What type and size of cable for a long underground run

[comclusive]

Hi there,

I have a customer (I'm in IT, not an electrician, and am just getting information at this stage before appointing a contractor early next year) who is planning on having fibre and a switch in a couple of fields for events. We will be providing an IP65 cabinet with mains power, a fibre patch panel and a single 48 port network switch. The power requirement is just for one switch and the fans in the cabinet, and will only be powered for the duration of event prep, during the event and on take-down.

We'll be using a cabinet with 19 inch rails for the fibre patch and switch, and a separate locked section for the mains ingress, which event organisers would not have access to. The switch socket will be in this locked section.

The runs will be in a trench and could be as long as 600m. The source is a large (huge) house with a big distribution board.

My question is, for a low power requirement, but with the length of run, what size cable would be best, and do we need to consider inspection points and/or joints? There will be up to 4 of these trenches to different areas of the site, but they may only use one at a time depending on the size of the event.

Also, is it bad to go direct from point A to point B if it involves crossing a field which may have LGVs crossing for event setup and take-down, should the trench go around the perimeter?

Thanks very much

Mike

 

[timhoward]

To give some idea - a 3 amp load (guess) and a 600m run works out at 10mm sq. SWA just to keep the volt drop under the 5% requirement. 5 amps would push it up to 16mm sq.

Have you thought about other options?
It may be worth asking the question how many events there are a year that would use this and whether a small genny is a viable option.
Or even charge up a UPS before the event - I'd imagine a modest UPS would keep that load alive for at least a couple of weeks.

 

[comclusive]

To give some idea - a 3 amp load (guess) and a 600m run works out at 10mm sq. SWA just to keep the volt drop under the 5% requirement. 5 amps would push it up to 16mm sq.

Have you thought about other options?
It may be worth asking the question how many events there are a year that would use this and whether a small genny is a viable option.
Or even charge up a UPS before the event - I'd imagine a modest £100 UPS would keep that load alive for at least a couple of weeks.

Thanks for the reply. We're having to trench fibre to each location so at the time made sense to run power lower down. Generators are most likely going to be ruled out by the home owner. A UPS could be a consideration but event organisers will be paying for the service and there are planned to be upwards of 8 major events each year lasting from 1 to 5 days.

I was aiming for a 5a load. I think if the cost is prohibitive then at worst we'd just run the fibre and let the organisers figure it out from there. That gives me some notion of cable size, and thanks for the other considerations.

 

[James]

Large house, if there is 3 phase available it may be worth looking at using 2 phases into a transformer with a 2:1 ratio.
this will give aprox 800v for the distribution run
this will reduce the current in the cable to less than 1/3 of what it would be at 230v
a transformer at the other end (in the cabinet) to bring 800v down to 230v

with a 600m run, the cost of the transformers should be offset by the lower cost of the smaller cable required.

volt drop is unlikey to be an issue as you will
a. be able to modify the voltage to acceptable values by altering the tapping's on the transformers.
b. not be a great issue in practice because most I.T equipment will happily run at a voltage range from 100 to 240v

 

[plugsandsparks]

A 48 port switch - simple one is around 25watts, Fibre converters again only a few watts, as for fans, do you really need them ? - So consumption in amps at 240V is tiny a few hundred milli amps.

When you price up the cable at that distance and then the cost of installation, batteries will appear very attractive.......

You can afford to reduce the size of the cable for that load but beware terminating with a general purpose socket to plug in the switch as it will be too attractive for anybody wanting to use it for a lawnmower, lol

 

[David Prosser]

These are the approximate cost for the cable delivered (but not unloaded).
6a 16mm £3K for 600m.
10a 25mm £4.5K for 600m
16a 50mm £8.5K for 600m.

It would be well worth putting extra ducts and draw pits if you are digging a trench anyway as they are always handy in future.

