Cable Selection And Connections

[Darkness]

I was speaking to a sparky this week and I am still not sure on tackling the situation that I have.

Coming from a 100A supply to an outbuilding is around 80m 16mm² with first half being T&E and the other half being SWA which goes to an outbuilding. The two are joined/connected via a CU. The T&E is mostly overhead cable because of restrictions preventing SWA underground.

Anyhow, I have built a large garage/shed and I am nearly ready to wire it but I am debating cable size. Equipment such as a powerwasher will be used so I doubt it will use anymore than 4000w.

Distance from the CU which connects the T&E/SWA, to the new building is no more than 150M due to route and I will most likely get 150m to be on the safe side. I had thought 25mm² would be ok but the sparky told me 16mm² would be better and most importantly he mentioned problems connecting 25mm² cable hence why he said to go with 16mm².

Not sure what to go for but I was thinking the distance and you never know what I might want to connect or use in the future. One thing I didn't ask was using a larger cable coming of a smaller cable i.e running 25mm² cable from 16mm² run. Is that allowed?

Cheers for any suggestions.

 

[ericmark]

At 2.8 mV/A/m 80 meters and 100 amp I make it 22.4 volt drop assuming you have lights then the limit is 3% or 6.9 volt with this I think working out the Earth loop impedance is pointless.
Eric

 

[333rocky333]

Is it off a 100 amp supply or fused down to a lower rating first.

16mm T+E is not rated at 100 amps

 

[ericmark]

Agreed 80 amp limit on 16mm T&E but 25 amp limit on volt drop I am sure earth loop impedance and prospective short circuit current will also be a problem I did not think it worth working out the rest.
Eric

 

[Lectrician]

It depends on the load you want at the new unit.

If you go for a 63 amp submain, which is going to be far more than you need by the sounds of it, then even 25mm is pushing it in terms of volt drop, it leaves verrrrrry little for the final circuits, and hence you consider going larger again.


Now, if you say your loading is less, and you went for a 40amp submain, then 16mm is tight, but 25mm fine.

You mention 4kw. What other kinds of loading are we looking at? You really need to get a current consumption in mind, and then size the cable accordingly.





As for the spark preferring 16mm to 25mm..........That just makes me question the ability of the sparks in terms of being able to deal with larger cables (and 25mm IS NOT large!), and his ability to understand cable selection.

 

[333rocky333]

I read it, that he is only replacing the second half of the run and will increase to 150 metres.

The first section of 16mm T+E he is going to reuse 40 metres

 

[Lectrician]

Ah - I completely missed that in the original post.

We really need to know the loading of the other out building then?

With a 150m run, even at 32amp for the submain from the end of the T+E, you are going to need a hefty size to combat the volt drop already lost from the T+E.

We need to know the loading on the T+E, and the loading of any other shed.

If the 16mm is protected by a 63amp fuse, and then the 150meter run by a 32amp, I reckon you are looking at 35 or even 50mm to combat the volt drop.

You really cannot guess this though - the loadings need to be determined. The protection being 63amp is likely to be way oversized I expect for the capacity you require, and as we are not there to see what you intend to power, or what the sheds are for, it is difficult to judge what sort of size submain is acceptable.

 

[ricicle]

.That just makes me question the ability of the sparks in terms of being able to deal with larger cables (and 25mm IS NOT large!), and his ability to understand cable selection.

Yes, I was wrestling with a 240mm² 4 core just this week

 

[Spark123]

Just been playing with a few PILC cables, old sizes but 2 were pretty big around the 125mm.

 

[RF Lighting]

That's nothing.

I've just done a 2L1.5 pyro

 

[Lectrician]

My Largest last week was only a 50mm 4c

Did some 400mm Alu armour single cores a while back - actually right on the switch from old to new colours.......so a fair while ago!

 

[ricicle]

That's nothing.

I've just done a 2L1.5 pyro

Poor you, you must be knackered !

 

[Spark123]

Did some 400mm Alu armour single cores a while back - actually right on the switch from old to new colours.......so a fair while ago!

Inside a substation?

 

[Lectrician]

Nah. From generator to changeover switch. I'm sure there are pics here somewhere.....

 

[Darkness]

Just to clear things up, I want to T off at the CU that connects the T&E/SWA. At the SWA end of run, output should not be anymore than 3500w used at the one time.

Between the existing run and the new run I want to lay, there should never be anymore than 5500w to be safe. Basically the powerwash is the item that will be shifted about now and again so that is what will use the most juice. Lighting consumption is included in that figure.

A friend of mine earlier said I should lay strong enough cable as if I was installing a shower at the new building. Maybe I should consider that statement than trying to guess the wattage.

I have a feeling the T&E is a stumbling block here but its something I can't really change. Oh, the T&E is coming from a 63A RCD. SF is 100A.

Anymore questions or I've missed something, please ask.