Circuit Wire For Long Run

[Cookie]

Ok, say you've got a lighting run with the last fixture being 1,280 meters away. 1,800 watts per phase 3 phase, 277/480Y volts though in this example we can use 230/400Y for the ease of discussion.

My question is- what size wire do I run for each phase and earth? What disconnection time do I use? Do I use a Type B, C or D breaker? What should my max R1+R2 come out to be?

 

[cliffed]

Street lighting would be an example in which the REC uses a concentric cable looping of the main ring supply cable.
Theres loads of software to calculate size required to local cutout in each fixture.

 

[pc1966]

I suspect this is a specialised case as above.

But...if you simply followed the UK domestic requirements for 3% drop and assuming the lights are spaced sort-of uniformly, the feed cable comes out at 25mm, for 5% drop (reasonable for a case like this with LED or similar lights that are not that voltage-sensitive) it is 16mm. Risers to each lamp could be much smaller of course!

As a final circuit you would be looking at 0.4s disconnection so a B or C curve breaker. With a lot of simultaneously-switched lights (central control) then probably a C-curve for that and 10A C-curve MCB has max Zs of 1.75 ohms.

Assuming 4C 16mm with armour as CPC, then R1 = 1.15 mOhm/m, and R2 = 3.1 mOhm/m so your R1+R2 = 5.4 ohms which is too much, as would 5C cable as well (2.9 ohm). In this case I would probably put on a 100mA instant RCD to meet disconnection, as then still selective if the main DB has a 300mA delay incomer for fire reasons, etc.

But I don't design street lighting, so the above might be complete nonsense!

 

[Cookie]

I suspect this is a specialised case as above.

But...if you simply followed the UK domestic requirements for 3% drop and assuming the lights are spaced sort-of uniformly, the feed cable comes out at 25mm, for 5% drop (reasonable for a case like this with LED or similar lights that are not that voltage-sensitive) it is 16mm. Risers to each lamp could be much smaller of course!

As a final circuit you would be looking at 0.4s disconnection so a B or C curve breaker. With a lot of simultaneously-switched lights (central control) then probably a C-curve for that and 10A C-curve MCB has max Zs of 1.75 ohms.

Assuming 4C 16mm with armour as CPC, then R1 = 1.15 mOhm/m, and R2 = 3.1 mOhm/m so your R1+R2 = 5.4 ohms which is too much, as would 5C cable as well (2.9 ohm). In this case I would probably put on a 100mA instant RCD to meet disconnection, as then still selective if the main DB has a 300mA delay incomer for fire reasons, etc.

But I don't design street lighting, so the above might be complete nonsense!

Thnaks, but I don't want to use an RCD for disconnection. RCDs can fail and all disconnection in TN systems must be done by the breaker (MCB) itself.

 

[pc1966]

Thnaks, but I don't want to use an RCD for disconnection. RCDs can fail and all disconnection in TN systems must be done by the breaker (MCB) itself.

If the lights are separately switched (e.g. each has its own photocell) you could get away with a 10A B-curve MCB fine as no massive inrush surge. That would allow you to meet the disconnection with 5-core 16mm (£9.37/m from Superlec) or just 4-core 25mm cable (£9.24/m).

1280m * 4-core 16mm at £6.84/m = £8,755 + RCD
1280m * 4-core 25mm at £9.24/m = £11,827
1280m * 5-core 16mm at £9.37/m = £11,994 (easier to wrangle, more reliable earth)

Of course that difference probably is trivial compared to the project cost.

 

[Cookie]

If the lights are separately switched (e.g. each has its own photocell) you could get away with a 10A B-curve MCB fine as no massive inrush surge. That would allow you to meet the disconnection with 5-core 16mm (£9.37/m from Superlec) or just 4-core 25mm cable (£9.24/m).

