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Armoured cable - what size would you reccomend

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dave train20/03/2017 17:45:51
22 forum posts

I want to power a shed with armoured cable. It will have a small lather and milling machine plus a pillar drill. It will also have 3 strip lights in it and its about 30m from the house.

Looking on ebay its seems you can get armoured cable from 1.5mm to 12mm so which one for me?

Brian Wood20/03/2017 18:04:51
1256 forum posts
34 photos

Hello Dave,

​I wired out the supply to my workshop with 6mm armoured cable. The total cable distance from the consumer unit and destination is comparable to yours and voltage drop calculated at under 5 volts. Remember to buy 3 core cable so that you have a copper cored earth, the armour is steel sheathing and not really good enough for a reliable earth. and will get failed if you put it up for test certification.

​You will also need the appropriate size of glands at each junction point.

​Regards
Brian

SillyOldDuffer20/03/2017 18:17:19
2150 forum posts
467 photos

I hope a proper electrician will advise on the details, but this online calculator suggests 4mm armoured for a 5kW load, (about 21A) over 30 metres. You need to add up how many watts you expect to burn in your shed, I guessed:

  • lighting 300W
  • Pillar Drill 700W
  • Small Lathe 1000W
  • Milling Machine 1000W
  • Radio, grinder, saw, etc perhaps another 1000W

You probably wouldn't have it all switched on at once but I'd still use 4mm to reduce the voltage drop.

Regulations apply if the circuit is permanently connected.

Edit : I see Brian got in first recommending 6mm for a 5V drop.  In comparison 4mm would drop 7V with a 5kW load.  If money is no object, buy the bigger cable.

Dave

 

 

 

Edited By SillyOldDuffer on 20/03/2017 18:20:41

Antony Price20/03/2017 18:19:36
15 forum posts
11 photos

its likely that volt drop will be the deciding factor, rather than current rating. There are a number of volt drop calculators on the www. I’ve found this one.

**LINK**

Assuming 10 kW load, and 30 metres, assuming 240 volts incoming at the meter position, it recommends 6 mm cable, for a 10Volt drop . For 6 kW the cable size can be reduced to 4mm.

To check your typical load, look at the rating of the motors on your lathe and milling machine, and pick the largest . Just a note of caution, the larger the volt drop, the bigger the “dip on the lights” when the motor starts, which can be annoying.

My recommendation would be to go for a minimum of 6mm, especially if you are digging a trench to bury it underground

Tony

Mark Rand20/03/2017 19:46:46
170 forum posts

The wiring regs call for a maximum 2% voltage drop on a circuit at its maximum load. So use that for the calculations.

I used 16mm^2 three phase + earth SWA cable over a 30m run, based on wanting to get a 3 phase supply and wanting to be able to run my 200A oil cooled welder. Haven't got 3 ph, because the cost of getting it installed was steeper than I liked and I got an excelent deal on a 23kVA inverter that feeds the entire shop.

Remember, if you get flickering lights, you'll regret it more over time than you'll regret the extra cost of going up a wire size or two.

Andy Ash20/03/2017 20:06:39
124 forum posts
33 photos
Posted by Brian Wood on 20/03/2017 18:04:51:

Hello Dave,

​I wired out the supply to my workshop with 6mm armoured cable. The total cable distance from the consumer unit and destination is comparable to yours and voltage drop calculated at under 5 volts. Remember to buy 3 core cable so that you have a copper cored earth, the armour is steel sheathing and not really good enough for a reliable earth. and will get failed if you put it up for test certification.

​You will also need the appropriate size of glands at each junction point.

​Regards
Brian

I'm not sure your advice about not using the armour as a protective earth is quite right there.

If you use SWA, it would certainly be bad practice not to use the armour as an earth conductor. It is very much more than sheathing. Indeed it is the very definition of an electrical protective earth. It is virtually impossible to damage a phase conductor without first cutting the protective sheath. The very act of encircling the phase conductors with the earth offers a method of electrical protection and essentially guarantees fast circuit breaker action if the cable is accidentally cut.

There is much uncertainty out there on this matter, so lack of awareness can be excused. One thing is certain, poor implementation will not be excused by mother nature under fault conditions.

People have generally noticed that corrosion can set in at glands and where the outer plastic sleeve becomes damaged. This can compromise the earth bond, and provide less than adequate protection.

Some recommend using an additional copper earth conductor inside the armour, others an additional copper earth conductor outside the armour.

Both schemes have problems;

1) If the armour is electrically damaged then neither scheme can guarantee to expose the earth conductor to the phase if the cable is cut. If the digger bucket scrapes along the length of the conductor then phase can be exposed without shorting to the protective earth. One is dependent on soil resistance to operate the circuit breaker in this situation, and it is not good.

