Voltage drop in 4 core 50mm SWA

[minkster100]

We aim to install a 25M run of 50mm 4 core SWA. The cable will be buried in a tube in the ground (Ref method D). Design current is 100A per phase. However i am struggling with the voltage drop calculation. Using BS7671 page 281 table 4D4B we can see three values of mV/A/m, namely: r (resistive) ; x (reactive) and y (impedance). Which of these (or combination of ) should i use? The on site guide gives values (none of which seem to cover Ref method D) which it states "consist of both the resistive and reactive elements" but i cant figure out how the numbers they give relate to the seperate figures in the regs. Any ideas?

Cheers

 

[ian932]

Is it PVC or XLPE your using?

 

[apprentice87]

Hi,

It is the "Z" value you need to use. "R" is the resistive as you say, "X" is the reactive, and "Z" is the combined result, the impedance of the cable.

So, from the table it is 0.81

Therefore it would be current [100] x run length [25M] x .81 = 2025

Divide this by 1000 to get it back to volts from millivolts = 2.025V

That is the "between the lines" volt drop mind you, so this comes off the 400V you started with to give 397.97V

This is a drop of 0.5%, so, as you are allowed 5% for power or 3% for lighting you are very well within limits.

As i say, this is the "three phase" volt drop if you like. If you are going to split it into three separate phases at the end of your cable, you must divide the volt drop above by 1.732 to get the "single phase" volt drop.

This gives 1.169V as a "single phase" drop if you like.

If you divide this drop [1.169] by 230V [the "single phase" voltage] and multiply it by 100 you will come up with the drop in % again, and of course we are back to 0.5%

You could use smaller cable than 50mm with 100A, why you using such a big one?

john.

---------- Post Auto-Merged at 16:51 ---------- Previous post was made at 16:41 ----------

Hi again, should have said, the volt drop in the tables is based on the cable being fully loaded and at its max working temperature [70c or 90c depending on what cable you use]

As your cable will not be at "full capacity" and be somewhat cooler than this, you will have even less volt drop, as the resistance increases with temperature, so you will be even better off again!!

Make sure the chart on page 281 you have been looking at is the right one for your cable.

Ah, yes, i see why you have chosen this size cable and not the next size down, it is in a duct!

Teach me to read things properly the first time!!

john..

 

[batty]

Sure its not 250mtr run for 50mm swa?

 

[ian932]

As most SWA these days is XLPE 90degC you could go down to a 35mm, VD would be 2.875V.

You would need check the rating of equipment connected to this cable and that they are rated for 90degC operation.

Also this increase in VD may affect the overall design of your final circuits. Not sure if this cable is direct to the origin or sub-mained so only you will know if you can save a few quid on dropping the size down at

 

[Curley]

Hi,

I use the solutions electrical software, makes design easy as cake, or piece of pie. its pretty cheap too, saved me weeks of messing with the BRB.

 

[minkster100]

Many thanks to all who have replied. Not sure if the cable is PVC or XLPE, i will have to check, but as stated in one of the replies, if it is XLPE we could drop to 35mm, if the equipment will tolerate the temp increase. The cable will be running from the origin to the main DB. Once again thanks for the help.

 

[apprentice87]

Hi, yes, as Ian932 has pointed out, if you use 90c XPLE cable you will be able to drop a cable size.

As Ian quite rightly pointed out you will have to check whether the equipment you are connecting up to is rated for 90c. Not entirely sure, but i do not think much actually is.... Other forum members will be able to help you with this though!!

I would go with what Ian says and stay with the 50mm myself. Who knows, in future you might expand operations and wish you had chosen the larger cable..... besides, r1 + r2 will be less and that always helps!

john

 

[Sidewinder]

There are other economic justifications for choosing larger cable sizes when (IF) they are heavily loaded over their design life.

IF that were the case it may even pay to go above 50mm.

This would depend heavily on the load magnitude/duty cycle.

 

[apprentice87]

Hi Sidewinder, can you explain a bit more as i like to learn.

Would i be right in thinking that this is because, as a larger cable will have a lower impedance in the first place, [due to being larger!] this allied to the fact that for the same current it will run cooler [and therefore have an even lower impedance again, as resistance goes up with temperature] that I2R losses in the cable will be less and therefore you are not paying to heat the ground up????

john..

 

[Chr!s]

Hi Sidewinder, can you explain a bit more as i like to learn.Would i be right in thinking that this is because, as a larger cable will have a lower impedance in the first place, [due to being larger!] this allied to the fact that for the same current it will run cooler [and therefore have an even lower impedance again, as resistance goes up with temperature] that I2R losses in the cable will be less and therefore you are not paying to heat the ground up????

john..

thats about it john, simples really

 

[apprentice87]

Hi Chris, thanks for that!!

john..