Shower cable size calc

[sparky05084]

Hey all. Hope everyone is well.

Ok, I'm currently doing a rewire on a ground floor flat. The entire job will be done in trunking and I have an apprentice with me. Now, my intention is to run a 10mm cable to the shower as it's 9.5kw (I do this as a rule of thumb).

This is also in our company spec that regardless of situation, a new shower install is supplied via 10mm T + E. I am helping the apprentice do his colleague work and he is currently learning about maximum demand and diversity. He asked me how I come to the conclusion that it needs to be wired in 10mm. Well, got the brain ticking and I attempted to do a cable calc there and then but had a "brain fart"

I explained that we need to work out the design current (Ib). To establish this we divide power by volts and we get the maximum demand (please correct me if I'm wrong). So, this gives me aprrox 41A. Now, I believe we then need to select an appropiate protective device size ≥ of our obtained value.

This is where, well, I kind of fall flat on my face and I'd greatly appreciate some help. To give you an idea of the situation the length of run is approx 5m and it will be run throughout it's length in trunking but alone so as to save space.

I don't think any other factors apply. I basically would like the calculation of how to work it out and the answer and then I can not only explain to the apprentice how we worked it out but also give him some literature for study.

Don't worry guys I won't claim it I will say I asked the experts. Thanks :)

 

[GLENNSPARK]

well for a start theres no diversity allowed for instantainious water heaters....which this is.......

 

[sparky05084]

Well, theres my first mistake *embarrassed look on face*

Ok then well could you please help with the calculation with regards to MCB size and cable size etc.

 

[GLENNSPARK]

a 40A type B will suit here......manufacturers are somewhat "generous" with the current draw of their products.......install,..test....then stick some juice in it.....get it clamped.....see what it really pulls.....noware near the 41.3A that the design calc gives....it will probably never be used at its capacity anyway...and in fact a 6mm cable would be ample here as well....but theres this thing about "future proofing" stuff like this.....but come on.....a 9.5KW shower is more than enough for the avarage sized family......

 

[johnboy6083]

you have already worked out Ib, and not you need to select In, the rating of your protective device. This must be equal or greater than Ib. You now need to derate yopur cable. Appendix 6 of the OSG is a great place to read up on this, and itys laid out in simple terms. This value with the factors taken into account is called It. now you need to select a cable that is capable of taking It. Once you have this cable, you then need top work out volt drop (mV/A/m x Ib)/ 1000. Youn then need to carry out an adiabatic check on the CPC, unless you comply with table 57G (i think from memory).

 

[nickblake]

Installation methods will also be required if the cable is covered or surrounded by insulation then the current carrying capacity of the cable is reduced check table 2.3.1 of the electricians guide to the building regs it is in the onsite guide and thats in my van and as its -5 out there at the mo there it stays ,look im a wimpy electrician lol

 

[Richard Burns]

Ib <= In <= It(Iz)

Ib: design current of load, with diversity allowance if applicable, in your case for 9,500W shower 41.3A

In: current rating of protective device theoretically 50A in this case because it is a short load time and the MCB will not trip for over an hour at 1.5 x rating you can use 40A.

Iz: current rating of cable, 10mm2 multicore cable has 70A in free air
It: current rating of cable with installation methods derating Iz. In your case 10mm2 multicore enclosed in surface trunking, not grouped, not in insulation, not at high ambient temperature, not in the ground, not on a BS3036 has CCC 52A.

Ib 41.3A <= In (40A) 50A <= 52A

 

[GLENNSPARK]

you have already worked out Ib, and not you need to select In, the rating of your protective device. This must be equal or greater than Ib. You now need to derate yopur cable. Appendix 6 of the OSG is a great place to read up on this, and itys laid out in simple terms. This value with the factors taken into account is called It. now you need to select a cable that is capable of taking It. Once you have this cable, you then need top work out volt drop (mV/A/m x Ib)/ 1000. Youn then need to carry out an adiabatic check on the CPC, unless you comply with table 57G (i think from memory).

i can understand the idea behind this post John.....and agree......but VD...over 5 meters?

 

[GLENNSPARK]

Ib <= In <= It(Iz)

Ib: design current of load, with diversity allowance if applicable, in your case for 9,500W shower 41.3A

In: current rating of protective device theoretically 50A in this case because it is a short load time and the MCB will not trip for over an hour at 1.5 x rating you can use 40A.

Iz: current rating of cable, 10mm2 multicore cable has 70A in free air
It: current rating of cable with installation methods derating Iz. In your case 10mm2 multicore enclosed in surface trunking, not grouped, not in insulation, not at high ambient temperature, not in the ground, not on a BS3036 has CCC 52A.

Ib 41.3A <= In (40A) 50A <= 52A

thank you MR Burns.

 

[Guest123]

http://www.electriciansforums.net/e...al-regulations/47059-cable-calc-template.html


Might be good for your apprentice.

 

[wildgoose]

I'm assuming you have a copy of the 17th Edition On-Site Guide?
Look towards the back and reference the tables against the fuse type, installation method and cable capacity.

If you don't own a copy of the OSG, then I suggest you buy one - Defo well worth the money.

 

[i=p/u]

id love ya to install a shower at my house,lol at lessty you making effort to find out..