2) If the parallel wired copper earth conductor leads an electrical test operative to believe that the armour is intact during test, then no-one may know that the armour has actually become damaged.

In conclusion; if the armour is not good enough to do the job because of corrosion, then it has not been monitored and maintained properly.

In most cases there is no reason why a steel wire armour cannot perform the function of a protective earth. If the calculations show that the armour can trip the fault circuit in the prescribed time, then my opinion is that it is safer only to use the steel wire armour.

Like all engineered installations it is important to periodically check and maintain equipment to ensure safety.

Edited By Andy Ash on 20/03/2017 20:13:54

dave train20/03/2017 21:03:45
22 forum posts

Phew, good reading and thanks for the replies.

So if not wanting to go too mad 4mm will be enough, like you say I will never have everything on at the same time.

I assume the electrician will but 3 core cable, will he not just use the earth wire withing the cable?

Total watts would be:

Lights 100

Pillar drill 550

Mill 1100

Lathe 1500

Grinder/Radio/Drill 500

Letrs say 4kw total

Using the calculator in the link based on 30m its saying 2.5mm, however to buy 4mm is only an extra £12 so worth it

Emgee20/03/2017 21:11:30
648 forum posts
151 photos

Hi Dave

If the shed is 30metres distant from your house the length of cable needed will be greater, allow 2.5M at least to come from the trench to the distribution board in the shed and then whatever from the entry point in the house to the source of supply at the metering position, the calculation for the cable is total length of the cable.

You don't state type of supply at source but if it is PME and you intend to use a core in the SWA to provide earth protection in the shed then the cable cpc (earth) csa has to comply with the supply authority PME requirement which may be a conductor csa of 10mm or greater.

Your request for help may go on for a good period of time and by the end you will not know what is the best information. The best advice I can offer is to find a local registered electrician and bite the bullet, spend some money to achieve a safe installation. You can save some money by offering to dig the trench and backfill when the cable is laid.

Emgee

Martin 10020/03/2017 21:49:59
166 forum posts
5 photos
Posted by Mark Rand on 20/03/2017 19:46:46:

The wiring regs call for a maximum 2% voltage drop on a circuit at its maximum load. So use that for the calculations.

It used to be 4% in the 16th Edition of BS7671, but the 17th edition (2008) altered that to 3% for lighting circuits and 5% for 'other uses'

Bear in mind you not only need to consider the votage drop but the earth loop impedence and the electrical protection for the cable (MCB /RCCD/RCBO) to ensure suitably fast clearance of any credible fault. Also the export of an earth from the main premises may not always be advisible, and you may need to leave the armouring isolated at the remote end and use a local earth spike.

This is a brief overview

**LINK**

You really need someone with specific knowledge of your existing installation particularly the earthing method in order to design something that meets the requirements of BS7671 and to test the final installation and thus ensure an adequate level of safety.

Edited By Martin 100 on 20/03/2017 21:50:53

Bazyle20/03/2017 23:34:26
avatar
3568 forum posts
157 photos

While you are digging the trench bung in a plastic pipe to carry a phone line, Ethernet, doorbell, and most important burglar alarm wires. Ok I know we are all wireless for everything now but you might find some of those don't work at the distance with electrical motor noise etc.

Mark Rand21/03/2017 00:07:46
170 forum posts
Posted by Martin 100 on 20/03/2017 21:49:59:

It used to be 4% in the 16th Edition of BS7671, but the 17th edition (2008) altered that to 3% for lighting circuits and 5% for 'other uses'

Bear in mind you not only need to consider the votage drop but the earth loop impedence and the electrical protection for the cable (MCB /RCCD/RCBO) to ensure suitably fast clearance of any credible fault. Also the export of an earth from the main premises may not always be advisible, and you may need to leave the armouring isolated at the remote end and use a local earth spike.

 

Bugger, you're right. I was mentally conflating the 2% frequency stability requirement on the grid. blush

 

Earth loop impedance comments very relevant. ISTR that the advice is now for RCBO for all ground floor circuits, but I only have them for the garage and an outside circuit.

 

Part P of the building regs will require inspection of the work for it to be cosher. (Bah! Humbug!)

Edited By Mark Rand on 21/03/2017 00:08:23

Toby21/03/2017 13:09:30
113 forum posts
17 photos
Posted by Martin 100 on 20/03/2017 21:49:59:
Posted by Mark Rand on 20/03/2017 19:46:46:

The wiring regs call for a maximum 2% voltage drop on a circuit at its maximum load. So use that for the calculations.

