# current capacity of SWA cable- combined cores?

#### frankychops

I've a 10mm 4 core SWA cable that feeds my garage.

I'm going to get a new 3 phase supply put in.

If this becomes the cable to feed my house(10mm 4 core), can I use the '4th' core to increase the CCC of the cable? Ie,10mm CSA effectively becomes 20mm CSA on the live? I'd guess if possible it/will be derated.

Given the dereated CCC of the cable, i'm looking at circa 50amps to run the house on, given the oven hob is gas, no electric showers, this should be 'ok'. However i'm trying to work out if there'd be headroom by using the 4th spare core.

Are you using the 10mm SWA for three phase?

Or is the three phase supply head in the the garage and you are only using one phase to supply the house?

Are you using the 10mm SWA for three phase?

Nope, a new 3 phase supply to a meter box(side of garage), then the 10MM SWA that runs from the garage to house will become the main supply cable for the house. Garage is the only 3 phase usage area.

Hmm. There are at least a couple of regulations regarding parallel conductors in BS7671. You’ll need to have a read.

in principle you could double then up, it is done a lot in commercial applications but
I have no idea if 2x10mm²=20mm² or what the current carrying capacity of such an arrangement might be. Some cable manufacturers may have tables on their web sites.
You’ll need to have a standard size fuse anyway so Let’s guess and say it’s 60amp.

Can you really run your house given that limitation???

Secondly, you have thought about doubling up on the L&N, but what about the earth? If you were to use the SWA armour, you would need to make some serious adiabatic calculations to ensure this meets the requirements, or you’ll have to run a separate main earth all the way, or convert the house installation to TT. That would be a backward step.

Hmm. There are at least a couple of regulations regarding parallel conductors in BS7671. You’ll need to have a read.

in principle you could double then up, it is done a lot in commercial applications but
I have no idea if 2x10mm²=20mm² or what the current carrying capacity of such an arrangement might be. Some cable manufacturers may have tables on their web sites.
You’ll need to have a standard size fuse anyway so Let’s guess and say it’s 60amp.

Can you really run your house given that limitation???

Secondly, you have thought about doubling up on the L&N, but what about the earth? If you were to use the SWA armour, you would need to make some serious adeabeic calculations to ensure this meets the requirements, or you’ll have to run a separate main earth all the way, or convert the house installation to TT. That would be a backward step.

It might make more sense just to run some new 16mm CSA cable given its cost.

Definitely. Really, the registered electrician who us doing, certifying and notifying this work is your best place to ask!

Definitely. Really, the registered electrician who us doing, certifying and notifying this work is your best place to ask!

I'm just doing the donkey work, I need to work out costs/hassle agaisnst the plus of 3 phase. also finding the route!

I suggest you just pencil in your ideas and agree them with your electrician.

he/she will be signing the cert to say they designed the installation. It won’t help if you present him with a coil of 16mm and he insists you need 25mm.

Hmm. There are at least a couple of regulations regarding parallel conductors in BS7671. You’ll need to have a read.
Indeed.
I have no idea if 2x10mm²=20mm² or what the current carrying capacity of such an arrangement might be.
I don't, either. I presume that the 3-phase CCC quoted for 4-core cable assumes that each of the three phase conductors may be carrying that CCC (simultaneously) but, beyond that, as you say, one would have to find some chapter and verse.
Secondly, you have thought about doubling up on the L&N, but what about the earth? If you were to use the SWA armour, you would need to make some serious adiabatic calculations to ensure this meets the requirements, or you’ll have to run a separate main earth all the way, or convert the house installation to TT. That would be a backward step.
All agreed. However, if I'm reading it correctly, the OP is suggesting that only the L would be 'doubled up' (using the '4th conductor), so maybe he was planning on using one core as an earth? However (2), although it would reduce voltage drop a bit, doubling up the L but not the N would not increase the CCC of the circuit (i.e. it would not increase the allowable current through the N) - so I'm not quite sure what is being proposed. As has been said, new 16mm² SWA would seem to be what's really needed. I don't think that we have been told the length of the SWA run.

Kind Regards, John

Indeed.
I don't, either. I presume that the 3-phase CCC quoted for 4-core cable assumes that each of the three phase conductors may be carrying that CCC (simultaneously) but, beyond that, as you say, one would have to find some chapter and verse.
All agreed. However, if I'm reading it correctly, the OP is suggesting that only the L would be 'doubled up' (using the '4th conductor), so maybe he was planning on using one core as an earth? However (2), although it would reduce voltage drop a bit, doubling up the L but not the N would not increase the CCC of the circuit (i.e. it would not increase the allowable current through the N) - so I'm not quite sure what is being proposed. As has been said, new 16mm² SWA would seem to be what's really needed. I don't think that we have been told the length of the SWA run.

Kind Regards, John

Hi John

Looking at it, I may as well put in new 16mm swa to feed the house off the 3 phase(proposed) distro behind the garage. it'd only be a 18M run.

it'd only be a 18M run.
Well. It may seem “only” 18metre run. But what’s the VD on 16mm @ 60amps. ??

It depends on the installation method. But my quick check says it should be ok. But, again, it’s for the designer electrician to calculate this, not a few hot people on an Internet forum.

Well. It may seem “only” 18metre run. But what’s the VD on 16mm @ 60amps. ??

It depends on the installation method. But my quick check says it should be ok. But, again, it’s for the designer electrician to calculate this, not a few hot people on an Internet forum.

xlpe, 16mm csa, 18m run, less than 4v drop when clipped so fagpacket maths says 72Amps

Great let’s risk it and crack on then

What about the adiabatic equation to ensure the earthing will be sufficient?

Why are you using xlpe?

Great let’s risk it and crack on then

What about the adiabatic equation to ensure the earthing will be sufficient?

Why are you using xlpe?

As that's what i've had some prices for. I'm as above i'm working out the realistic costs/whats involved of having a 3phase supply put in then how that alone knocks onto other things. I'm not running around wiring up old ladies houses in 1.5mm T+E.

Out of interest.
If the house has a supply now. You could just use it

Or don’t you want two standing charges. ?

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