Ok iam about to buy 70meters of 25mm swa, i need to buy the gland packs but it has a code on it which one will i need for 25mm? What size mcb in house will i need to buy its got 40amp at min is that ok? If house has rcd the fuse box in barn ok with out one? One last thing what kw shower do u recommend for the size of cable? Cheers guys
buying for job
- Thread starter uklad
- Start date
What does the cable manufacturer say? Not all cables are the same size, so you might need different sized glands depending on the exact cable.
And also:
why don't you just let the electrician get on with it?
No. You'll lose all the circuits, and have to trek back to the house to rest it. See 314.1 & p22 of the OSG.
How can you be asking questions like that, given this:
- ?
The knowledge you got from reading and understanding that information would mean that you would not need to ask.
So either:
1) You couldn't be bothered to read it, i.e. you can't be bothered to acquire the necessary competence. In which case you must stop now and get an electrician.
or
2)You did read it, but didn't understand it, i.e. you are not capable of acquiring the necessary competence. In which case you must stop now and get an electrician.
Before you go head on into this project, why not see if you can find a friendly local spark who is happy to let you get involved and do some of the donkey work while they complete the work and sign it off?
I undertsand your want to get this done as cheaply as possible but installing DIY submains in 25mm² SWA is not really the best place to start.
At least you've accepted the importance of getting the correct cable for the job...
and I would say that for a 70m run you will need more than 25mm SWA if you're putting an electric shower on the end of it!!
Sounds fine to me, although that's just off the top of my head, I haven't checked the volt drop. It's irrelevant, as I don't think anyone is interested in helping the OP any more after his blatant lies.
you don't say if it's 2 core, 3 core or 4 core..
it's one of those borderline sizes..
one chart I googled says that 2 core 25mm SWA will fit a 25mm, and 3 core will fit a 32mm..
usually this means that the 2 core is right on the limit of the 25mm and a different manufacturer from the gland maker could mean it's too tight..
there is usually a chart on the back of the glandpack to tell you what size gland for what size cable..
it's one of those borderline sizes..
one chart I googled says that 2 core 25mm SWA will fit a 25mm, and 3 core will fit a 32mm..
usually this means that the 2 core is right on the limit of the 25mm and a different manufacturer from the gland maker could mean it's too tight..
there is usually a chart on the back of the glandpack to tell you what size gland for what size cable..
please find a tidy spark to help you with this.
You've got this wrong uklad.
Your electrician will work out what you need. He won't start with a cable and work out what goes each end.
Youe calculate the load, this gives you the MCB size and form this you work out the size of cable required.
You may be lucky, in that the cable may be the right size. leave it to the spark.
Your electrician will work out what you need. He won't start with a cable and work out what goes each end.
Youe calculate the load, this gives you the MCB size and form this you work out the size of cable required.
You may be lucky, in that the cable may be the right size. leave it to the spark.
I did a job a few months ago
9Kw shower
2kW downflow
28w 2D light
SELV extractor fan
All fed off a 35m 25mm 3 core SWA submain and an 800KMF with a 50A fuse fitted (TNS supply)... and it only just passed the calcs!
I assume volt drop was not the limiting factor? From table 4D4B, the volt drop on 25mm 3/4C 70degC SWA is 1.5mV/A/m. According to appendix 4.6, we would ideally factor in inductance when conductors >16mmsq are involved, but using the basic resistive (r) values from 4D4B will, if anything, lead to a more conservative answer than if they were factored into the equation.
So, Ib is in the region of 11,000W/230v = 47.8A.
47.8A * 35 metres * 1.5mV/A/m = 2.5v
So, I can only assume the issue was with calculated loop impedance or touch voltage? Certainly, in terms of volt drop, 25mm is a far larger conductor than necessary for the installation you describe above.
EDIT: I've thought about this some more, and would be very interested to see your calculations, because by mine, your circuit would be well within limits for EFLI. The following are my calcs on the assumption that the 3rd core was used as earth:
Distribution circuit, 50A BS88 fuse, 5s required disconnection time. Maximum permitted Zs from table 41.4 = 1.04 Ohms
TNS supply, assumed worse case Ze = 0.8 Ohms
Maximum allowable (R1+R2) = Max permitted Zs - Ze = 1.04 - 0.08 = 0.24 Ohms
From OSG table 9A, (R1 + R2) of 25mmsq copper line + CPC conductors @ 20degC = 1.454 mOhms/metre
From OSG table 9C, temperature correction factor for PVC cable is 1.2, therefore maximum allowable (R1 + R2) = 1.454mOhms/m * 1.2 = 1.74mOhms/metre
For 35 metres of cable, R1 + R2 including correction factor will be 1.74mOhms/metre * 35 metre = 0.0609 Ohms
So, Zs at the DB where the end of your 35m run of SWA terminates will be Ze + (R1 + R2) = 0.8 + 0.0609 Ohms = 0.8609 Ohms
In any case, this is clearly well under the maximum 1.04 Ohms allowed for your protective device.
EDIT 2: Ok, I'm an idiot - completely forgot to apply x0.8 for measured Zs values. Point taken, I can see how it only just passed. However, I'm not sure x0.8 applies here, as the CPC is not of a reduced CSA. The installation could still be acceptable under the alternative method described in appendix 14.
Can you please elaborate? I've not heard the x0.8 rule before.
The Zs figured quoted in BS7671 do not include any correction factors for increase in conductor temperature under fault conditions. Therefore, it is stated in both the OSG and appendix 14 of the regs, that the measured Zs during testing should be no higher than 0.8 x calculated/tabulated Zs.
Of course, appendix 14 does also give other methods of compliancy when the 0.8x rule cannot be adhered to, but they involve a little more effort and calculation and you're better off applying the 0.8 rule of thumb where possible.
In the OPs case, his 'electrician' will still be able to use the cable purchased, but 'he/she' may need to consider alternative methods of fault current protection to a fuse/MCB alone for the submain in the event that the installation already has a particularly high Ze.
The 80% (or divide by 1.24) comes from the difference in resistance between the conductors at the temperature when they are tested (i.e. 10ºC) and their maximum operating temperature (70ºC).
The maximum measured impedance should not be more than 80% of the maximum tabluated value in BS7671.
Where a conductor is being used at 90ºC different numbers need to be used, i.e. divide by 1.32 or multiply by 75%.
Where the conductor temp is not 10ºC then additional corrections need to be taken into account.
Have a peep at the last few pages in guidance note 3.
The maximum measured impedance should not be more than 80% of the maximum tabluated value in BS7671.
Where a conductor is being used at 90ºC different numbers need to be used, i.e. divide by 1.32 or multiply by 75%.
Where the conductor temp is not 10ºC then additional corrections need to be taken into account.
Have a peep at the last few pages in guidance note 3.
Thanks fellas.
You'll be glad to know I don't do design by profession, only act as the client.
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