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Very long run of LV Cabling

Hi all,

im looking at a project where the landlords LV cut out is approx 850 metres from the proposed tenants installation. The earthing arrangement is TT and protected by a time delayed 300ma RCD and a 63A MCCB.

The landlords sub main is going to be 2 x 4C 95mm2 in parallel buried in the ground to supply 20KVA worth of power. This achieved a volt drop of 1.6% which leaves 1.9% VD for lighting. From my Amtech calcs I also get about 1ka at the load Dis board (Phase fault)

im not worried about Earth Loop because of the RCD, but i am thinking about the adibatic of the phase conductors. If the tenant puts a long run of cabling in, this will comply from a Zs perspective, but could there be issues with the line under a phase to neutral /phase to phase fault as the cable could slowly cook under a lowish load without tripping the breaker. Is this a valid concern? Amtech isn’t throwing up any faults, but just wanted your thoughts.

thanks in advance

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0 David Stone over 3 years ago

The cheapest option is 2km of DNO overhead line. Going up in voltage is possible, but the change to under a kV is probably not worthwhile and does have the snag of continuous losses, probably around 500-1000 VA for 2 20 kVA transformers. This would cost around £2000 a year at today's prices, similar to 4000 litres of red diesel which would operate a 20 kVA generator on little load for at least 2000 hrs. The next thought is the load profile, how much, how long and when is important. If it is power 24/7 the DNO line is certainly best, if little load for a few hours a day the generator might be cheaper overall, if occasional full load but usually nothing, again the generator would work.
The next point is that if you use regulated LED lighting you can allow much more volt drop, it will not cause flicker with load changes. For most other appliances it makes little difference and electronics doesn't care at all. So we could say that 10% regulation is ok, saving a large amount of money, but with the snag that we will lose some power in the resistive loss in the cable. The cost of this loss can again be calculated and informed choices made.
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0 David Stone over 3 years ago

The cheapest option is 2km of DNO overhead line. Going up in voltage is possible, but the change to under a kV is probably not worthwhile and does have the snag of continuous losses, probably around 500-1000 VA for 2 20 kVA transformers. This would cost around £2000 a year at today's prices, similar to 4000 litres of red diesel which would operate a 20 kVA generator on little load for at least 2000 hrs. The next thought is the load profile, how much, how long and when is important. If it is power 24/7 the DNO line is certainly best, if little load for a few hours a day the generator might be cheaper overall, if occasional full load but usually nothing, again the generator would work.
The next point is that if you use regulated LED lighting you can allow much more volt drop, it will not cause flicker with load changes. For most other appliances it makes little difference and electronics doesn't care at all. So we could say that 10% regulation is ok, saving a large amount of money, but with the snag that we will lose some power in the resistive loss in the cable. The cost of this loss can again be calculated and informed choices made.
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Very long run of LV Cabling

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