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Twenty years ago, electric showers drew less current than they do now, but I fitted a 10mm T&E cable as my Dad, who is a now-retired electrician, advised me to, as it would future-proof the bathroom. Only problem is, it goes to a Wylex CU which takes 40Amp max MCBs. so I now have to unbury the cables so I can put a bigger CU next to it.

is this a statement?

Your dad is right, a 10mm cable will carry enough current for a larger output shower!

Some pics would be good

Mooncat wrote:Twenty years ago, electric showers drew less current than they do now, but I fitted a 10mm T&E cable as my Dad, who is a now-retired electrician, advised me to, as it would future-proof the bathroom. Only problem is, it goes to a Wylex CU which takes 40Amp max MCBs. so I now have to unbury the cables so I can put a bigger CU next to it.

Hi Mooncat. Are you saying that you need to unbury the cable so you can move it from the Existing Wylex CU into another CU which will accept an MCB larger then 40A because you need to have a >40A MCB?

If you take UK voltage as a nominal 240V which it is still is for just about everyone even though it is often referred to as 230V, (mine is consistantly 246V in fact), then doing the maths, a 9.5Kw Shower will draw 39.5A at full power, so I would have thought that a 40A MCB would be fine for your needs.
I know that Wylex have stopped the B45 MCB and the B40 is being suggested as the preferred replacement.
Even if you do draw slightly over 40A, a 40A MCB will not trip unless the overdraw is maintained for a fair amount of time.

Also, when you say "unbury the cable", a 10mm cable buried in plaster has a recommended max rating of 43A I think, and using an MCB larger then the the cable rating would not be a good thing, so it seems to me that a 40A MCB is spot on


Be worthwhile getting an opinion from one of the sparks here before you start rewiring - you may be able to save yourself some work

My thanks to all. Looking at the shower unit it is rated at 9.8KW not 9.5KW, indicating a current of 42A. So it MIGHT be OK. The last couple of metres of cable are in a plastered, thick stone outside wall. I'll have to leave that end til last as I can't have two rooms in pieces.

I have two of the Prodigit Electronics mains monitors, one shows 256V, the other 225V, so one or both are faulty.

Mooncat wrote:... Looking at the shower unit it is rated at 9.8KW not 9.5KW, indicating a current of 42A. So it MIGHT be OK.....

On this one, I am hoping one of the sparks will comment when they are here, but I underatand that a MCB is actually expected NOT to trip for approximately an hour of use (the 'conventional time') if the current draw is no more then 1.45 x the MCB rating

So in other words, if you draw upto 1.45 x 40A (i.e. ~55 Amps) on a circuit that has a 40A MCB for say 15 minutes, the 40A MCD will not trip.

Sounds surprising, doesn't it, so hopefully a sparks will confirm/advice as appropriate

Mooncat wrote:I have two of the Prodigit Electronics mains monitors, one shows 256V, the other 225V, so one or both are faulty.

Have you tried swapping them over which each other? or run them side by side? It would be a hell of a voiltage drop on a cable, but never know!

Interestesting previouse post Hoovie ( 1 +.5 btl's port!, worsed for where! ) but I woud be very interseted to know, if the con's unit is one of those that will take take a slighty shorter fuse? you know, the ceamic coated jobbies?

If so? I can well belive it, the "Idian" buddies of mine of mine have had a simmilar prob, but with a 5amp lighting curcit, the ballast or transformer in a flou fitting over loaded and partialy melted.

Would be interesting to get a pro elec's oppinion?

Hoovie wrote:a 10mm cable buried in plaster has a recommended max rating of 43A I think

64A, actually....

ban-all-sheds wrote:

Hoovie wrote:a 10mm cable buried in plaster has a recommended max rating of 43A I think

64A, actually....

I based the info on this table...

1) That's not the right table for twin & earth - you should be looking at 4D5A.

2) Even if that were the right table you're looking at the wrong column. The latest version on p222 of the regs has more descriptive detail, and for Installation Method 1, which is clipped direct, OR, if you look at Table 4A1 on p210, sheathed cables embedded directly in masonry, brickwork, concrete or plaster, 4D2A shows 63A. The figure you're quoting of 43A for "enclosed in a wall" is actually for Reference Method 4, which is enclosed in an insulated wall....

Thanks for the update and correction

i just read through this entire post and now my head hurts

Probably best if you steer clear of electrical work then...