Calculating Voltage Drop on a Ring main

[fire8789]

Hi Guys, I'm fairly new to the industry and have sat and passed all the relevant exams to become a Domestic installer. I'm a little stuck with one of my pieces to be assessed by NICEIC. I'm doing a full rewire at my dads house and would like to check that I'm calculating the cable volt drop correctly.

So for radials it seems to be fairly simple.
(mV/A/m) x Ib x L / 1000

So for a light circuit using 1.5mm 2 core I got
29 mV/A/m x 6 amps x 30meters = 5220 / 1000 = 5.22 volts, so would be ok as it's within 3% (6.9V) voltage drop.

The bit I'm not sure on is how you calculate this for a ring main?

I had a guess here so please let me know if this is correct or would be a suitable method to use?


18 mV/A/m x 32amps x 50meters.

So what I did was to half the Ib ampage to 16A as it's on a ring so the current can be divided between the incoming and outgoing cables, then half the length also for the same reason.
So my formula came to
18mV/A/m x 16amps x 25m = 7200 / 1000 = 7.2v which would be acceptable.

Had I not done this the voltage drop would have been way too high (18mV/A/m x 32 x 50 = 28800 / 1000 = 28.8v).

Help would be much appreciated. cheers. Rich

 

[chrish_1977]

I'm pretty sure volt drop is on radials eg for a supply to a shed. With rings I think (from memory) that's its done on square meterage, which I think is 85 door a ring. Please do correct me anybody though, stuck at a kids party, boring

 

[jamesBoater]

OSG Table H2.1

 

[jay sean]

100m2 floor area for a ring final cct

 

[jay sean]

+1 for a 5year apprenticeship with a old school electrician

 

[Geoffsd]

18 mV/A/m x 32amps x 50meters.

So what I did was to half the Ib ampage to 16A as it's on a ring so the current can be divided between the incoming and outgoing cables, then half the length also for the same reason.
So my formula came to
18mV/A/m x 16amps x 25m = 7200 / 1000 = 7.2v which would be acceptable.

Had I not done this the voltage drop would have been way too high (18mV/A/m x 32 x 50 = 28800 / 1000 = 28.8v).

You have arrived at the correct answer so your thinking is good.

However, you should have just divided the total length by 4 (same as for the resistance).

Also, on radial circuits you can use the actual load instead of the mcb rating.
For lighting circuits the v.d. will decrease along the length.

 

[fire8789]

Thanks for the replies guys. Table H2.1 seems to be what I had overlooked!
Also cheers for the tip Geoffsd. Might be able to squeeze an extra few meters out of my radial lighting circuit using the design current rather than mcb rating then.

 

[Clem Fandango]

Hi Guys, I'm fairly new to the industry and have sat and passed all the relevant exams to become a Domestic installer. I'm a little stuck with one of my pieces to be assessed by NICEIC. I'm doing a full rewire at my dads house and would like to check that I'm calculating the cable volt drop correctly.

So for radials it seems to be fairly simple.
(mV/A/m) x Ib x L / 1000

So for a light circuit using 1.5mm 2 core I got
29 mV/A/m x 6 amps x 30meters = 5220 / 1000 = 5.22 volts, so would be ok as it's within 3% (6.9V) voltage drop.

The bit I'm not sure on is how you calculate this for a ring main?

I had a guess here so please let me know if this is correct or would be a suitable method to use?


18 mV/A/m x 32amps x 50meters.

So what I did was to half the Ib ampage to 16A as it's on a ring so the current can be divided between the incoming and outgoing cables, then half the length also for the same reason.
So my formula came to
18mV/A/m x 16amps x 25m = 7200 / 1000 = 7.2v which would be acceptable.

Had I not done this the voltage drop would have been way too high (18mV/A/m x 32 x 50 = 28800 / 1000 = 28.8v).

Help would be much appreciated. cheers. Rich

Divide the mv/a/m by 4

 

[Clem Fandango]

Divide the mv/a/m by 4

(Mv/a/m \4) X IB X L
—————————- = vd in a ring
1000

you 1/4 the mv/a/m figure because it’s a ring.

Ib can be the mcb/rcd size eg 32a
Or 26amps because that’s a rings design current in the on site guide.
What I’ve used below.

(18/4= 4.5) x Ib x L
—————————
1000

4.5mv/a/m x 26amps x 50 meters
_______________________________ = 5.85v
1000

 

[DPG]

(Mv/a/m \4) X IB X L
—————————- = vd in a ring
1000

you 1/4 the mv/a/m figure because it’s a ring.

Ib can be the mcb/rcd size eg 32a
Or 26amps because that’s a rings design current in the on site guide.
What I’ve used below.

(18/4= 4.5) x Ib x L
—————————
1000

4.5mv/a/m x 26amps x 50 meters
_______________________________ = 5.85v
1000

Just to note, he's not been on for 10 years.