Volt drop from radial spurred from ring main.

[Monkeyblaine]

I'm installing a gate supply at the end of a customers garden. My plan is to spur off internal ring main and run a radial roughly 100m to end of garden. Providing the long run cable is of adequate size to sustain 230v to the end of the garden am I right in thinking that I can tee off again at gate with 2.5mm cable? As the long run cable will not struggle to hold 230v so in theory its just adding another small spur to original ring main. Also same with garden lights. Big enough cable to surround perimeter of garden and hold 230v then tee off where lights will be situated with 1.5mm flex. Just reassuring myself. I will obviously carry out correct calculations to make sure supply cable is adequate for load.

 

[Amp David]

Only one spur allowed from your ring final.

Your vd is all dependent on what load you'll be pulling.

 

[chopper76]

Unless you fuse it

 

[Amp David]

Of course, the fact hes thinking of running a 100m spur, then i don't reckon fusing it was ever in the equation

 

[FrankUK]

Don't forget that if you are taking a supply to outside it must be RCD protected. Is the FRC RCD protected at present?
Also, any fault on the outdoor circuit would affect the FRC. I personally, would provide the outdoor requirements, gate, lights etc, with their own dedicated circuit. If I was an electrician of course :icon10:

 

[stuartcourt]

ha ha glad I am not your customer

 

[Amp David]

I'd love to know what load this adition is designed to carry

 

[Outspoken]

I'm installing a gate supply at the end of a customers garden. My plan is to spur off internal ring main and run a radial roughly 100m to end of garden. Providing the long run cable is of adequate size to sustain 230v to the end of the garden am I right in thinking that I can tee off again at gate with 2.5mm cable? As the long run cable will not struggle to hold 230v so in theory its just adding another small spur to original ring main. Also same with garden lights. Big enough cable to surround perimeter of garden and hold 230v then tee off where lights will be situated with 1.5mm flex. Just reassuring myself. I will obviously carry out correct calculations to make sure supply cable is adequate for load.

A spur 100m long....well that says it all really.

Take a sub-main from the main board to a IP rated enclosure by the gate that would be sufficient to handle the loads placed upon it and ensure full compliance with VD requirements. Install in the enclosure a suitable board with a main isolator and an RCBO for each circuit. Considering the length of run involved you will need to ensure the cable is buried and protected in accordance with the Regs and that it is protected by a suitably rated RCD.

 

[Monkeyblaine]

In an ideal world of course I would run a sub main to new CU but its not something they can afford as main CU is bang centre in the middle of the house. The run is actually 40m slight exaggeration at first. (I was just over compensating for my theory) and my plan is to use and RCD switched fused spur to supply gate. All I wanted to know was in fact the way I was thinking about voltdrop was correct. I.e large enough CSA cable to deal with length of run to keep 230v to the gate then back to using smaller cables once distance was achieved. I guess it's no different to running a sub main and then using smaller cables from sub main board tho

 

[Monkeyblaine]

Except in this instance it's coming off an internal ring so it will be like adding a small radial off that in comparison. Plus there will already be a voltdrop on that circuit so I would need to take that into consideration.

 

[Outspoken]

Monkey, it is totally irrelevant whether they can afford it or not, if they cannot, then they open the gate manually or buy a smaller home. If they can afford the home then they can afford you.

The cable will need to be adequately buried and protected regardless, the cable should be SWA regardless (cheapest option for this job) and the costs of adding a few RCBO's is not really here nor there.

Do you know the maximum load that will be drawn along this supply? Are you able to measure..there is a world of difference between 40m and 100m, I would suggest you measure your route properly before pricing this job up.

However lest assume you have a 20A load on that cable, taking into account all factors I would hazard a guess at using 4mm² SWA as your absolute bare minimum, but personally I would use 6mm² SWA as your guaranteed compliance and it will remain the case even if more load is put upon it..within reason of course.

