3 phase cabling and DB

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GatemasterNz
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3 phase cabling and DB

Post by GatemasterNz »

hi just wondering if someone could help me please
first off i am not a registered electrician but a DC technician so any subsequent work will be done by an electrician

we have one main DB supplying a number of sub boards on the property
i have a 100m 2 core and neutral screen running from the main DB to a sub board at the pump shed

this cable passes right by our main workshop that i a trying to get a 3 phase feed to. this is to operate 1x 3 phase motor

questions as follows
can i use the 2 core plus neutral screen to create a 3 phase supply to the workshop. (cable core sizes are12mm as is the neutral core)?
can i supply a secondary sub board off another sub board?
can i have 2 x sub boards in the same workshop (in this case one SB supplying the 240volt equipment and the 3 phase SB supplying one 3 phase motor- for that matter do i even need a 3 phase sub board in the workshop or can i come direct off the main DB with 1 x 3phase isolating switch wired in series?

just trying to create a bit of a budget and workout how extensive a installation this could potentially be

thanks for your time

BTW if anybody is looking for gate automation or advise i would be happy to help -we import supply and install
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gregmcc
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Re: 3 phase cabling and DB

Post by gregmcc »

GatemasterNz wrote:
Sun Oct 03, 2021 11:58 am
hi just wondering if someone could help me please
first off i am not a registered electrician but a DC technician so any subsequent work will be done by an electrician

we have one main DB supplying a number of sub boards on the property
i have a 100m 2 core and neutral screen running from the main DB to a sub board at the pump shed

this cable passes right by our main workshop that i a trying to get a 3 phase feed to. this is to operate 1x 3 phase motor

questions as follows
can i use the 2 core plus neutral screen to create a 3 phase supply to the workshop. (cable core sizes are12mm as is the neutral core)?
can i supply a secondary sub board off another sub board?
can i have 2 x sub boards in the same workshop (in this case one SB supplying the 240volt equipment and the 3 phase SB supplying one 3 phase motor- for that matter do i even need a 3 phase sub board in the workshop or can i come direct off the main DB with 1 x 3phase isolating switch wired in series?

just trying to create a bit of a budget and workout how extensive a installation this could potentially be

thanks for your time

BTW if anybody is looking for gate automation or advise i would be happy to help -we import supply and install
Q1, 2 Core neutral screen is a single phase supply (phase, neutral and earth), you could not get 3 phase from this. a 3 phase would require 4 cores IF you didn't run an earth and put in an earth pin at the sub board.
Q2, you can supply another sub board of one, tend to run in to volt drop issues amongst other things
Q3, you can, but things get messy. Without seeing exactly what you have it's hard to give specific and correct answers, time to talk to your electrician.
GatemasterNz
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Re: 3 phase cabling and DB

Post by GatemasterNz »

Cheers for that

just one question

i have read that you can do 3 phase from 3 wires wired as a star connection (sqroot of 3 * 230 volt = 398v)
if you needed an unbalanced load ie plan to take a 240 volt supply as well then you need a 4th neutral wire

or have a got my wires crossed :)
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Re: 3 phase cabling and DB

Post by TPower »

Two core neutral screen cable, and you’re thinking of using the two cores and the screen all as active conductors is that right?

You won’t be able to do that, it’s unsafe and against the rules. The screen is reserved for a neutral or earthing conductors only.
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gregmcc
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Re: 3 phase cabling and DB

Post by gregmcc »

GatemasterNz wrote:
Sun Oct 03, 2021 6:07 pm
Cheers for that

just one question

i have read that you can do 3 phase from 3 wires wired as a star connection (sqroot of 3 * 230 volt = 398v)
if you needed an unbalanced load ie plan to take a 240 volt supply as well then you need a 4th neutral wire

or have a got my wires crossed :)
That's the way it comes from the distribution transformer in to an installation. Your right on the balanced part. If you are doing a 3 phase submain balanced load or not the neutral is required and this has to be tied to installation earth through a MEN link to the earth so that any fault current to earthed metalwork can return thru the neutral and activate the protection device (Circuit breaker/fuse).

So a 3 phase subboard HAS to have 4 wires 3 phase + Neutral, your can run a 5th wire for the earth OR put in an earth pin and MEN link.

I see where you are coming from on using the 2 core neutral screen - this adding up to 3 usable cores - 3 phase, but the Neutral screen cannot be used for an active conductor (phase) as it would only be a single layer of insulation.

Basically you are going to need to run a new suitability sized circuit for your 3 phase supply, be it a new submain or a circuit for a 3 phase socket.
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Re: 3 phase cabling and DB

Post by t92300 »

GatemasterNz wrote:
Sun Oct 03, 2021 11:58 am
this cable passes right by our main workshop that i a trying to get a 3 phase feed to. this is to operate 1x 3 phase motor
What size is the motor?
You may be able to supply it from single phase using a VSD. If that is the only thing you need 3 phase for and already have single phase there.
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