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Discuss Question about sharing Neutral.. in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

A

atomic

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Hi All, Just registered here as I have a question about the installation of an electrical oven.

I am now living overseas, but the principle is pretty much the same as in the UK.

I have recently laid 2 x 3x2.5mm cables for the installation of a new oven with induction hob. As I believed both units connected to a normal 3-phase oven connection. (I had no 5x2.5mm available)

Now it turns out, that the induction plate, has a normal 3 phase connection, but the Oven itself requires a seperate normal plug socket.

so I have 6 x 2,5mm wires now available to complete this.

My plan is to connect the induction hob using the normal 3 phases, Neutral and Earth, using 5 of the available wires. Then using the remaining wire for the plug socket for the oven, and sharing the Neutral and Earth between the 3 phase hob and the single phase socket.

Is there any risk I am overloading neutral doing this ?


The oven is 2.2kw and the Induction Hob is 7.2kw.


Hope someone can help, look forward to hearing from you.
 
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W

wayne

  • Thread Starter Thread Starter
  • #2
in the uk its one neutral to one circuit you have two circuits there ,however is this question for overseas if so whats their regs?
 
R

rumrunner

  • Thread Starter Thread Starter
  • #3
in the uk its one neutral to one circuit you have two circuits there ,however is this question for overseas if so whats their regs?
I wouldnt think the neutral does much anyway .best thing do it turn it all on and test current in neutral ,i dont think it will be very much ;)
have fun:)
 
A

atomic

  • Thread Starter Thread Starter
  • #4
Thanks for the answers, I haven't a clue what the regs are, but I am sure, like you say, its one neutral per circuit.

I also don't expect much weight to be on neutral, but unfortunately can't test it yet as the new hov/oven hasn't arrived, and when it does it will be too late to change the cable.

At least nobody has written back "YOU CAN'T DO THAT, ITS A TERRIBLE IDEA" so I feel partly re-assured :)

Regards

Atomic.
 
S

Shakey

  • Thread Starter Thread Starter
  • #5
Thanks for the answers, I haven't a clue what the regs are, but I am sure, like you say, its one neutral per circuit.

I also don't expect much weight to be on neutral, but unfortunately can't test it yet as the new hov/oven hasn't arrived, and when it does it will be too late to change the cable.

At least nobody has written back "YOU CAN'T DO THAT, ITS A TERRIBLE IDEA" so I feel partly re-assured :)

Regards

Atomic.
Is the hob wired as single phase hobs of each of the three phases, or they actually three phase elements? If so, they will be star connected and there will be no (or neglible) neutal current.

even if not, you can apply diversity to the hob, so 7.2kw is about 2.5kw per phase (roughly) so about 10 A per phase. But of course not all phases can peak at the same time, so you still find your neutral current is preety low. 2.5 is roughly 27A (with no correction factors,) so you are well within

So

A. You wont oveload the cable
B. Cant say if its 'legal' against your regs, but would almost certainly be against ours because of the borrowed neutral
 
I have to disagree with shakey
Only single core PVC single phase has a rating of 27A clipped direct.

You have 2nr 3 core cables which abroad will typically be wired through conduit
This at 3 phase only has a rating of 20A (table 4D2A col 5) which then has to be derated by 0.8 for a group of 2 = 16A

Without diversity you could get 21A at 220V (2.4kw + 2.2kw = 4.6kw).

With diversity though, which you can legitimately use, this will be much less so I would say the cables won't get too hot. They should all be wired back to a 20A TP circuit breaker. Using one neutral for the 2 items does not class as a borrowed neutral provided both circuits connect to one circut breaker. A borrowed neutral is where two separate circuits with separate fuses/CBs share a neutral.

Mike
 
W

wattsup

  • Thread Starter Thread Starter
  • #7
A shared n is not the same as borrowed as above, so long as the breaker controls the shared n, no problem. 3-phase ovens, the ones i've seen anyhow, are not truely 3-phase, elements are single phase, they just have a 3-phase supply. You should be able to tell from the connection terminals. Just power the whole thing up and use tongue tester to load test.
 
R

rumrunner

  • Thread Starter Thread Starter
  • #8
Hi
Most industrial ovens ive worked on have been wired as "4 wire star" which is the old bs way of doing it ,euro stuff might be different,and if it is 3 seperate neutrals ,which i think is uncommon then that means he needs 8 cores plus cpc so i think it must be conventional wiring
 
B

Bane

  • Thread Starter Thread Starter
  • #9
Well...I think it's a terrible idea.
 
S

Shakey

  • Thread Starter Thread Starter
  • #10
I have to disagree with shakey
Only single core PVC single phase has a rating of 27A clipped direct.

You have 2nr 3 core cables which abroad will typically be wired through conduit
This at 3 phase only has a rating of 20A (table 4D2A col 5) which then has to be derated by 0.8 for a group of 2 = 16A

Without diversity you could get 21A at 220V (2.4kw + 2.2kw = 4.6kw).

With diversity though, which you can legitimately use, this will be much less so I would say the cables won't get too hot. They should all be wired back to a 20A TP circuit breaker. Using one neutral for the 2 items does not class as a borrowed neutral provided both circuits connect to one circut breaker. A borrowed neutral is where two separate circuits with separate fuses/CBs share a neutral.

Mike
erm....I know what a borrowed neutral is thanks:p

the way he was describing it, using the 'spare cable' for the supply to the socket, and using the three phase supplies neutral for the return, I assumed he was going to put the socket cable on a seperate CPD, in which case it WOULD be a borrowed neutral.

If he was taking the lot of a three phase breaker, and just tapping off one of the phases for the socket, then he wouldnt be needing the extra cable

and i did say it was 27A 'roughly with no correction factors'
 
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A

atomic

  • Thread Starter Thread Starter
  • #11
If he was taking the lot of a three phase breaker, and just tapping off one of the phases for the socket, then he wouldnt be needing the extra cable

This is correct, If I was just taping one of the phases I wouldn't care. But I am using a completely seperate cable for the phase, and just borrowing the neutral.

There are 4 circuit breakers in play here, three from the 3 phase oven, and one for the socket phase.

I don't have the appliances yet, so can't do any loads tests, and I won't get them until after the floor is laid, and then its too late for me to run any new cables if needed. Hence the speculation now.

Thanks for all your comments. I think I am going to run another 3x2,5mm anyway.

Regards

atomic
 
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