 

[comclusive]

Large house, if there is 3 phase available it may be worth looking at using 2 phases into a transformer with a 2:1 ratio.
this will give aprox 800v for the distribution run
this will reduce the current in the cable to less than 1/3 of what it would be at 230v
a transformer at the other end (in the cabinet) to bring 800v down to 230v

with a 600m run, the cost of the transformers should be offset by the lower cost of the smaller cable required.

volt drop is unlikey to be an issue as you will
a. be able to modify the voltage to acceptable values by altering the tapping's on the transformers.
b. not be a great issue in practice because most I.T equipment will happily run at a voltage range from 100 to 240v

Thanks. I think there is 3 phase, will need to check. I think even with having to dig a trench, the cost of electrical cabling will make them think again!

 

[comclusive]

A 48 port switch - simple one is around 25watts, Fibre converters again only a few watts, as for fans, do you really need them ? - So consumption in amps at 240V is tiny a few hundred milli amps.

When you price up the cable at that distance and then the cost of installation, batteries will appear very attractive.......

You can afford to reduce the size of the cable for that load but beware terminating with a general purpose socket to plug in the switch as it will be too attractive for anybody wanting to use it for a lawnmower, lol

Thanks. Yes, batteries may well be the way forward!

 

[comclusive]

These are the approximate cost for the cable delivered (but not unloaded).
6a 16mm £3K for 600m.
10a 25mm £4.5K for 600m
16a 50mm £8.5K for 600m.

It would be well worth putting extra ducts and draw pits if you are digging a trench anyway as they are always handy in future.

Thanks indeed for the costings. I think this may make them rethink!

 

[James]

1.5mm 2 core SWA is £1 per meter
if you can deal with the volt drop by using a transformer or 2
the maximum load you could support using 800v transmission voltage is about 16Kw

however, i would think that specifying for a 1000w (1.25A @ 800v) load would be more realistic, just in case you want a 250W anti frost heater inside the coms rack.

 

[Strima]

Solar and batteries or a genny, cost would just be ridiculous for this type of installation.

 

[7029 dave]

Have you considered a new metered supply closer to the site, may be worth getting in touch with the DNO for a quote.

 

[pc1966]

Realistically if you are looking at just a switch and maybe a small defrost heater then 1A / 230V is enough and you could use 2.5mm 2-core SWA with an RCD for supply disconnection (as it would fail on the typical min 6A OCPD for domestic style boards). If the house is PME supply and you are looking at outdoor stuff you might even want to make this a TT arrangement.

Using transformers for lower drop as @James suggests is possible, but I would suggest unnecessary complexity if you don't really need it.

Cable works out at about £1/m plus VAT.:

Superlec 6942X 2CORE 2.5MM SWA BS5467 STEEL WIRE ARMOURED CABLE - HARMONISED CORES

Please note: If you require advice on which cable to use, how and where to install it, it is recommended that you will need to contact the relevant qualified tradesperson for the job as the Superlec Direct Team are not qualified electricians.

www.superlecdirect.com

You can either use SWA network cable and both can be buried directly in the ground but I would strongly advice using some twinwall duct for both power and fibre and you can use any outdoor style fibre that is strong enough to be pulled through. Mains SWA is safe enough beside even normal copper network cable as it has an earthed screen in the form of the armour. This is the sort of thing (but shop around, this was just my first result that was what I was trying to illustrate) so around £1.60 per meter plus VAT:

Flexi Duct CCTV Fibre Optic - 63mm (O.D.) x 50mtr Green Coil

Flexi Duct CCTV Fibre Optic - 63mm (O.D.) x 50mtr Green Coil

www.drainagepipe.co.uk

Typically you would want an access chamber around every 200m to allow cable pulling and potentially to allow a section to be repaired. It also allows you to have one run out and then split at the box to serve other end points.

BT/Openreach typically use this system for pavements or footpaths (not on a road where heavy concrete or cast iron cover and a concrete or brick box is then needed for the loading):

BT Quadbox Chambers

We stock the BT Quadbox Chamber Access System approved by British Telecom. Order online today, free delivery.

www.drainagepipe.co.uk

However, you can see that a complete box (say 4 * stacked sections to get 60cm depth, and lid) you are looking at around £600 per box.

