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Armoured cable sizing

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Armoured cable sizing

Postby the bear » 25 Nov 2017, 18:49

! wouldn't normally ask this here but I''m having problems getting hold of my electrician, I have a feeling he is on holiday, and I need the answer.

I'm currently renovating the house and have a trench open that goes from the house right past the detached garage. This garage is destined to be my new workshop if I ever get my machines out of storage. The current supply is way too small for the machines I have so this is an ideal time to upgrade the armoured cable even if it sits unconnected for while. Eventually I will install and connect it to a new consumer unit in the garage. The rush is due to the fact it was -6C here last night and part of the trench also carries the water main. With the overnight temperature I want to get the trench filled in asap to prevent it freezing and bursting.

Everything is single phase. The biggest woodwork machines I have are 3hp (2200kw) I will only ever use one at once but will also have dust extractors running at the same time. The main extractor is also 2200kw and when I run the table saw I also have a 1800kw extractor running as well. (prob the only time I run both extractors together). In addition there are the lights, power tool batteries charging, radio, a fridge freezer and I will in future use this supply to provide power to the garden for lights and hopefully one day electric gates.

Distance from house consumer unit to garage consumer unit is about 30m, but maybe up to about 40 if I put the garage unit where the current small one is.

Any sparks, electrical engineers, competent persons know what size cable I need?

I addition I'm considering dropping an armoured cat 5/6 cable in as well for future use. Is it ok to put in same trench as the supply or are there interference or any other issues?

Thanks

Mark
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Re: Armoured cable sizing

Postby 9fingers » 25 Nov 2017, 19:08

Use the online calculator on the TLC website. I has all the regulations built in.

To get the the right current need, find the highest motor load and ignore that to start with and work out the maximum of the things like to be on simultaneously then add the starting current for that highest load item.

hth

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Re: Armoured cable sizing

Postby the bear » 25 Nov 2017, 19:45

Bob

I've tried using the TLC calc as you suggest but unsure of your explanation. It requires the total kw not current (amps?) Biggest machines are all 16amp and 2.2kw, extractor is 16amp 2.2kw, other extractor is 13amp 1.8kw. One machine plus both extractors is most I will turn on at once. Running in back ground is lights, battery charging, fridge, outside lights, possibly elec gates.

From what I've put in I read i can use a 6mm cable as long as the total power of everything on is no bigger than 9kw? More than 9kw I need a 10mm cable?

Mark
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Re: Armoured cable sizing

Postby 9fingers » 25 Nov 2017, 20:45

Firstly you have stated non lighting in your use of the calculator but you will be having lights on the circuit?
This tightens up the voltage drop requirement.

You need to consider the the starting load of the biggest motor. a 3hp planer for example could easily draw 40-50 amps which is why I suggested the calculation method.
Once the current is calculated in this way, use of the approximation of 4amps per kW is reasonable - I'd forgotten the calculator asks for kW :oops:

You might consider it excessive to rate the cable for the starting load but the voltage drop under these high current condition tend to be the root of most single phase starting problems as the start up takes too long and starter windings and capacitors can get damaged.

Dont forget to protect the supply with a type C Breaker in the house CU or via a Henley block in the feed from the meter.

HTH
Bob
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Re: Armoured cable sizing

Postby RogerS » 26 Nov 2017, 12:25

Forget all the calculations.

Over that distance 4mm isn't man enough.

50 m of 6mm from TLC-Direct will cost you £81 + VAT

50 m of 10mm will cost you £109 + VAT.

For the extra dosh, I'd stick in 10mm and be done.
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Re: Armoured cable sizing

Postby Deejay » 26 Nov 2017, 16:35

Afternoon Mark

I agree with Roger about the size. If you put the load up to 10 KW, the calculator suggests 10 sq mm cable.

IIRC, the drop must be less than 5%, so with 6 sq. mm cable you'd be very close to it.

As far as screening goes, the armour forms a metallic skin round the UTP within. Connect one end of it to an earth.

Put the cables on the opposite sides of the bottom of the trench.

Cheers

Dave
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Re: Armoured cable sizing

Postby DaveL » 26 Nov 2017, 20:31

RogerS wrote:Forget all the calculations.

Over that distance 4mm isn't man enough.

50 m of 6mm from TLC-Direct will cost you £81 + VAT

50 m of 10mm will cost you £109 + VAT.

For the extra dosh, I'd stick in 10mm and be done.

This is the sensible option, think about having to dig the trench again if the the smaller cable gives problems and therefore needs to be replaced. You will of bought both sizes of cable and dug the trench a second time, not a good option.

This is one of the reasons when data cables (both copper and fibre) are installed, there is always spare capacity, the cable is relatively cheap compared with the cost of installing another cable.
Regards,
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Re: Armoured cable sizing

Postby the bear » 27 Nov 2017, 18:03

Thanks all
I purchased the cable this morning, its in and partially backfilled, along with the water main.
Despite telling me they had it in stock, when I got there they had run out of 3 core so gave me 4 core for the same price. Is there anything I can use the fourth core for?

Mark
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