10.5kw shower on 40a mcb

[Custom45]

hi
as per title.. is this safe?

its on a 10mm² cable laid across loft on top of insulation, approx 8m run (1m from shower to 45a isolator switch, then 7m from switch to c.u). I know it should be on a 45a mcb, but it's not. never had a problem with it tripping etc.

shall I replace to 40a mcb with a 50a (they don't make a 45a for my unit)? if I put a 50a mcb in do I need to change the 45a isolator switch to 50a too?

many thanks

 

[EFLImpudence]

as per title.. is this safe?

Technically it is not right. 43.8A @ 240V but 42A @ 230V so just over.
Not unsafe as such but might weaken the MCB after a time.

its on a 10mm² cable laid across loft on top of insulation, approx 8m run (1m from shower to 45a isolator switch, then 7m from switch to c.u). I know it should be on a 45a mcb, but it's not. never had a problem with it tripping etc.

It won't trip in normal use.

shall I replace to 40a mcb with a 50a (they don't make a 45a for my unit)?

Yes.

if I put a 50a mcb in do I need to change the 45a isolator switch to 50a too?

No.

 

[Custom45]

Technically it is not right. 43.8A @ 240V but 42A @ 230V so just over.
Not unsafe as such but might weaken the MCB after a time.
.

many thanks for your reply. I'll get the 50a, it's only a couple of quid. I must've done my calcs wrong, at 230v I made it 45a?

 

[bernardgreen]

What voltage is quoted for the 10.5 kW rating, it will be either 230 volts (A) or 240 volts. (B)

(A) 10.5 kW at 230 volts is 45.65 Amps Element resistance 5.03 Ohms

(B) 10.5 kW at 240 volts is 43.75 Amps Element resistance 5.49 Ohms

The maximum supply voltage of 253 volts ( 230 +10% ) must be taken into account when calculating the trip time of an MCB .

(A) For the 5.03 Ohm element the current when the supply is 253 volts will be 50.29 Amps A 40 Amp MCB would trip after approx 10 mins

(B) For the 5.49 Ohm element the current when the supply is 253 volts will be 46.08 Amps A 40 Amp MCB would trip after approx 1 hour

 

[Custom45]

From the manufacturer manual...



Putting in a 50a mcb still best way forward then?

 

[bernardgreen]

Yes putting in a 50 Amp MCB is sensible, ( 10mm cable is just acceptable in terms of voltage drop but safe ) )

the 50 Amp MCB would not trip even when the supply was 253 volts.

 

[Custom45]

thanks, will do that for now then.

will get someone in at a later date for a proper overhaul as the c.u is a bit of out date now, it's a plastic casing with mcbs...

6a lighting
40a shower (be 50a soon)
32a ground floor sockets
32a top floor sockets
32a kitchen sockets
20a loft socket
20a bedroom socket
20a bedroom socket
80a 30ma rccb main switch on the end

and have a cooker on separate 50a rcbo.

 

[JohnW2]

The maximum supply voltage of 253 volts ( 230 +10% ) must be taken into account when calculating the trip time of an MCB . ...

Indeed - except that 'we' don't !!

As I say so often, in a sensible world, we would use the maximum or minimum (as appropriate) possible supply voltages for safety-critical calculations - but 'we' usually don't - we use the (arbitrary) 'nominal' voltage instead.

After decades of ignoring this issue, the Wiring Regs / BS7671 eventually almost addressed this in terms of 'maximum Zs calculations' (by introducing 'Cmin'), but they have taken no similar action to 'correct' the way we calculate in relation to required cable CCC and OPD rating etc.

Kind Regards, John

 

[JohnW2]

( 10mm cable is just acceptable in terms of voltage drop but safe ) )

I think people get too hung up with the guidance/recommendations in Appendices of BS7671.

No matter how high the voltage drop, it would only be 'unacceptable' (in terms of compliance with BS7671) if that voltage drop "impaired the safe operation of the shower" - and I think that one would probably struggle to think of a situation in which VD, however great, could have that consequence.

Kind Regards, John

 

[scousespark]

The maximum supply voltage of 253 volts ( 230 +10% ) must be taken into account when calculating the trip time of an MCB .

I was taught to 16th edition and the nominal voltage used in cable calcs (Uo) was 240v and the only change I can recall is to drop this to 230v in later editions. Is the 253v requirement new?

 

[EFLImpudence]

10mm cable is just acceptable in terms of voltage drop but safe

Why?

According to TLC's calculator, 6mm² only drops 5V.

 

[JohnW2]

I was taught to 16th edition and the nominal voltage used in cable calcs (Uo) was 240v and the only change I can recall is to drop this to 230v in later editions. Is the 253v requirement new?

As I just wrote (and as I'm sure you know), there is no "253V requirement" in relation to calculations (nor a "216.2V requirement" in relation to other calcs - give or take the fact that the recently introduced Cmin almost achieves that for some calcs) - but, as I also wrote, I strongly believed that there should be!

Kind Regards, John

 

[bernardgreen]

Is the 253v requirement new?

Probably not a formal requirement but it makes sense to use the worse case voltage to ensure that the instalation is safe at that voltage. Even more so now that domestic solar panels are feeding into the local network and sending the local supply up towards the 230+10% limit

 

[bernardgreen]

According to TLC's calculator, 6mm² only drops 5V.

That is about 2% lost heating the cable. What is the maximum current rating for 6 mm ?

 

[EFLImpudence]

47A method C.

I was just pointing out that 10mm² would be even less than that as opposed to your "just acceptable".


So, again, Why "just acceptable"?