Need some advice on an MCB size please

[freedomrun]

TN-S. Domestic 1.5 kW shower on a 6mm2 T&E clipped direct for the most part and in some plastic trunking for about 200 cm Total cct length is about 15M. The manufacturers label inside the shower itself states that it is a 9.6kW shower at 230V - should i take this as the power rating or the one also stated as 10.5kW at 240V (now we operate to 230V maybe i should take this as the power rating?) But I refer here to the 10.5kW rating. I've not got the Manufacturers instructions available.

Trips out a 32A MCB no problem (DIY installed shower and shower upgraded from previous). Told the customer i've got to include 30mA RCD and increase the MCB size. From what i can work out, the maximum i can go to will be at 46A. I would rather use a lower size cct protection at 40A, but am i likely to find this will trip out with the power rating here.

How much of an issue is the 200cm of cable enclosed in the plastc mini trunking, will this significantly decrease the current rating in this case, or at this length not an problem?

The customer is going to have the hump when i tell him i've got to put Main Bonding in as none present, not keen on swaping out the shower for a less powerful one. Changing the T&E from 6mm2 to 10mm2 is the obvious solution, but wanted to double check the MCB situation - would i be risking it using a 45A MCB on a cct when the regs say 46A is the maximum this cct design can take?

 

[gavindrummond]

All this should be stated in the 17th and the on site guide.

Ie: trunking size, ambient temp, volt drop, cable size.

 

[binky]

40A MCb would probably work for about 20mins. 45A MCBs are a little hard to come by. Trunking I wouldn't worry about as long as it has some air movement in it. It's at the top limit for 6mm to use 45A MCB, but I've done this without any issues before, cable ratings tend to be for continuous use, so shower working for say 30mins max is unlikely to cuse issues, and cable ratings do have some safety factor built in. Best option would be to change shower for 9.5kW, but customer probably doesn't like that.

 

[Technician]

The Electrician's guide to Part 'P' will have

the information you need, all set out in a

tabular form, in the section on standard circuits.

The installation methods are also included.

Each table is set out for BS 3036 (old hat)

circuit breaker types and sizes, and if you

look along the line you will also see maximum

circuit length for given conductor sizes.

 

[freedomrun]

Thanks, I'm OK on cable length and so on. My question seems to be answered best by Binky, the Regs state that this cct on 6mm2 T&E can go up to 46A, just wondered if i would be pushing my luck going for a 45A MCB here. Is there a stated value for the minimum length of T&E allowed in trunking before down rating needs to occur? Also, the cable for this cct is in a GFF, goes under the floorboards and then up the wall in trunking.

 

[Technician]

What table in appx 4 are you in?

---------- Post Auto-Merged at 14:49 ---------- Previous post was made at 14:45 ----------

What is a GFF?

 

[Sharpend]

Ground floor flat?

 

[binky]

Thanks, I'm OK on cable length and so on. My question seems to be answered best by Binky, the Regs state that this cct on 6mm2 T&E can go up to 46A, just wondered if i would be pushing my luck going for a 45A MCB here. Is there a stated value for the minimum length of T&E allowed in trunking before down rating needs to occur? Also, the cable for this cct is in a GFF, goes under the floorboards and then up the wall in trunking.

Been there too many times. Max length is dictated by voltage drop. TLC have a useful on-line tool in their technical section if you can't be bothered to work it out yourself, though a bit of practice helps keep it fresh in your mind.

 

[freedomrun]

Ground floor Flat. Tble 4D2A REF C

 

[Technician]

Doing conductor sizing...

Ib is 42A. ( for 9.6 kW)

Thus...45A would do it but rare.

Take 50A and the de-rating factors

would be 1 for the CPD and the ref

method is C. (inst. method 20) BUT

if in mini trunking for 0.2m method

B might apply.

With values in Table 4D5 (I assume)

It (max tabulated current) could be

as high as 47A and as low as 35A.

Iz (In service current carrying capacity)

could therefore be as high as 47

and down to 35 A.

In other words, 6mm may be a wee bit small.

At 10.5kW it is deffo too small.

Then you have the volt drop to consider.

---------- Post Auto-Merged at 15:11 ---------- Previous post was made at 15:06 ----------

Is it FTE; Flat twin and earth?

 

[freedomrun]

Thanks for that, yes FTE 6mm2. Should i consider the cable installed under the floor as method B or C. I have no access under the floor - maybe i should assume worst case.

 

[Technician]

Best of luck Freedom...leave it a bit and someone

might come along with something a little more

reassuring.

 

[freedomrun]

I think this has been a good lesson for me, i must have been lucky up to now and not had this issue right up to the margins of what the regs allow. I would rather have a bit of breathing space - so only options are to get the customer to get a shower more suitable for the cable in place as it's going to be a big job swapping out the present cable for 10mm2 T&E. On top of that, i have to include 30mA RCD protection & install Main Bonding.

 

[Technician]

Freedom;just a bit of a heads up.

It was the end of the week- sorry.

When doing the assessment for size

and the CPD is 50 Amp any derating

factors will increase the current.

Thus Iz will be greater than In,

the rating of the CPD. Thus, even if

all derate factors were 1, Iz would

still be 50A which means or reinforces

the reality that 6 mm is too small.

Apologies, but I think I said the right

thing with a bit of flawed reasoning.

I can post to you full calcs by PM next

week.

Best of luck though.

 

[steptoe]

stick it on a 40,

she'll be right,

sometimes we make work and problems that dont actually exist,

 

[OnOff]

Doing conductor sizing...Ib is 42A. ( for 9.6 kW)

Thus...45A would do it but rare.

Take 50A and the de-rating factors

would be 1 for the CPD and the ref

method is C. (inst. method 20) BUT

if in mini trunking for 0.2m method

B might apply.

With values in Table 4D5 (I assume)

It (max tabulated current) could be

as high as 47A and as low as 35A.

Iz (In service current carrying capacity)

could therefore be as high as 47

and down to 35 A.

In other words, 6mm may be a wee bit small.

At 10.5kW it is deffo too small.

Then you have the volt drop to consider.

---------- Post Auto-Merged at 15:11 ---------- Previous post was made at 15:06 ----------

Is it FTE; Flat twin and earth?

Doesn't the OP say 200cm i.e 2 METRES not 0.2m?

 

[steptoe]

stick it on a 40,she'll be right,

sometimes we make work and problems that dont actually exist,

Doesn't the OP say 200cm i.e 2 METRES not 0.2m?

still applies

 

[freedomrun]

Yes the length of cable enclosed in trunking is 2 METRES. What is the minimum ammount of cable permitted in trunking before factors need to be applied?

 

[steptoe]

I think its <500mm is disregarded, but, as with most things, Im probably wrong.

 

[park1]

Its simple Iz>In>Ib or if we assume no overload Iz>Ib