three phase

[spidersparks]

Hi chaps got a job coming up wiring in a kiln. We put it a single phase one in a wile back ran in two three core swa's one did the kiln the other did the a consumer unit. Customer now wants a three phase kiln put in and do away with the consumer unit as its no longer feeding anything, both swa's are 10mm cables but my question is can I run the three phase down one cable and the neutral and cpc down the other? the armoured will be earthed on both cables

All points be great cheers

David

 

[Guest55]

just from what i`m aware....the neutral should always be within the same containment as the phase/s....now this is for metal containment...same with knockouts....so wouldn`t an SWA be a earthed mettallic containment Biff....yes yes i know its insulated but....

good point glenn.
but the magnetic heating effects can be massively reduced if the cables are glanded onto non metallic surfaces , and ran closely together ( ie tie-wrapped to each other )
edit ; eddy currents increase the further apart the conductors are.
another edit lol ; if this was a 300amp , 100mtr circuit - it might be an issue but with 40 odd amps to a back yard kiln i dont see to many probs. ;-)

 

[Knobhead]

its very simple.
x2 3 core swa cables , both 10mm csa.
cable 1 - lines 1 , 2 & 3 - colour coded brown , black , grey.
cable 2 - neutral & earth and 1 unused core , colour coded blue , green/yellow.

No way, sorry Biff but that is a NoNo.
Depending on how the kiln is wired 3ph delta or star or 3ph+N star connected, neutral could end up with quite a bit of current on it

 

[Knobhead]

good point glenn.
but the magnetic heating effects can be massively reduced if the cables are glanded onto non metallic surfaces , and ran closely together ( ie tie-wrapped to each other )
edit ; eddy currents increase the further apart the conductors are.
another edit lol ; if this was a 300amp , 100mtr circuit - it might be an issue but with 40 odd amps to a back yard kiln i dont see to many probs. ;-)

What are the armours made from? The entire cable will heat up. Have a look on google for AWA cables, it's the only way it could be done.

 

[GLENNSPARK]

What are the armours made from? The entire cable will heat up. Have a look on google for AWA cables, it's the only way it could be done.

yes Tony....its magnetic hysteresis isn`t it....

 

[Knobhead]

yes Tony....its magnetic hysteresis isn`t it....

No, magnetic induction :smilielol5:

 

[Guest55]

The entire cable will heat up.

how much heat would you expect considering its probably only 40ish amps ? , with an ambient temp no higher than 20 degrees for most of the run ? , and with a cable that is probably only pushed to 70% of its capacity ?
I strongly suggest that the effects from this type of scenario are being greatly exagerated.
Will it trouble a cable rated for 70 degrees continuous service ?
( 90 degrees if its xlpe )
i find that hard to believe.

 

[Knobhead]

The armours will act as a 1/1 CT so in your case 40A would be flowing all be it at miniscule voltage. It's the current that generates the heat.

 

[ayjay]

i think you may get probs with eddy current /heat/ ect
check whether the new kiln needs a neutral first, it may not, if it does why not consider using 2 cores as l1-l2 and the third core as neutral
then in the other armoured one core as l3 1core as neutral and one core as earth
(double up the neutrals at each end)
what do you reckon?

 

[Sintra]

Just do it right and replace with a 4 core SWA if a neutral is required.

 

[MDJ]

Ive never heard of that done in nearly 30 years in the trade, even 2 phases in one armoured and the thrid core as earth and 1 phase and neutral in the second with the third core as an earth is 100 times better than 3 phases in one armoured and the neutral with an earth in the second, just don't do either, both are just wrong, run a 4 core armoured and an earth ty wraped to it, thats my view anyhow.

 

[spidersparks]

Thanks chaps my feelings were towards not to do it but I was asked if it was possible so I thought I'd get confirmation to my own doubts really pleased you guys are on the same wave length

Many thanks once again

David

 

[loop3]

That's the question. Is it a balanced three phase load? It's only serving one piece of equipment so find out. In a balance three phase system the primary function and only function of the neutral is to carry away out of balance current. If it is, use the there cores for phases. Then I would personally pull in a separate cpc. If you choose to use the armour you can find it's resistance in the blue book. Design guide.

 

[Engineer54]

I don't understand any self respecting electrician even contemplating such a supply set-up!! , Yes there are way's around the problem your creating, but why would you want too, rather than doing the job right in the first place!!

Also remember, any losses due to circulating/eddy currents are paid for at the DNO'S meter, They don't come free!! lol!!!

 

[ayjay]

i just though it was a novel idea ive never had to do it
i reckon it would be ok though
anyone for a pop quiz?