Voltage drop and 3phase

[keniff]

I'm having a bit of a blonde moment, I'm trying to work out the volt drop on a 3 phase Submain, the Submain is to be supplying a 3phase board loaded with rcbo's all single phase loading, I can't for the life of me remember how to do it I think it's working out the single phase vd and multiplying by 0.86 but have gone blank! Any help appreciated

 

[ackbarthestar]

Check the AC volt drops for 2-core and 3-4core cables in appendix 4 BS7671 and you will see a difference of rt3/2 (0.866) as you have stated.

eg. 25mm2 2-core cable has an overall volt drop of 1.8mv/A/m but for 3-4 core it is 1.55mV/A/m.

 

[keniff]

Ah right ok so that's already done on the stated values that makes sense, so if I use the largest load and work out volt drop on that value ( it's a very hard one to balance as swimming pool is pulling a large load on one phase and circuits are spread out on the other 2 phases) thanks alot

 

[ackbarthestar]

Assume the worst if its that unbalanced and treat the sub main and final circuit as both having a neutral volt drop. (2-core)

Otherwise its calculating the sub main and then adding it to each final circuit in turn and taking the highest value.

 

[keniff]

Assume the worst if its that unbalanced and treat the sub main and final circuit as both having a neutral volt drop. (2-core)

So use the largest load and use the vd figures for 2 core cable out the bgb? Its the neutral size I'm concerned of iv always been told keep it the same size as the phase unless if cost is an issue then start calculating to see how small you can get it!!

 

[Sintra]

I personally would keep the neutral conductor the same size regardless of the savings. To reduce the neutral requires a very precise design knowing all the constant phase loadings etc. Then applying this formula to determine what current will be drawn by the neutral.

In=√((IL1²+IL2²+IL3²)-((IL1xIL2)+(IL1xIL3)+(IL2xIL3)))

 

[Silly Sausage]

Then applying this formula to determine what current will be drawn by the neutral.

In=√((IL1²+IL2²+IL3²)-((IL1xIL2)+(IL1xIL3)+(IL2xIL3)))

Then you've got the harmonics content to worry about!

 

[Knobhead]

Harmonics only really start to cause problems when VFD’s or SMPS’s are in use.
3½ core cable is normally for larger sizes. Now a days I would use 4 core no matter what.

 

[Silly Sausage]

Lured him in!

 

[Knobhead]

And I fell for it :90:

 

[Knobhead]

I still wonder how all the 185mm and 240mm 3½ core cables I put in are fairing after 30 years.

 

[keniff]

I still wonder how all the 185mm and 240mm 3½ core cables I put in are fairing after 30 years.

Jesus Christ and I'm getting all flustered over terminating a 35mm lol! What sort of equipment are they supplying?

 

[Knobhead]

Sub boards out on the plant. It's no different to what you're doing, just needs a few more animals to pull the cable in.
25mm or 240mm it still needs just the same skills to make off correctly. You just need the animal power :90: