Volt drop and cable calculations

[Sav]

Hi,
does anybody have a quick or easier way to work out volt drop and cable selection calclations ??
I always refer to the BS7671 and OSG, but wondered if anyone had an easier way to work these out.
Cheers guys and speak soon
Sav

 

[electro]

Hi

The easy way is to use software, a cable calc takes about 1 to 2 mins, and you can play with the figures untill your happy with the cable size. The way i did it before i had sofware was to calculate max and mins for cable sizes, this can work well for most jobs and is a big help when quoting.

Regards

Regards

 

[Phil Mumford]

I would respectfully suggest that you undertake a course that covers this as such a course covers other areas that you would need to know.

Calculating voltage drop is simple if you know the formula and the relevant information such as design current, length of cable and voltage drop per amp per metre of the cable in question. There is no other way to work it out as length and design current are variables - very few calculations will be the same.

You can find voltage drop per amp per metre in BS 7671 or in manufacturers data sheets for specific cables, but I find BS 7671 easiest to use as it covers all scenarios on one page once you have identified the correct cable type and size.

 

[electro]

Hi Phil


A very hard line reply, suggesting he should go on course, he was asking for short cuts, rule thumb and ways picked up by experience to quicken the process of cable calculations. With experience you can determine a cable size, while on site, then I do the calcs to confirm the size, or if the size could be reduced or increased dependant on design.


Regards

 

[Phil Mumford]

Sorry if my post was too harsh but I believe all electricians are trained to work this out and that there is no quick way to work it out.

I don't carry a copy of the regs with me but would find it impossible to work out voltage drop without it.

For example, I know it wont be a problem on 6mm cable under 20 metres at about 30 amps. Wouldn't know at 40 metres and 34 amps. You therefore need to know hpw to work it out and there are other factors that make a rule of thumb difficult with cables in a non domestic scenario.

I think it better to be able to cover all bases and know how to obtain and use the information as necessary. Certainly stands you in good stead in an industry that refuses to stand still !

Apologies if I misunderstood the question.

Just found this though in testing and inspection forum:

Approx R1 + R2 values for 1.5mm T&E are 0.29 ohms / 10 meter run.

1.0mm T&E 0.38 ohms / 10 meter run.

Maybe a few calcs like this might give you a method to have a rule of thumb method - just multiply by the design current to get an idea of the voltage drop?

 

[old dog]

Useful In...odt (26.1 KB


sorry cant get the link to work.

Cable​

1m​

5m​

10m​

15m​

20m​

25m​

30m​

35m​

40m​

45m​

50m​

1.0/ 1.0​

0.036​

0.18 Ω​

0.36 Ω​

0.54 Ω​

0.72 Ω​

0.9 Ω​

1.09 Ω​

1.27 Ω​

1.45 Ω​

1.63 Ω​

1.81 Ω​

2.5/ 1.5​

0.02 Ω​

0.10 Ω​

0.20 Ω​

0.30 Ω​

0.39 Ω​

0.49 Ω​

0.59 Ω​

0.68 Ω​

0.78 Ω​

0.88 Ω​

0.98 Ω​

4.0/ 1.5​

0.017​

0.08 Ω​

0.17 Ω​

0.25 Ω​

0.33 Ω​

0.42 Ω​

0.50 Ω​

0.58 Ω​

0.67 Ω​

0.75 Ω​

0.84 Ω​

6.0/ 2.5​

0.01 Ω​

0.05 Ω​

0.1 Ω​

0.15 Ω​

0.21 Ω​

0.26 Ω​

0.31 Ω​

0.37 Ω​

0.42 Ω​

0.47 Ω​

0.52 Ω​

10/ 4.0​

0.006​

0.03 Ω​

0.06 Ω​

0.09 Ω​

0.13 Ω​

0.16 Ω​

0.19 Ω​

0.22 Ω​

0.26 Ω​

0.29 Ω​

0.32 Ω​

 

[electro]

No problem mate, I do tend to size the cables first by experience then I do the calculations to see if they are the correct size. Properly not the best way but it seems to work for me. This is all prior to installation. There is a rule of thumb about the ZS can't quite remember. I Do agree you should do your calculations, must admit you start to forget when you use software all the time.


Regards