What type and size of cable for a long underground run

[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