Help with corrct MCB Rating on shower upgrade

[Maus]

Hi,

Hope you can help.

I want to upgrade an existing shower installation to a 9.8kw unit. This requires a 45 Amp MCB to replace the current 40 Amp.

The current installation connects from the CU with a 40a Mcb to the shower via the DP pull switch using 10mm2 cable.

The cable runs through trunking and from the CU to the DP switch, and I believe is chased from the switch to the shower unit.

Wylex (who manufacture my CU) do not make the reccommended 45a MCB, only 40 & 50.

Will the 50a MCB be too high a rating given the cable size and routing etc?

Should I be looking for another compatible make of 45a MCB? If so any suggestions on what compatible?

This is the CU installed.
http://www.tlc-direct.co.uk/Products/WYNHRS9SL.html

Many Thanks

 

[JohnD]

If I were you I'd run it on the existing MCB for a while, I doubt it will trip.

Amps load is probably about 42.6 depending on how they measure it and what the voltage is in your area at the time of day you have your showers.

Does the documentation say how many Amps it draws?

 

[sm1thson]

and it depends what voltage they specify the power at.

if its 9.8kw @ 240V and you supply it 230V (what the county is suppose to run on these days) then it will only use 39.13A, or if your voltage is 238v (national average) then it will still only use 40.49A.

 

[davy_owen_88]

The stated shower ratings are almost always given in both 240 and 230V. At 240V your going to be drawing 40.8A - the MCB will take a very long time to trip under this tiny overload, far longer than you will be in the shower.

But if you find it does start tripping, 10mm² cable in trunking on a wall is rated for 52A so assuming the trunking is of a suitable size and there are no other derating factors (insulation, bunching etc) you could upgrade to a 50A MCB, but do as JohnD says and try it first to save yourself the hassle.

 

[Maus]

If I were you I'd run it on the existing MCB for a while, I doubt it will trip.

Amps load is probably about 42.6 depending on how they measure it and what the voltage is in your area at the time of day you have your showers.

Does the documentation say how many Amps it draws?

Thanks for the reply. Was also tempted to run it on the existing to start. Cant see it will draw the full load unless it's cranked up to full temp, which is unlikely.

The documentation states:-


9.8kw @ 240v
9kw @ 230v

It says "The 9.8kw model requires a 45 amp fuse".

Thanks

 

[sm1thson]

Cant see it will draw the full load unless it's cranked up to full temp, which is unlikely.

actually typically they have a cold setting and 2 heat settings, then to control how hot it is the flow of the water is restricted so it gets hotter. so it will usually be on full power when in use, but that’s ok as a 40amp breaker is designed to take 40 amp constantly at 60 amp it will blow in 1 minute, at 45 amp it will take something like 50 minutes (well you cant give an exact value in this region so call it 40 mins to 2hours) and at 40.8A it should be will over an hour, if ever. so you should be fine, try it and see!

9.8kw @ 240v
9kw @ 230v

=39.13A@230V as predicted

 

[Steve]

They ought to put a warning in electric shower installation instructions, along the lines of

"THIS ELECTRIC SHOWER DRAWS ITS FULL CURRENT ALL OF THE TIME IT IS OPERATING, REGARDLESS OF THE WATER TEMP SETTING. DESIGN ITS CIRCUIT FOR FULL LOAD"