Wiring an electrical shower

At that length 6mm sq will be fine (voltage drop is only 0.6V)

Given the nominal load is only 39.58A at 240V I would stick with the current MCB.

..except shouldn't the rating be at nominally 230v these days, i.e. 41.3A, or are showers rated at the highest mains value rathter than nominal ?

showers rated at

The nominal rating of the shower will be 230V +10%/-6% although the actual voltage that it will be run on will obviously be 240V in reality.

If you do the sums for 230V then you are still in budget on the cable (assuming it is clipped direct and not in insulation or buried etc) and get a current rating of 41.3A. A 40A MCB will supply that indefinitely. It would also be worth doing a check on the earth loop impedance to make sure you get the required disconnect time etc.

If the cable is in insulation or subject to other derrating factors then an upgrade ought to be in order (or a smaller shower).

Thanks for that - I reached the same figure with my calculations.

The cable run is a follows: 1.5m in a very large cavity (clipped in a space above a false ceiling, some 1m from the original one), and the last 0.5m in a lath and plaster wall with no insulation.

By reckoning it should be ok with 9.5kW. What do you think?

Thanks again.

or are showers rated at

There's usually two ratings. The load is largely ohms law resistive. It will normally be quoted at 240V and 230V. You should plan for the 240V rating. The 230V rating will actually draw less current, so you'll undersize the protection. Normally to 240V rating is the headline figure. On 230V, a 9.5kW shower will be an 8.7kW shower and draw about 38A.

Remember, it won't trip at 40.00001A anyway. There's LOADS of leeway. As long as it is nominally below the rating by calculation, it doesn't matter if factors (i.e. supply overvoltage, element overrating) leads to an amp or two over).

Christian.

or even 10 or 20!

From a current point of view that sounds fine. Same goes for the MCB.

(If you were planning on a larger shower (i.e. 10.5kW or bigger) then replacing the cable would probably be required)

Is the shower going on a RCD protected feed?

Well, but you shouldn't plan for it to above the nominal rating. If the actual turned out to be 10 or 20A over, then your planning was a little deficient, maybe?

Christian.

Yep, 82204 on

I should point out that I was not suggesting that you design with that sort of overload in mind, I was more highlighting that, contrary to popular belief, a 40A MCB will happily supply a 25% or 50% overload for a significant time without tripping.

That saves needing to get into too much detail on earth fault loop impedances and disconnect times then ;-)