudence]

Everything was alright as it was originally. Hence the ten years.

Now it is not right even though nothing untoward may happen.

'Even though the cable is 6mm²' doesn't apply. It is correct.

 

[Kido44]

Everything was alright as it was originally. Hence the ten years.

Now it is not right even though nothing untoward may happen.

'Even though the cable is 6mm²' doesn't apply. It is correct.

I may get another spark out to check but on this basis the 10.5kw 40 a mcb just needs to be changed to a 45a mcb and the 8.5kw from a 32a to a 40a. That means only one 45a mcb is needed to be purchased.

Is it safe to use in the mean time until I get someone to do that?

 

[JohnW2]

Isn't it safe to also assume either shower won't be used at its maximum load ( 8.5kw = 35a & 10.5kw = 44amp) and are only used at about 80%.

Shower elements usually are either 'on' or 'off', so the 'maximum load' is always going to be '100%', even if the average over a period of time is lower (because it has been switching on and off).

There isn't a 35a mcb for the 8.5 kw so wouldn't 40amp mcb be to high? And there isn't a 45a mcb for the 10.5 kw so wouldn't a 50a be too high?

No. The MCB is there to protect the cable, regardless of the load. If the cable were big enough to carry 100A, then a 100A MCB would be acceptable for either of your shower circuits.

Kind Regards, John