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

 

[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