So for 600m run you are looking at around 4 grand for duct, 3 boxs, and SWA cable delivering 1A power (before you add fibre and network switch). However, if fibre is needed out there then most of that cost is going to apply anyway.

 

[pc1966]

Also, is it bad to go direct from point A to point B if it involves crossing a field which may have LGVs crossing for event setup and take-down, should the trench go around the perimeter?

If the cable or duct is buried deep enough (say 0.4-0.5m below surface) then it is not important. However, if the field is ever used for agriculture then you really need around 1-1.2m depth.

If this was a permanent system then you might want it routed around the edge of fields to allow for future changes in land use (e.g. adding building) without having to change this, however, that might not be an issue for the owner here.

 

[comclusive]

Have you considered a new metered supply closer to the site, may be worth getting in touch with the DNO for a quote.

I think that may cost even more given the terrain, distance from the road and the 4 areas being spread over a wide range. Something to add to the thinking though, thanks!

 

[comclusive]

Realistically if you are looking at just a switch and maybe a small defrost heater then 1A / 230V is enough and you could use 2.5mm 2-core SWA with an RCD for supply disconnection (as it would fail on the typical min 6A OCPD for domestic style boards). If the house is PME supply and you are looking at outdoor stuff you might even want to make this a TT arrangement.

Using transformers for lower drop as @James suggests is possible, but I would suggest unnecessary complexity if you don't really need it.

Cable works out at about £1/m plus VAT.:

Superlec 6942X 2CORE 2.5MM SWA BS5467 STEEL WIRE ARMOURED CABLE - HARMONISED CORES

Please note: If you require advice on which cable to use, how and where to install it, it is recommended that you will need to contact the relevant qualified tradesperson for the job as the Superlec Direct Team are not qualified electricians.

www.superlecdirect.com

You can either use SWA network cable and both can be buried directly in the ground but I would strongly advice using some twinwall duct for both power and fibre and you can use any outdoor style fibre that is strong enough to be pulled through. Mains SWA is safe enough beside even normal copper network cable as it has an earthed screen in the form of the armour. This is the sort of thing (but shop around, this was just my first result that was what I was trying to illustrate) so around £1.60 per meter plus VAT:

Flexi Duct CCTV Fibre Optic - 63mm (O.D.) x 50mtr Green Coil

Flexi Duct CCTV Fibre Optic - 63mm (O.D.) x 50mtr Green Coil

www.drainagepipe.co.uk

Typically you would want an access chamber around every 200m to allow cable pulling and potentially to allow a section to be repaired. It also allows you to have one run out and then split at the box to serve other end points.

BT/Openreach typically use this system for pavements or footpaths (not on a road where heavy concrete or cast iron cover and a concrete or brick box is then needed for the loading):

BT Quadbox Chambers

We stock the BT Quadbox Chamber Access System approved by British Telecom. Order online today, free delivery.

www.drainagepipe.co.uk

However, you can see that a complete box (say 4 * stacked sections to get 60cm depth, and lid) you are looking at around £600 per box.

So for 600m run you are looking at around 4 grand for duct, 3 boxs, and SWA cable delivering 1A power (before you add fibre and network switch). However, if fibre is needed out there then most of that cost is going to apply anyway.

Thanks for the detailed analysis. much appreciated.

 

[comclusive]

If the cable or duct is buried deep enough (say 0.4-0.5m below surface) then it is not important. However, if the field is ever used for agriculture then you really need around 1-1.2m depth.

If this was a permanent system then you might want it routed around the edge of fields to allow for future changes in land use (e.g. adding building) without having to change this, however, that might not be an issue for the owner here.

It's parkland so no agri involved, but always best to plan for the unexpected!

 

[pc1966]

Realistically if you are looking at just a switch and maybe a small defrost heater then 1A / 230V is enough and you could use 2.5mm 2-core SWA with an RCD for supply disconnection (as it would fail on the typical min 6A OCPD for domestic style boards).

