Cable Calc Question

[jdes]

Hi,

I'm hoping someone can clear something up for me.

I'm about to install an electric shower in my own house. I previously ran a 10mm cable in when re-wiring but now trying to decide how powerful the shower can be. For the sake of this calculation I'll use a 10.5kw shower.

the cable run consists of roughly: 6 meters under the floor (not insulated)

                                                 2.5 meters up 2x2inch trunking with 3 other cables (socket radial, smoke, lighting cables)

                                                 8 meters in loft below 100mm insulation

                                                 1 meter clipped direct behind tiles/ plaster

so to begin:

10500/230 = 45.7a

MCB should be 50a

Now its the rating factors i'm getting a bit confused with.

I'm assuming ambient temperature is 30 so.....................Ca is 1

Looking at table 4c1 in regs...........................................Cg is 0.65

I cant find any rating factor for the Ci. can only find table 4D5 which says 10mm cable can take 45a for method 100 (under 100mm insulation)

BS60898 mcb being used so no rating factor there?

At this point i'm getting a bit confused with how to work out. Do i actually need to apply a rating factor for Ci???

I know i could probably get away with using a 40a MCB and expect the shower not to exceed 40a but I was hoping to use this job as an assessment for NICEIC DI and would like to justify my actions much as possible.

I know i should know this but never really had the need for cable calcs working for other people all my career! And never was good at it at college either.

thanks

 

[Atty]

Hi

I'll try and help...

523.9 (OSG p141)

For a cable installed in thermal insulation as described in Tables F4(i), F5(i) and F6 no correction factor is required (Ci=1)

Table F6

Table 4D5 reference method 100 (above a plasterboard ceiling covered by thermal insulation not exceeding 100mm in thickness) also requires the cable to be in contact with the plasterboard ceiling, wall or joist.

Using a BS60898 so Cf = 1.

Read OSG p68

Grouping of circuit cables. (Cg)

Hope this helps

 

[binky]

personally i would lift cable above insultion. However that's not what you are asking. The insulation is clearly biggest issue in derating cable, and is the part of the run you need to focus on as this is the worst case scenario. If table says cable can carry 45A under the condtions of installation described, then IMHO you can work with that. 45A MCBs are available, MK do one,and will probably cope perfectly well with the 45.7A load calculated at 230V - the point is correctly protecting the cable, and not so much the loading. But you will have to be wary of adding further insulation to loft (270mm is now recommeded), so i think I would go back to lifting cable above insulation, which means you could use easily available 50A MCB and never have to worry.

 

[SPECIAL LOCATION]

so to begin:

10500/230 = 45.7a

I know i should know this but never really had the need for cable calcs working for other people all my career! And never was good at it at college either.

thanks

Just for the record...

If 10.5kw is the manufactures definition then with is most likely quoted at 240v NOT 230v

as almost all electric shower ratings are still quoted ref 240v...

and..  The actual real supply voltage is going to be nearer 240v than 230v anyway!!!

Possibly not a good omen not been good at cable calcs or college if looking at designing circuits...

May be good to get a few old books and sample papers to re-cap a few calcs to get yourself a bit more up to speed...

Agreed 100% with Binky about the cable under the insulation...

WHY not move it above the insulation???????

:C

 

[jdes]

Just for the record...
If 10.5kw is the manufactures definition then with is most likely quoted at 240v NOT 230v

as almost all electric shower ratings are still quoted ref 240v...

and..  The actual real supply voltage is going to be nearer 240v than 230v anyway!!!

Possibly not a good omen not been good at cable calcs or college if looking at designing circuits...

May be good to get a few old books and sample papers to re-cap a few calcs to get yourself a bit more up to speed...

Agreed 100% with Binky about the cable under the insulation...

WHY not move it above the insulation???????

:C

Thanks all.

I was planning on moving it above the insulation but wanted a discussion on it to get a better understanding so thanks for the responses.

I was going through old college notes before posting but found them a bit lacking in certain areas. Don't worry i have identified this as something that I need to improve...I'm sure we all have some shortcomings.