I would suggest you price up both ways, explain the merits of both ways of doing it, and the negative, and then all make a joint decision.

 

[Monkeyblaine]

I completely understand and will go back to the customer and explan all of this and I was always going to bury it correctly and use the correct type of cable. But I am right in thinking at the gate end we can go back to smaller cable as 230v will be achieved at this end due to the 6mm SWA (for argument sake) carrying it to there with no problem. The job may or may not happen but it's the theory I want to get correct first for my own piece of mind. So forget the job. Do I have the other bits correct?

 

[Amp David]

Again, if you tell us the load this spur has to carry then someone will give you and answer to your vd question.

 

[Richard Burns]

I completely understand and will go back to the customer and explan all of this and I was always going to bury it correctly and use the correct type of cable. But I am right in thinking at the gate end we can go back to smaller cable as 230v will be achieved at this end due to the 6mm SWA (for argument sake) carrying it to there with no problem. The job may or may not happen but it's the theory I want to get correct first for my own piece of mind. So forget the job. Do I have the other bits correct?

You are calculating volt drop as if it is only needed to comply at one point.
When you calculate volt drop you are doing this to the end(s) of the circuit.
If you run 40m of 6mm for lights and calculate your volt drop to be 6.8V at that point and then run another 10m of 1mm out to a light you then need to add the volt drop that this 10m of 1mm causes; if, in this theoretical case, the volt drop were more than 0.1V then the circuit would not comply.

You will know the current taken, the distance and the mV/A/m of each cable you are planing to use, add up all the volt drops (reducing the current at the appropriate points if the loads are split) and see if it complies. If it does not comply increase the csa at various points to compensate.

 

[Outspoken]

I completely understand and will go back to the customer and explan all of this and I was always going to bury it correctly and use the correct type of cable. But I am right in thinking at the gate end we can go back to smaller cable as 230v will be achieved at this end due to the 6mm SWA (for argument sake) carrying it to there with no problem. The job may or may not happen but it's the theory I want to get correct first for my own piece of mind. So forget the job. Do I have the other bits correct?

Monkey, as Richard has said you have to treat the entire installation as part of the circuit, the rules do not change because you put an MCB in line.

If you want help with this then I suggest you tell us all the details of the circuit such as;

1. Exact length of the run from the Main CU to the IP Enclosure near the gate,
2. The exact length of the circuit from this IP Enclosure to the gate motor/controls
3. Exact length of the lighting Circuit
4. Exact length of any other circuit you intend to install
5. The maximum load required by the Gate
6. The total load required by the lights
7. Total load of any other circuit attached.

Only when we know the details of the circuit can we for sure give you a definitive answer on this and what worries me is that your having to be asked for this information when this should be the initial information you provide.
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Example version of your question;

Guys, I wonder if you could help me as I am a little rusty. I have been asked by a client to install a couple of new supplies for lights and a gate supply in their garden and need to confirm I am correct.

From the CU in the house I intend to run an SWA buried in the ground to an IP66 rated enclosure fixed to a small pole, this will be some 48m long.
In the IP65 enclosure I was going to install a 4 way enclosure in which I would mount a double pole isolator and two RCBO's, one rated at 10A 100mA for the gate supply and the other rated at 6A 100mA for the lights.
The Circuit for the gate will be 4m long and a maximum load of 6A
The circuit for the lights will have a total length of 25m and a maximum load of 4.5A
For the purposes of compliance I have assumed the circuit has a total length of 48+4+25 = 77m and a maximum assumed load of 4.5+6=10.5A

Now I have calculate the volt drop using Table 4D4A, Reference method D as a 4mm² SWA with a tabulated volt drop of 9.5mV per A per M giving me a total volt drop along the circuit of 7.6V.

Could someone please tell me if this is correct and does it comply with the Regs?

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Now for me that is the way you should ask the question, you cannot really expect people on the forum to do all the design work for you because if you do not understand how to do this then you should not be doing this in the first place.