Help with three phase power

[gavin26]

Hi I need some help with 3 phase wiring. I'm fairly confident I'm right but haven't done it since I was 18. Would I be right in saying that normal 3 phase power for sockets are wired in star and you should end up with 5 cables at each outlet? L1, L2, L3, N & E? With a 3 pole isolator for phases.Also do they have to be wired separately? Or can they be wired as radials?

I've been asked to do some industrial work (which I don't normally do!) what sort of thinks do I look for in regards to sized of cables etc what is the formulas for cable calcs on three phase?

I have a picture of the way I think they are wired but can't upload it because I haven't posted 5 times! Will upload once I can!

Appreciate responses

 

[telectrix]

i think from your post you really ought to get someone with 3 phase experience to help you on this job. it's a lot more complex than just wiring up a radial with a few sockets.

 

[gavin26]

So am I wrong in the way it's wired?

 

[bugsy]

1 What are the sockets to be used for
2 What load on each socket

Personally I would wire each outlet on seperate radial with a switched / isolated s/o (M.K. Commando range 16a - 125a ) but as Tel say's if your not sure ?

 

[trev]

I'm sorry mate but you're taking on work that you're not sure how to do. This is wrong no matter which way you dress it up.

 

[gavin26]

I have done 3 phase sockets as an apprentice but never been incharge of the design. I know it's different calcs to single phase etc. probably best getting somebody in with me then!

 

[i=p/u]

spead acrooss phases equally is no.1 or you neutral will carry tom much cureent.... BEWARE

 

[The Ranger]

jesus some off you's need to chill out

it's really not that hard but need a few bits of info

what size sockets?
how far is the cable run?
how is the cable installed?
grouped cables how many?
proposed protective devices?
ambient temperature?

 

[trev]

Ranger you're right it's not that hard but if you didn't know that would you take a job on that required it?

 

[The Ranger]

Ranger you're right it's not that hard but if you didn't know that would you take a job on that required it?

we all learnt once!

 

[trev]

we all learnt once!

Under supervision

 

[gavin26]

Hi ranger thanks for response. I know what the guys are saying but would just like a head start. I will in the end hire someone with 3 phase experience.

I couldn't tell you length of runs etc yet but it will more than likely be ran in conduit. Could you tell me the 3 phase formulas

 

[telectrix]

we all learnt once!

which is why i suggested he got some expert help with the job. it may be to power motors or other industrial equipment. get it wrong and you're in it up to the neck. apart from that, public liability insurance may not cover him for 3 phase.

 

[Knobhead]

The formulas for what?

 

[i=p/u]

1.7 x cos o<i x pf

 

[i=p/u]

there is a we bit to observe and i am only learning it so sorry i cant help you as i dont fully understand it cause im a we fish in a big pond but i bark

 

[UNG]

which is why i suggested he got some expert help with the job. it may be to power motors or other industrial equipment. get it wrong and you're in it up to the neck. apart from that, public liability insurance may not cover him for 3 phase.

How many electricians check their insurance before every job to see if they are covered, would seem to be a stupid exclusion for an electrician

 

[The Ranger]

1.7 x cos o<i x pf

surely for design current

In = P/(400x1.732) = ?A

note: no power factor given

 

[telectrix]

How many electricians check their insurance before every job to see if they are covered, would seem to be a stupid exclusion for an electrician

it's common. just a way for the insurance companies to screw more out of you.

 

[Knobhead]

surely for design current

In = P/(400x1.732) = ?A

note: no power factor given

In if your referring to 3Ph neutral current it is

In = √((Ia²+Ib²+Ic²)-((Ia*Ib)+(Ia*Ic)+(Ib*Ic)))

 

[i=p/u]

yous boys are good

 

[i=p/u]

pure inductive load , and what about these super conductors in sern

surely for design current

In = P/(400x1.732) = ?A

note: no power factor given

 

[Knobhead]

If you were using just one out of the three you’re almost right. Try 400/1.732 instead.

But if you have unbalanced load on each phase interacts with the others giving the 1/3[SUP]rd[/SUP] harmonic current.

For a balanced load (P/3)/(V/√3) = In. but the currents cancel themselves out making the neutral unnecessary. Run a neutral to a star connected motor and it won’t be a happy bunny.

 

[Edward Amsden]

Get some decent cable calculation software (cd). It requires the inputs you need most. As with alot of things you don't know. Do your research. Add it up. If all what your powering is 3 phase then n is ok.. So long as your circuit is rated to your breaker then you should be ok..
As I said do your research, and if its safe it is safe!!!

 

[The Ranger]

pure inductive load , and what about these super conductors in sern

my questioning comes from a lack of understanding on my part not questioning yours but as i was taught

Ib = P/(400x1.732xpf) example 10kW so 10,000/(400x1.732x0.8)= 18.04

selection of protective device

In= 20A

then It = In/rating factors ca cg etc

then on to VD

mVxIxLoR/1000