It used to be 4% in the 16th Edition of BS7671, but the 17th edition (2008) altered that to 3% for lighting circuits and 5% for 'other uses'

Bear in mind you not only need to consider the votage drop but the earth loop impedence and the electrical protection for the cable (MCB /RCCD/RCBO) to ensure suitably fast clearance of any credible fault. Also the export of an earth from the main premises may not always be advisible, and you may need to leave the armouring isolated at the remote end and use a local earth spike.

This is a brief overview

**LINK**

You really need someone with specific knowledge of your existing installation particularly the earthing method in order to design something that meets the requirements of BS7671 and to test the final installation and thus ensure an adequate level of safety.

Edited By Martin 100 on 20/03/2017 21:50:53

imho the best advice so far, especially the bit about needing specific knowledge of the existing installation. You are only guessing if you don't know what the existing earthing arrangements are and have measured the Ze and Ipf.

Posted by Bazyle on 20/03/2017 23:34:26:

While you are digging the trench bung in a plastic pipe to carry a phone line, Ethernet, doorbell, and most important burglar alarm wires. Ok I know we are all wireless for everything now but you might find some of those don't work at the distance with electrical motor noise etc.

 

Also excellent advice, although I did just this about 5 years ago and still have an empty pipe running under my lawn. I really must get around to using it....... ;(

duncan webster21/03/2017 18:29:34
avatar
1234 forum posts
22 photos

I'm about to show my total ignorance here! If burying a pipe (should that be duct?) cant you just use 4 or 6mm twin and earth (not armoured) inside the duct?

Toby21/03/2017 19:07:08
113 forum posts
17 photos
Posted by duncan webster on 21/03/2017 18:29:34:

I'm about to show my total ignorance here! If burying a pipe (should that be duct?) cant you just use 4 or 6mm twin and earth (not armoured) inside the duct?

duct or conduit

To be clear we you should not put T&E (or any other "mains" cable) in the same conduit as signal cables like ethernet etc. If you want both you need two conduits.

Yes, you can use T&E but...........

1) T&E is strictly speaking only approved for "dry or damp" conditions, not submersion in water. If there is any chance your duct could fill up with water then using T&E is suspect. That said, I have never seen T&E degrade due to sitting in water.

2) The ducting really should ideally be good enough to provide mechanical protection, so a typical 20mm plastic conduit isn't really up to the job. Plus it will be a bit small to pull any reasonably sized cable through.

3) Taking 2 into account SWA is almost certainly cheaper than T&E plus decent ducting.

That said, ducting is a good idea if you ever think you might need to replace the cable. Just make sure it is large enough as there is nothing worse than trying to pull a stiff cable through ducting that is too small for it!

Nathan Sharpe21/03/2017 19:18:55
83 forum posts

I was taught that SWA armour was to be used as an equipotential bonding conductor and should be connected to the circuit protective earth. The Reg's still state this. On periodic inspection testing the continuity of Bonding, Circuit Protective Earth and Phase Conductors is carried out on individual cores/wire armour to make sure that all potential current paths are proved to be ,A) continuous, and B) meet minimum insulation resistance requirements. This will show up any existing and any potential faults with the cable if the IR is less 1.5megohm. The Reg's state that while a circuit showing IR of 1.5 megohm is acceptable further investigation should be undertaken to find any latent faults within the circuit. Nathan.

Toby21/03/2017 19:38:04
113 forum posts
17 photos
Posted by Nathan Sharpe on 21/03/2017 19:18:55:

I was taught that SWA armour was to be used as an equipotential bonding conductor and should be connected to the circuit protective earth. The Reg's still state this. On periodic inspection testing the continuity of Bonding, Circuit Protective Earth and Phase Conductors is carried out on individual cores/wire armour to make sure that all potential current paths are proved to be ,A) continuous, and B) meet minimum insulation resistance requirements. This will show up any existing and any potential faults with the cable if the IR is less 1.5megohm. The Reg's state that while a circuit showing IR of 1.5 megohm is acceptable further investigation should be undertaken to find any latent faults within the circuit. Nathan.

Sorry, but the regs do not state that

The armour *can* be used for bonding but only if it is sized correctly for the bonding, again, it is down to the individual circuit design. It certainly does not have to be used for bonding but as a minimum it should be properly terminated and earthed at the supply end as for any exposed conductive part. Of course the first job is to determine whether bonding is needed or not

Speedy Builder521/03/2017 21:07:11
1279 forum posts
91 photos

OPS !!! How will you heat your shed, if electric, double your consumption perhaps?? if oil or gas, buy loads of rust inhibiting oils. - Just a thought Dave.
BobH

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