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.