3 Phase cable sizing help?? Proceedure?

[Smudge]

i'm after a little help with cable sizing. I have tried reading my IEE design guide, and still struggling to get to grips with the correct proceedure to follow for sizing cables. I have always struggled a little with this, but thus far my knowledge goes (albeit shaky)...

It>_ In/(Ca x Cg x Ci x Cc)

then,

Vd = ((mV/A/m) x Ib x l) / 1000

I'll go no further as i'm already turning grey at the thought of embarrassing myself. Can someone please take the time out to help me understand the full proceedure. And what happens whent the 3 different values for mV/A/m appear in the regs??

Or direct me to the link in another thread where this may be explained.

This is not for a job/ nor a project etc, i'm finally admitting my weakness in this area and after some much needed advice on the theory side.

I'm sure with the wealth of knowledge on this site, someone will be able to knock some sense into me.

 

[malcolmsanford]

R= resistive load
x= inductive reactance
z= impedance

These are the values you find in the volt drop tables for 3 phase supplies. Theoretically speaking temperature and power factor are two things that generally affect voltage drop, when designing a circuit these are the two generally unknowns.

There are basically 2 formulas for volt drop

<16mm²

(mV/A/m) x Ib x L / 1000

>25mm²

(mV/A/m)z x Ib x L / 1000

As you see if your dealing with cables 16mm and less it is quite straight forward, but cables normally 25mm and over you need to know your loading and what type of loading you have, as this will affect greatly your cable choice

The figures in Appendix 4 are calculated with a temperature factor of 1 for 30 deg, this would almost certainly be the factor you would use for cables of less than 16mm.

Bigger cables in normal circumstances would operate at much lower temperatures therefore voltage drops are less than those ado vacated in the tables, say for instance you had a cable operating in an ambient temperature of 20 deg then you could use a reduction factor in the voltage drop of say 0.985.

Really bottom line is unless your really having to go into saving money on cable sizing on a huge project equation m/V/A/m(z) x Ib x L / 1000 will do for any cable over 25mm

 

[i=p/u]

thats what i was taught upto 25mm you have no reactance on cables.. or something like that..

i dont do much 3phase work other than add to existing board..... but i will take it up again but hopefully when working with the right guy

 

[dagrat]

also to calculate Ib the formulae is Ib=p/sqrt3 x vl
and if power factor and efficiency are involved then Ib = p x 100/sqrt3 x vl x pf x eff

 

[i=p/u]

Say you have 8kw 3phase coffee machine does this mean the8kw is decided between 3 phases ie. 2.7 kw each phase ????

 

[stef]

No, just the current of 8kw is split into the three phases, overall power consumption remains the same.

 

[CSEE]

I use Amtech Single Cable Calc Software, it is great just enter all load and cable details etc and length of run and it calculates the correct size etc
simples!
Darren

 

[i=p/u]

Is there any good ones for iPhone? You know of?

 

[Mark.W]

Draka on the iPhone isn't too bad!