Looking at the 2-core SWA cable, for 1.5mm the armour is lower R than the conductor!

But for 2.5mm considered by myself the R1 = 7.41 mOhm/m and R2 = 8.8 mOhm/m, so R1+R2 totals around 9.8 for 600m, say Zs approx 11 ohms allowing for TN-S supply.

That would meet disconnection on a 3A FCU as Table 41.4 has Zs for a 3A BS 1362 fuse as 22 ohms. It still needs RCD protection, but it is always nicer to know you can safely disconnect on the OCPD!

 

[pc1966]

That would meet disconnection on a 3A FCU as Table 41.4 has Zs for a 3A BS 1362 fuse as 22 ohms. It still needs RCD protection, but it is always nicer to know you can safely disconnect on the OCPD!

That was 5s as a sub-main. Probably it would be considered a final circuit where it is Table 41.2 for 0.4s disconnection and then a max Zs of 15.6 ohms for 3A fuse, so still OK.

While they oly give 3A and 13A in that table, you can also get smaller fuses (1A & 2A) but for typical network switch with switch-mode PSU you run the risk of switch-on surge weakening or taking out too small a fuse.

 

[loz2754]

That was 5s as a sub-main. Probably it would be considered a final circuit where it is Table 41.2 for 0.4s disconnection and then a max Zs of 15.6 ohms for 3A fuse, so still OK.

While they oly give 3A and 13A in that table, you can also get smaller fuses (1A & 2A) but for typical network switch with switch-mode PSU you run the risk of switch-on surge weakening or taking out too small a fuse.

@pc1966 Are you purposefully not taking volt drop into account? If so, may I ask please what your reasoning is behind this?

 

[pc1966]

@pc1966 Are you purposefully not taking volt drop into account? If so, may I ask please what your reasoning is behind this?

The voltage drop is taken in to account for the planned 1A max load.

 

[pc1966]

The voltage drop should be met for the planned use of a circuit, if that is a general purpose one where anything up to the max OCPD rating can be used, then you would size the VD to the OCPD rating. For a fixed load you can size it for the load.

Here the load is small and known, or at least my assumption is it is a network switch which is typically 20W to 150W depending on PoE support, and maybe a very small heater against condensation, like the 40W or 60W things you see for greenhouses and cold frames to prevent frost.

So the OCPD is there only for fault protection, not for overload and not to place any specific limit on the usage. So a 3A fuse is going to meet disconnection on OCPD under fault conditions without relying on any RCD action (and clearly also for overload on 2.5mm!)

 

[pc1966]

I would also add that generally speaking, if you meet 5% VD for the OCPD you will normally also meet disconnection times as the PFC would be 20 times the OCPD (ignoring Ze) hence even a D-curve MCB will trip in the 'instant' region.

For high current circuits you may not meet Zs as Ze is too high and/or the supply impedance itself is a factor in limiting the PFC.

But if you are sizing VD for a much lower current than the OCPD you run a risk of faults not clearing adequately. In many cases the circuit will have an RCD so faults to earth are going to clear by that route, but it leaves L-N faults that may not disconnect at all! The regs don't require a time for L-N faults to clear, only that there is no fire...

That is not something I am happy about in general, so while it is not a requirement, I would still always plan that a L-N fault clears in under 5s and see what means can be used to achieve it. Sometimes the old fuse is your best friend!

 

[loz2754]

The voltage drop is taken in to account for the planned 1A max load.

I see. Thanks for that.
Just did my own calculation on a max design current of 1 amp, with 2.5mm² 2 core swa.
Comes out at 4.97%. Cutting it close, but as has been said, most electronic equipment is designed to operate within a wide voltage range anyway.

Having said that, if going to the trouble and expense of digging trenches and installing ducting, it might be more sensible to design for a higher design current to allow for other equipment to be used.

