Shower related: watts and amps.

[amfisted]

I need just a little bit more advice before proceeding at last with the shower swap (best get on with it, because the 2 year warranty on the replacement shower runs out this June).

I had other electrical work done a few weeks ago, and whilst he was here the electrician informed me that the shower is fed by 4mm cable (32a MCB, cable run from CU to shower is about 7 metres). This has never caused any obvious problems in the five years that we've been using the old shower, and its only the erratic operation of the old unit itself which has persuaded me not to leave things alone.

But now I'm concerned that when I fit the new shower (8.5kw, same rating as the old one) I might start to experience problems because of the 4mm feed. Am I worrying unnecessarily?

 

[JohnW2]

... the electrician informed me that the shower is fed by 4mm cable (32a MCB, cable run from CU to shower is about 7 metres). .... But now I'm concerned that when I fit the new shower (8.5kw, same rating as the old one) I might start to experience problems because of the 4mm feed. Am I worrying unnecessarily?

Assuming that the 8.5 kW is quoted at 240V (which it usually is), it will draw about 33.9A at 230V (which is the voltage at which we undertake calculations). If installed in the most usual way (not in conduit or covered with thermal unsulation), 4mm² cable can carry up to 37A - hence no problem.

33.9A is fractionally more than 32A, so in theory a 32A MCB is not requite adequate but, in reality, is unlikely to cause any problems (except, perhaps a very slight reduction in lifespan of the MCB) - it certainly is required not to trip at currents less than 36A or so.

So, the cable is (if installed as I described) OK for an 8.5 kW shower, but the 'official' position is that the 32A MCB is not. However, to change to, say, a 40A MCB would involve the hassle of upgrading the cable to 6mm² - so it's up to you to contemplate the options!

Kind Regards, John

 

[EFLImpudence]

433, anyone?

 

[JohnW2]

433, anyone?

I was waiting for that to show its face ...

Yes, one could upgrade the MCB to 40A (or even 400A, if one existed) and (provided that requirements for fault protection was satisfied) then argue that 4mm² cable was still OK (if one believed that the nature of the load was such that overload of the cable was unlikely). That's true in terms of the regs (i.e. 'compliant') but I suspect that, in practice, a good few electricians would nevertheless be 'nervous' about invoking such an argument.

Kind Regards, John

 

[winston1]

Assuming that the 8.5 kW is quoted at 240V (which it usually is), it will draw about 33.9A at 230V (which is the voltage at which we undertake calculations

So these lies we are told about the UK voltage rear their dangerous head again. He could have over 250v, so using your assumption (even though we should never assume) it would draw over 37 amps. Not good for the MCB. It might even trip with a long shower.

 

[JohnW2]

So these lies we are told about the UK voltage rear their dangerous head again. He could have over 250v, so using your assumption (even though we should never assume) it would draw over 37 amps. Not good for the MCB. It might even trip with a long shower.

I've often expressed the view that the regs are wrong in requiring calculations to be undertaken at an arbitrary voltage within the 'permissible range', rather than in the 'minimum permissible' or 'maximum permissible' supply voltage (depending upon what we are calculating. Relatively recently, they have )(almost) corrected the decades-long anomaly in relation to fault protection (by introducing 'Cmin'), but the whole of the regs is full of similar, ongoing, anomalies.

However, for better or for worse, that's what the regulations require. However, it's nothing to do with any discussion about what 'within-range' figure (e.g. 230V or 240V) one uses but, rather relates to the fact that, in electrical (and 'safety') terms, all calculations really should be done using one or other (depending on which calculation) of the extreme ends of the permitted supply voltage range (i.e. currently 216.2V or 253V in UK).

In passing, in the days before 'Cmin', one would have put a higher proportion of householders at theoretical risk by determining the adequacy of their fault protection on the basis of 240V, rather than 230V. It really should be 216.2V, and Cmin has got almost there by effectively requiring the calculation to be done for 218.5V

Kind Regards, John

 

[amfisted]

Many thanks, John. Part of my reluctance to get this job done is down to the fact that I've done bits of electrics, I've done bits of plumbing, but never combined the two in one job. Its a step up for me, even if a very minor job for someone else. But I'm reassured by your calculation that the 4mm cabling is unlikely to create any problems with a new shower of the same rating. I have the weekend to consider whether I hit the ground running on Monday, or keep the old shower limping along a little bit longer.

 

[ban-all-sheds]

So these lies we are told about the UK voltage rear their dangerous head again.

It's not a lie winston,

Lying is what you do.

ST*U with your lies, which you keep on spewing out because you refuse to accept what words mean.

 

[bernardgreen]

33.9A at 230V (which is the voltage at which we undertake calculations).

But the worse case situation is not considered when the calculations are done.

The "official" calculations are restricted to using the "nominal" voltage and ignore checking that the current taken when the voltage is nominal +10% is still within the safe capacity of the cable and other parts of the circuit.

 

[winston1]

It is clearly a lie.. To say our voltage is 230 when it has always been and is still 240 is clearly a lie.

 

[Taylortwocities]

Winston, you really are a complete waste of space.

 

[winston1]

You may think that. Many don't.

 

[EFLImpudence]

Nowhere does it or anyone say our voltage is 230.

That is just what we use for calculations - otherwise you have to alter a lot of the tables in 7671.

 

[winston1]

Nowhere does it or anyone say our voltage is 230.

That is just what we use for calculations - otherwise you have to alter a lot of the tables in 7671.

As I have said before the people who say we have to use an incorrect figure for calculations are stupid, mad even.

 

[Taylortwocities]

Obviously Winnie’s hobby horse needs some exercise.
Shall we do transformers today as well, or manufacturers instructions are wrong, or maybe let’s put a 13A socket on a circuit that is designated for lighting.