Sizing of cable for 3 phase equipment

alphonse mephisto

Can you help me out here a little please? I have to do a test theory and practical and have have done my cable sizing through the regs but need a second opinion from someone who knows what they are talking about.

3 phase 1.5Kw motor, run in trunking. lenght of run 5 meters. id like to use nymj. ambient of about 20 degrees. run with 3 similar circuits.

also..

3 phase 1Kw gas heater, length of run about 4 meters, the rest same as above.

Id imagine they would both be happy on type c 10A

Any help would be fantastic!

2011

alphonse mephisto wrote: »

I have to do a test theory and practical and have have done my cable sizing through the regs but need a second opinion from someone who knows what they are talking about.

Post up your calculations, this will allow other posters to comment on your method.

alphonse mephisto

1.5Kw, 400v, 3.75A, 3 similar circuits, in trunking, 6 meters, nymj, 20degrees.

Ib<In<Iz

Ib=3.75

In=10A type C

Installation method B2 (A52e)

Ct(A52g1)= 1.12

Cg(A52f)= 0.70

Iz= In / (Ct x Cg)= 10 / (1.12 x 0.70) = 12.7A

From A52f1 1.5mmsquare will do 15A

max VD = 16v (400 x 0.04)

From A52j1 1.5 has VD of 25mv per amp per meter

VD = ( mv x Ib x cable lenght) / 1000 = (25 x 3.75 x 6) / 1000 = 0.56v

So 1.5mm square should be fine to use.

I used the exact same method for the gas heater. Id imagine if one is right the other should be fine too but ill put the other up aswell if needed. One other question I do have though is will the 1.5 allow for the motors starting current?

frankmul

You don't have to take the motor starting current into consideration if the motor takes a long period to run up to speed, standard motor is up to speed in less that 5 secs.
How did you work out ib
P=root3 x Vl x Il x pf
400 x 3.75 = 1500. Where is pf or root3

alphonse mephisto

frankmul wrote: »

You don't have to take the motor starting current into consideration if the motor takes a long period to run up to speed, standard motor is up to speed in less that 5 secs.
How did you work out ib
P=root3 x Vl x Il x pf
400 x 3.75 = 1500. Where is pf or root3

You are of course correct, my knowledge of motors is limited and I have been away from this for a fair few years and it is taking a little time to come back. I cant get near the motor to see the rating plate until the day of the test, the only info I got was 1.5Kw and that it is 3 phase. So i took Ib as 3.75A purely on the basis that it will not run that high anyway? Do you feel 1.5mm would be enough? There is a reg that says fixed appliances must be wired with a minimum of 2.5mm do you feel this would come into play here?

frankmul

I get ib = 2.7A assuming a pf =0.8

Iz = Ib/(Ig * It), you use In for Ib

1.5mm should carry the load current no problem

The minimum size for fixed wiring is 1.5mm, think about the PVC/PVC singles used for lighting circuits.

Are you doing the equivalency test

alphonse mephisto

I actually done the same thing after reading your reply yesterday, it does seem to work out alright..
Thanks very much for the help. Much appreciated.
Im actually doing a thing called competency determination for fas. I went looking for my cert about a year ago, I have all seven phases passed and they don't dispute that. It turned out though that a few of the lads I worked for never registered me, so from fas point of view I don't have enough weeks served. They won't take probation into account either so this is the way out they have given me!

frankmul

It's a pain when that happens, best of luck with the test