 

[pc1966]

Having said that, if going to the trouble and expense of digging trenches and installing ducting, it might be more sensible to design for a higher design current to allow for other equipment to be used.

It is certainly something to discuss with the customer to see if they do expect anything else to be added, or if it really is only that.

With duct you have the option to pull in another cable later if that changes, but it is a pain to do so and better to size properly up front.

@James suggestion of step-up and down transformers would be another route, but they have extra losses consuming power even when no load, and it complicates matters as well for anyone who ever comes along later. It might be an option to get ~4 times the power for around £400 to hardware, but you might be better to put that in to running 4mm or whatever to begin with!

 

[SJD]

I haven't noticed any mention of the nature of the events at this site, but can't help wondering what will be powering all the equipment to be connected to the 48 port network switch. I guess that is maybe not the site owners concern, but it would seem a little odd to provide the network connections but no power?

 

[pc1966]

it would seem a little odd to provide the network connections but no power?

Yes, it is indeed very odd.

But if they are looking at powering much out there it is looking to be VERY expensive in cable, or looking at something a bit out of the ordinary like 3-phase maybe going 400/690V on the cable and back down again which ain't going to be cheap either.

If it is for stuff like web cameras then having a PoE switch is really the way to go, and then you are typically in the 150W-ish range, but depending on switch size and total load you might be looking at 500W so around 2A.

 

[littlespark]

Plan your trenches carefully.

If it’s for events, then marquees? I’ve seen a 2ft long anchoring pin burst a 400v 3ph armoured cable

 

[pc1966]

That is a good reason to look at going close to 1m deep even if not agricultural. You can hire digging machines for just this sort of job:

Tracked Trenchers (For Hire)

From soft and sandy to hard as a rock, these tracked trenchers are designed to perform - digging 100mm trenches up to 1.0m deep quickly and

hsstoolshop.co.uk

 

[pc1966]

Out of curiosity I took 2.5mm = 1A on VD exactly and put in advertised costs for cable from Superlec site (ex-VAT) and came up with currents for various voltage options:

CSA \ Volts​	230​	400​	480​	630​	800​	£/m​	For 600m​
2.5​	1​	3.0​	4.4​	7.5​	12.1​	£0.95​	£570.00​
4​	1.6​	4.8​	7.0​	12.0​	19.4​	£1.25​	£750.00​
6​	2.4​	7.3​	10.5​	18.0​	29.0​	£1.80​	£1,080.00​
10​	4​	12.1​	17.4​	30.0​	48.4​	£2.42​	£1,452.00​
16​	6.4​	19.4​	27.9​	48.0​	77.4​	£3.73​	£2,238.00​
25​	10​	30.2​	43.6​	75.0​	121.0​	£5.75​	£3,450.00​

Here I was thinking of @James suggestion and considered:

• 230V is standard single phase (230-0V)
• 400V assumes N is common and L stepped up/down (autotransformers at both ends) 400-0V
• 480V assumes L is mains and other 'N' is opposite phase L (could be isolator uses as autotransformer) so 230-0-230V
• 630V assumes L is direct and 'N' is opposite stepped up to 400V so 230-0-400V
• 800V assumes 230V to 400-0-400V transformers at both ends

No losses or transformer drop considered here, but if you needed something like 16A for powering other equipment remotely it is looking like a cost-effective way either 4mm & 800V, 6mm & 630V, or 10mm & 480V, etc.

However, I don't have any transformer costs to hand, other than a 230V 500VA isolation transformer is around the £150 mark.

