3phase volt drop.

Simonslimline · Jun 3, 2014

This is one of a weekly question by sparks magazine posted on Facebook to win free tools each week. Everyone has answered B. 8Volts. I am not sure this is correct. Please could anyone help me with this. I have posted it up in here rather than just the trainee section so more people will see it. Not bothered about the prize. I just have a feeling that 8volts is wrong, but would happily be corrected on this.

Thanks.

To comply with IEE Wiring Regulations the voltage drop on a three phase 400v Circuit should not exceed:
a) 4v
b) 8v
c) 20v
d) 32v

snowhead · Jun 4, 2014

D Skelton said:
It would be. Do the math

"No need for workings, (or Math,) people are looking way too much into this."

If the Max neutral current was ever going to higher than the Max current in a single phase then the Neutral cable in a 3 ph installation would have to be larger than any of the phase cables.

Try and find a 4 core SWA with one core larger.

Or another way;

If the Neutral current is 5 amps with only 1 phase carrying current and Zero with all 3 phases balanced carrying 5amps each, then with only 2 phases carrying 5amps each the neutral current must be between Zero and 5amp

D Skelton · Jun 4, 2014

snowhead said:
"No need for workings, (or Math,) people are looking way too much into this."

If the Max neutral current was ever going to higher than the Max current in a single phase then the Neutral cable in a 3 ph installation would have to be larger than any of the phase cables.

Try and find a 4 core SWA with one core larger.

Or another way;

If the Neutral current is 5 amps with only 1 phase carrying current and Zero with all 3 phases balanced carrying 5amps each, then with only 2 phases carrying 5amps each the neutral current must be between Zero and 5amp

Read my post at #72 again mate. I never said the N current would be higher than the P current. If two phases are carrying 5A then the N current would be 5A. It's the square root of 50-25.

Chr!s · Jun 4, 2014

D Skelton said:
No need for workings, people are looking way too much into this.

Imagine a TP+N board. On this board is one single balanced three phase load pulling 10A per phase, what is the current in the supply neutral? 0A right? Add a 5A single phase load onto that board, what is the current in the supply neutral then? 5A. This would be both the total, and the maximum amount of current in the neutral.

Add another 5A single phase load to a different phase as the first 5A load, the total neutral current is now 10A, but the maximum amount of current in the neutral at any one time is still 5A.

not to sure of the point being made, but i agree with the neutral current being 5 Amps in the two instances mentioned.

Going further though, in reality we need to consider P.f and also harmonics. The important point being the Harmonic current adds to the unbalanced neutral current.

chris

happysteve · Jun 4, 2014

D Skelton is correct. If you draw a phasor diagram of the two 5A currents, your resulting phasor (representing your neutral current) becomes the third side of an equilateral triangle.

[ElectriciansForums.net] 3phase volt drop.

And the thing about equilateral triangles, is they have sides of equal lengths (despite my efforts to disprove this in the sketch above).

Huh. Every day...

Simonslimline · Jun 5, 2014

So sparks magazine announced the answer today as being 20volts maximum volt drop on a 400 circuit.

They spotted that i answered correctly and everyone else seemed to think it was 8volts.

I won the competition and got some irwin sidecutters lol

Silly Sausage · Jun 5, 2014

Well done!

I knew my theory was total nonsense, I'm just surprised nobody ripped it to pieces!

Just looked at the website, might have to join if there's free tools up for grabs.

Simonslimline · Jun 5, 2014

Archy Styrigg said:
Well done!
I knew my theory was total nonsense, I'm just surprised nobody ripped it to pieces!

Just looked at the website, might have to join if there's free tools up for grabs.

Free tools every week. I imagine you will be up to the questions aswell Archy:50: