Mains cable size.

[Russell griffiths]

Sitting here bored, I thought I would ask this as I'm sure someone can answer it easily 

 

So so we have a mains feed that comes in from the road and runs to a brick building with our metre in and a main switch. We have 3 phase in this building. 

So from this building I need to run a new cable to the new house consumer unit. 

 

Question. What size cable do I need to run a fairly substantial family home. 

Distance from building to house is 100m 

house will have ashp, solar on roof and a good size workshop where I have a good size compressor and a mig welder, also will need power for sewage treatment plant, I like lots of power points 

hope this isn't to vague. 

Cheers russ. 

Edited August 11, 2017 by Russell griffiths

[PeterW]

According to Doncaster Cables calculator you would need 70mm² which is a bit beefy for 100m and 120A...   If its across your own land you may want to see what you can do about getting it moved by the DNO and dig the trenches yourself ..?

[Russell griffiths]

Hi @PeterW I'm not sure I understand, move what and why. 

You say 70mm is a bit beefy for 100 m, again why. 

What does 70mm look like in terms of outside diameter. 

[Russell griffiths]

If I'm going to install this in a duct does it need to be swa or can I run singles. 

Just looked at some prices and I'm still laying on the floor trying to get up. 

[Onoff]

You want to consider running that 3p all the way to the new house. Much more choice on machinery for your new workshop and you can spread the load of the "substantial" house.

[ProDave]

The issue is not the current carrying capacity of the cable, if it was then 25mm² would be fine.

 

Rather, over that length, it's the voltage drop that will dictate the size of cable that you use.  It would not surprise me in the least to find you do indeed have to use something as big as 70mm, I would need to look it up and do the calcs.

 

The DNO use larger cables, here the main running down the road is 95mm² which then they tap off a 25mm feed for the short tun to each property.

 

If you get the DNO to move the supply closer to the house, the cost will be for such a large caable, and the non contestable work of them making the connections, which they will almost certainly do live.  It will cost a lot more than you just buying suitably large cable.  Unless of course moving it closer to the house would mean shortening the DNO's cable run? It depends where it comes from?

 

+1 to getting a 3 phase meter fitted and running 3 phase to the house. By spreading the load over 3 phases you might be able to reduce your max demand per phase and use a smaller cable, though of course more cores in the cable.

 

I would use SWA buried direct in the ground.  Single cores in conduit is acceptable, but I am not sure I would describe an underground duct as "conduit" so I would not use singles.

[PeterW]

One of the qualified ones will be along soon however...

 

You need a minimum cable size to cope with voltage drop over distance. Longer distance = fatter cable. Big house loads (ie upwards of 100A) means you then need bigger cables again...

 

 

[PeterW]

Ahh and they have spoken !

 

Is the garage near the house..?? you could get away with 35mm to the house and 25mm to the garage if you can live with 80A to the house and 50A to the garage.

[Nickfromwales]

Won't 2 cable runs be more money? Plus 2 switched fused isolators too to separate the two different rated cables ?

One big cable to the house, or if the garage is significantly closer, go there first for the new meter location and then onto the house so you don't have such a big incomes going into the house. 

100m is a long old run, so get the chequebook ready

 

[Nickfromwales]

Running 3 x 70mm SWA singles or 1 x 70mm 3-core SWA doesn't really make any difference to the price. Still around £1500.00  

One example for 3 core

And singles

[Jeremy Harris]

Before going too far down the path of looking at massive cables, do a quick diversity calculation.  It's very unlikely that you'll be running all the high current loads at the same time, especially if there's only one or two people working in the workshop at any one time.  If I add up all the loads in my workshop, and assume they can all be on all the time, I'd need a cable three times bigger than the one I ran down there.  Normal diversity rules aren't really suitable for a one-man workshop, so what I did was work out the highest possible load that could be on at the same time.  This turned out to be the compressor, a room heater, all the lights, the metal bandsaw (which can be left running when cutting big lumps of stock) and my big pillar drill.  The load then came down to around a third of what it would have been with everything on. Even then I think I was pessimistic, as the compressor could only run for a very short time if I was busy cutting and drilling, so I could probably have safely taken a small hit on voltage drop and assumed around 50% of the compressor load.

[Nickfromwales]

As this will have to be signed off by a sparky, it won't be down to interpretation, it'll have to be by the book. If the house AND the workshop are on the end of the 100m run, whatever configuration, its going to be a biggy. I'd certainly run at least a second phase up and have the workshop on its own phase so the house lights don't show activity in the workshop. Over that run, and with things running at peak loads I'd fear there would be some noticeable drop otherwise, even with the bigger cable. 

[Onoff]

So that connected phase on the incoming 3p feeds what? Existing building?

[Russell griffiths]

16 minutes ago, Onoff said:

So that connected phase on the incoming 3p feeds what? Existing building?

 

Im not a sparky so I could be wrong, but the 3 phases come to the brick cabinet but only a small feed comes out as it is just feeding a small wooden cabin at the moment. 

[Russell griffiths]

How does every one feel about running a swa cable under water????

 

 

wtf I hear you say

 

the cable that feeds the cabin at the moment runs directly across the lake underwater. 