 

[pc1966]

These folks can do any special transformers you need:

Single phase transformer manufacturer

R Baker (Electrical) Ltd is a transformer manufacturer based in Liverpool and is one of the leading single phase transformer manufacturers in the UK.

www.rbaker.co.uk

 

[pc1966]

Doh! Table had 480V not 460V for the split-phase set up. Try again:

CSA \ Volts​	230​	400​	460​	630​	800​	£/m​	For 600m​
2.5​	1​	3.0​	4.0​	7.5​	12.1​	£0.95​	£570.00​
4​	1.6​	4.8​	6.4​	12.0​	19.4​	£1.25​	£750.00​
6​	2.4​	7.3​	9.6​	18.0​	29.0​	£1.80​	£1,080.00​
10​	4​	12.1​	16.0​	30.0​	48.4​	£2.42​	£1,452.00​
16​	6.4​	19.4​	25.6​	48.0​	77.4​	£3.73​	£2,238.00​
25​	10​	30.2​	40.0​	75.0​	121.0​	£5.75​	£3,450.00​

 

[James]

Remember @pc1966 the volt drop can be bigger than 5%
if you set up the transformers correctly so you have maximum allowed voltage when at zero load, you will have to drop a lot before coming out of the nominal allowed range.

 

[SJD]

If all you need to power is some fixed equipment in a cabinet, rather than worrying about a 5% or whatever voltage drop, can you perhaps not just install a power conditioner / voltage stablizer?

These seem to exist from quite small loads e.g. 3A to 10A or 20A at quite reasonable costs £100 to £300. A typical input range might be 140-260V, with a 230V output +/- 6%. You might even find one that operates from 100V or less up to 260V or more, a bit like many AC adapters that work off both 110V and 230V.

Obviously this is not going to be some BS7671-compliant supply, but perhaps you treat the whole cabinet & very long supply cable as a fixed load from wherever it is supplied from?

But feel free to tell me this is a daft idea!

 

[pc1966]

If all you need to power is some fixed equipment in a cabinet, rather than worrying about a 5% or whatever voltage drop, can you perhaps not just install a power conditioner / voltage stablizer?

These seem to exist from quite small loads e.g. 3A to 10A or 20A at quite reasonable costs £100 to £300. A typical input range might be 140-260V, with a 230V output +/- 6%. You might even find one that operates from 100V or less up to 260V or more, a bit like many AC adapters that work off both 110V and 230V.

It is a very good suggestion. In fact, many IT systems would be planned to include a UPS and usually they will 'condition' the normal input range to come out at a since stable 230V.

Not all UPS are very well behaved in this, the expensive double-conversion sort (continuously converting incoming AC to the DC battery bus, and back to stable AC again) are good, but typically if you dip below nominal input the revert to battery operations.

I can see an oscillating cycle in the case of a high Z supply where it reverts to battery, AC demand goes to zero, volts recover, ...

Obviously this is not going to be some BS7671-compliant supply, but perhaps you treat the whole cabinet & very long supply cable as a fixed load from wherever it is supplied from?

But feel free to tell me this is a daft idea!

Here would be a reasonable case for a departure from BS7671 as it has no safety implications (high cable drop not related to exceeding the CCC) provided the design of RCD/OCPD is such that disconnection under fault conditions is still met.

 

[pc1966]

Looking briefly for transformers, if the house has 3-phase so you can have L1 & L2 on the feed for 400V, then this at the far end would give to 4A 230V out with 4mm feed cable:

https://uk.farnell.com/schneider-electric/abl6ts100u/transformer-1kva-1-x-230v/dp/2070208

So then cost around £1k, plus any extra for enclosure and added RCD on transformer output (with the pre-RCD neutral to the supply cable armour for earth referencing).

Supply OCPD is probably a DP/3P MCB and it is unlikely to meet disconnection times, so possibly added cost of a 3-phase RCD as well.

 

[Mikegh]

Seems like major work for such small loads ,.I dunno

Will there not be gennys arriving on-site at these locations for the events ?

Any other options like wind/solar or silent inverter gennys ?

 

[pc1966]

It seems a bit vague: power for networking, but not anything else?

So maybe the customer needs to think this through much more and plan on remote power from generator/solar+battery/etc so only fibre ducted through, or look at biting the bullet and putting in a usable amount of power.

Which IS going to cost a lot, even if tricks with transformers and UPS are considered.

 

[Mikegh]

Good thinking on the stepup/step down anyhow

And the UPS