1280m * 4-core 16mm at £6.84/m = £8,755 + RCD
1280m * 4-core 25mm at £9.24/m = £11,827
1280m * 5-core 16mm at £9.37/m = £11,994 (easier to wrangle, more reliable earth)

Of course that difference probably is trivial compared to the project cost.

Right, but a trip and reclose from then utility would engage all of them at once I think.

Do you have the ohms value for each conductor?

 

[pc1966]

Last page of this has both conductors and armour resistances:

https://uk.prysmiangroup.com/sites/default/files/atoms/files/BS5467.pdf

Yes, if you had a supply interruption at night it might well power all lamps simultaneously. I know some UPS allow random-timed start-up to avoid that sort of issue, no idea if there is a similar idea for lamps to get round the inrush problem without having to push the MCB trip point up.

Which you can do, just it gets very difficult/expensive for the cable if you do it that way and still want to meet the required disconnection Zs at the end of a long cable.

If your lights were in a region so they could be ring-connected that would reduce the max Zs value (and drop as well, of course).

Or you live with the RCD to avoid the Zs limit of the MCB being an issue and put in two in series for redundancy, accepting they don't have selectivity so both need reset on fault.

But as mentioned at the start, there probably are folks/companies/tools that specialise in street light solution. I'm just guessing!

 

[Cookie]

Your guess is as good as mine

FWIW I'm getting a minimum size of 33.62mm2 copper Phase and Earth.

 

[pc1966]

Also to add if the supply is not RCD protected (so no selectivity issues) you could even use a 300mA delay RCD, still meet the 0.4s disconnection time for shock, and be pretty robust against false trips.

So you could have a 300mA delay then a 100mA instant and retain selectivity unless the 100mA fails.

 

[Cookie]

Does BS7671 allow an RCD to meet disconnection times in TN? I just can't bring myself to do it.

 

[Cookie]

Reason I ask about disconnection times is that the ground around an energized lamp goes up in voltage (not my pic):

https://www.reddit.com/r/electricians/comments/mgze4b

 

[Strima]

Here's what your R1+R2 would be (Column D):

 

[pc1966]

Your guess is as good as mine

FWIW I'm getting a minimum size of 33.62mm2 copper Phase and Earth.

For what aspect, volt drop or meeting MCB disconnection R1+R2?

 

[pc1966]

Does BS7671 allow an RCD to meet disconnection times in TN? I just can't bring myself to do it.

It allows it but I would normally see it as poor practice unless there are no reasonable alternatives.

With over 1km of cable going to something as big as 35mm to get MCB disconnection and good inrush tolerance is pushing cost and assembly/repair effort up massively, so it moves towards "no reasonable alternative" territory.

It is possibly worth looking at other aspect normally taken for granted. For example the usual Zs limits allow a 0.8 factor for cable heating - that is not an issue for a heavy cable needed to meet VD when very long.

While usually seen as old fashioned, sometimes it is worth considering fuses (e.g. switched-fuse feed) as they often have a better combination of surge handling and Zs limit. Also one fuse blowing means you still have 2 out of 3 lights on!

 

[pc1966]

Here are some examples for SWA data and for various OCPD assuming no significant cable heating and 1280m length:

OCPD​	Zs (from OSG)​	Zs No heating (Ohm)​	R1+R2 (mOhm/m)​
10A BS88-2 fuse​	3.7​	4.63​	3.61​
10A B-MCB​	3.5​	4.38​	3.42​
16A B-MCB​	2.2​	2.75​	2.15​
16A BS88-2 fuse​	1.9​	2.38​	1.86​
10A C-MCB​	1.75​	2.19​	1.71​
SWA choice​	R1​	R2​	R1+R2 (mOhm/m)​
16mm 4C​	1.15​	3.1​	4.25​
16mm 5C​	1.15​	1.15​	2.30​
25mm 4C​	0.727​	2.3​	3.03​
25mm 4C + 16mm​	0.727​	1.15​	1.88​
25mm 5C​	0.727​	0.727​	1.45​