replacment shower

[Garibaldi]

very old triton T80 7kw shower,being fed by 6mm cable prob running along joists and plas in wall.30amp wylex type MCB and CU has ELCD built into it,Looking at replacing with triton Zante 8.5kw.Is this possible ,or will the cable and MCB not take increase in KW

 

[ban-all-sheds]

very old triton T80 7kw shower,being fed by 6mm cable prob running along joists and plas in wall.

You can't do this on the basis of "probably"...

30amp wylex type MCB and CU has ELCD built into it,Looking at replacing with triton Zante 8.5kw.Is this possible ,or will the cable

It might.

Depends on how it's installed.

and MCB not take increase in KW

Is 8500 ÷ 240 more than 30 or less than 30?

 

[Garibaldi]

Thankyou for the reply Ban-all-sheds, Ok so I will say the cable is definitely running in plaster un insulated wall and running along joists,as to wether it is bunched with other cables at some point ,I dont know? But if for arguements sake we say it is, would that mean it is unsafe to install 8.5kw shower.The answer to the sum you gave me is more than 30, excuse my ignorance , I`m guessing that means the 30 A MCB is not suitable. and am I write in thinking that you should definitely not up the MCB? All in all making this job a no no.

 

[Softus]

'Scuse me butting in, but I'd like to test my own rudimentary understanding of what's going on here.

If one took a pessimistic view of your 6mm^2 cable cable run, such that it were classed as running all the way "in conduit in thermal insulation", then it would be rated at 34.0A. An 8.5kW shower would require 35.4A at 240V, so the cable is not adequate for sustained operation.

10mm^2 cable, in the same conditions, could sustain 46.0A, which is enough for your shower.

I don't know how to determine the correct breaker for this scenario, but clearly it would have to be rated at more than 35.4A and less than 46.0A.

I've also seen mention of considering the length of the cable run to determine the voltage drop from supply to appliance. However, I don't claim to know what the permitted, or advisable, limit is on the amount of the drop.

Have I got this right?

 

[Spark123]

'Scuse me butting in, but I'd like to test my own rudimentary understanding of what's going on here.

If one took a pessimistic view of your 6mm^2 cable cable run, such that it were classed as running all the way "in conduit in thermal insulation", then it would be rated at 34.0A. An 8.5kW shower would require 35.4A at 240V, so the cable is not adequate for sustained operation.

10mm^2 cable, in the same conditions, could sustain 46.0A, which is enough for your shower.

I don't know how to determine the correct breaker for this scenario, but clearly it would have to be rated at more than 35.4A and less than 46.0A.

I've also seen mention of considering the length of the cable run to determine the voltage drop from supply to appliance. However, I don't claim to know what the permitted, or advisable, limit is on the amount of the drop.

Have I got this right?

Sort of, if the conduit is run directly in thermal insulation for over 500mm it will be subject to a 50% correction factor so a 6mm multicore cable in conduit installed directly in insulation for over 500mm is rated 19A before other corrections, (the figures in table 4D5A for method 15 are where the conduit is in contact on one side with thermal insulation.) There may also be other de-rating factors to consider such as the use of semi-enclosed rewirable fuses, ambient temp and grouping factor.
The volt drop limit for a 240v system is 4% which is 9.6v.

 

[Adam_151]

'Scuse me butting in, but I'd like to test my own rudimentary understanding of what's going on here.

No probs

If one took a pessimistic view of your 6mm^2 cable cable run, such that it were classed as running all the way "in conduit in thermal insulation", then it would be rated at 34.0A.

Unfortunatly a pessimistic view would result in a much lower capacity than this, thats just reference method 15 (IIRC) which is running through a loft or plastered into a wall, if I bunced it with loads of others I might have to multiply it by a derating factor see: http://www.tlc-direct.co.uk/Book/4.3.5.htm

There are factors for grouping, thermal insulation (if I bury cable in insulation all around it for more than 0.5m it looses half its capacity!), and ambient temp

An 8.5kW shower would require 35.4A at 240V, so the cable is not adequate for sustained operation.

True, but it will work in a bodged type of way for years and years FWIW

10mm^2 cable, in the same conditions, could sustain 46.0A, which is enough for your shower.

Yup, very few occasions when 10mm wouldn't be suitable

I don't know how to determine the correct breaker for this scenario, but clearly it would have to be rated at more than 35.4A and less than 46.0A.

a B40 would be ideal

Thats it, you just have to check that the Z's at the end of the cable are low enough to trip the magnetic part of the breaker (off the top of my head 230/(40*5) approx 1.15 ohms)

I've also seen mention of considering the length of the cable run to determine the voltage drop from supply to appliance. However, I don't claim to know what the permitted, or advisable, limit is on the amount of the drop.

Allowed limit is 4% of 230 =- 9.2v, volt drops can be got from tables and they are stated as millivolts per metre, per amp http://www.tlc-direct.co.uk/Figures/Tab4.6.htm

 

[Garibaldi]

thanks to all of you for your comments, but now I`am totaly confused The cable run is no more than 15mtrs,very old stone cottage,walls not insulated.

 

[ban-all-sheds]

Given the length you can work out the volt-drop from that table on the TLC website....

 

[Garibaldi]

Have visited suggested site even more confused now Web site is for average electrician,never claimed to be this,think I better get a sparkie in,I know new regs dont allow average jo blogs to carry out this type of work anyway,but the way things are going you`ll need to get a sparkie in to plug the tele in Just wanted a quick answer so I could rush out and buy this really cheap shower and fix it in myself ,starting to smell abit now thanks to all who replied

 

[Spark123]

Thats it, you just have to check that the Z's at the end of the cable are low enough to trip the magnetic part of the breaker (off the top of my head 230/(40*5) approx 1.15 ohms)

240/(40*5)=1.2ohms (design), 1.2*0.8=0.96ohms (measured@10deg C)