[Russell griffiths]

If I do a quick add up of all sockets,lights and then a list of tools in the garage will this give us a better idea of load

 

I agree with @JSHarris in that I won't be using a grinder at the same time as the pillar drill, the workshop is just for me and only for diy home fixing of things. The days of spending hours welding excavator buckets up have gone

 

the thing that won't change is the distance from the main switch to the building plot, I am going to have the garage connected to the house rather than detached and going to put the cu in the garage on the wall closest to the brick building to keep the cable as short as poss. 

[Russell griffiths]

The yellow line will be the new cable, the blue line is where it runs now, on a sunny day you can see the cable laying on the lake bed in about 4 feet of water. 

[Onoff]

31 minutes ago, Russell griffiths said:

 

Im not a sparky so I could be wrong, but the 3 phases come to the brick cabinet but only a small feed comes out as it is just feeding a small wooden cabin at the moment. 

 

You have 3 BS1361 fuses there on the red/yellow and blue phases incoming. The red and yellow are unused and your current single phase supply is taken off the blue incoming phase.

 

The red and black coming from the meter are the tails.

 

Underwater runs.....out of my depth there.....  (I do as an aside know a lad who works for Flag Telecom that specialises in shore transfer stations for undersea stuff).

[Jeremy Harris]

Have a look in the book with regard to cable sizing.  There are two main cable sizing criteria, this one (excuse the scan - I only have a paper copy of the 17th Ed):

 

 

The voltage drop limits are here in the regs:

 

 

and the diversity guidance, which is non-existent in the regs (but is in additional guidance, but it doesn't really help much for a one-man workshop):

 

 

There is some older guidance on diversity on the TLC website, here: 

https://www.tlc-direct.co.uk/Figures/Tab6.2.htm

that may be useful, although it relates to the 16th Ed it is still valid, I think.  I don't have a copy of the guide on this, maybe one our electricians has an up to date copy.  Bear in mind that diversity calcs are often not set in stone in the regs, and the worst consequence of getting them wrong is cable under sizing and nuisance tripping - there isn't a hazard from underestimating the load, as the protective device will be sized to protect the cable, not the load.

 

I opted to do my own diversity calcs based on how I thought I'd use the workshop, with enough of an allowance to be pretty sure that I'd rarely, if ever, get a nuisance trip.

 

One final thing to consider is the effect on earth loop impedance if you are thinking of exporting the PE down the cable.  My personal view was that I was happier with my workshop being TT'd, so I didn't export the earth down the long cable from the external CU that supplies it.  I fitted a long earth rod through the workshop floor, measured it to make sure it was OK and have used that as the workshop PE.  I didn't need to, as the earth loop impedance on the exported earth was within limits, but I felt happier knowing that the concrete workshop floor had a low impedance "connection" to the PE.

 

 

 

 

 

Edited July 29, 2017 by JSHarris

[ProDave]

Just now, Russell griffiths said:

The yellow line will be the new cable, the blue line is where it runs now, on a sunny day you can see the cable laying on the lake bed in about 4 feet of water. 

I assume your meter box is at the top of the picture? What size is the existing blue cable? it has after all served the existing house for some time. If it's large enough, why not re use it?

[Russell griffiths]

2 minutes ago, ProDave said:

I assume your meter box is at the top of the picture? What size is the existing blue cable? it has after all served the existing house for some time. If it's large enough, why not re use it?

Yes meter box at the top of pic, old cabin covered by blue and yellow blobs in pic,

the existing cable feeds the cabin which is very basic probably a dozen double sockets and an electric oven is all that's in there. 

I would like to get the cable out of the lake as it bothers me where it is as I did hook hold of it one day whilst dragging some weed out. 

Can swa be joined? Could I use the existing cable to run the garage and a new cable to run the house. 

 

Is this silly as by as by the time I buy a 20m piece and get it joined correctly and then buy a new 100m piece for the house I would be better off just getting a nice new bit run the whole length. 

I was thinking if I pull out that old swa I could use it to feed the sewage treatment plant and also a small garden potting shed the wife wants. 

[Jeremy Harris]

SWA is OK submerged, and yes it can be joined, using potted joints (I have a buried join where another run is tee'd off).  Having said that, a new run may well be the best bet, and is what I'd do.

 

Whether the old cable would be OK for the garage depends on its size and condition. 

[ProDave]

SWA can be joined and if joining underground you get a joint box that is then filled with liquid resin that sets and encapsulates the joint.

 

As to re use, depends what size it is. My guess is volt drop did not figure in the original install, they just wanted something that "worked"

 

EDIT: Typing at the same time as Jeremy and said much the same thing

[Russell griffiths]

That existing cable had been under water for 30 years it gets lots of light in the summer and I am concerned if I take it out of the lake it could end up being a bit brittle or soft or whatever. 

Bugger just going to go for a whole new length, just need to work out size as jumping to 70mm is a big cost. 

[Jeremy Harris]

100m of 50mm² at 100A (absolute max per phase that's allowable by the DNO, I suspect) would give a voltage drop that's OK (3.7%).  Not sure where the 70mm² came from, AFAICS 50mm² SWA looks fine, well within spec.

 

 

 

Edited July 29, 2017 by JSHarris