verification of voltage drop on innitial verification

[Simonslimline]

When ticking the box for verification of volt drop on an innitial verification whats the procedure for this ?

Would you not need loads to be running to get an accurate reading?

Thanks simon

 

[telectrix]

or you could calculate it using design current/s.

 

[Silly Sausage]

At the risk of public humiliation...
An R1RN measurement times the Protective Device rating would give an indication.

 

[Rockingit]

You've already got your R1/N/2 values and you know what size the conductors are, so you can work out the length......

 

[Simonslimline]

Yeah i know the volt drop formula using milivolts per Amp per metre /1000 .

I asked this question at college and was told to just test the voltage at the end of final circuits etc. But surely if there is no load then there is no significant volt drop.

Sorry if this sounds stupid. I may have misunderstood something somewhere .

 

[telectrix]

yep. if you are doing it by measurement, the circuit needs to be loaded up to the design current.

 

[Simonslimline]

Thanks that is what I thought.

 

[SirKit Breaker]

Yeah i know the volt drop formula using milivolts per Amp per metre /1000 .

I asked this question at college and was told to just test the voltage at the end of final circuits etc. But surely if there is no load then there is no significant volt drop.

Sorry if this sounds stupid. I may have misunderstood something somewhere .

You have been given duff info mate. The only realistic way you can accurately measure volts drop is by taking a measurement at the origin of the circuit, at the load on the circuit, and with the instantaneous load current present, and you need to do all this at the same time. So measurement is not practical.

At Initial verification, is is sufficient to confirm that the voltage drop calculations have been done. If you are the designer, then it is you who does the calculations, but this will depend a lot on your current experience and knowledge, as there are an awful lot of things to consider to calculate volt drop. (mV/A/m) x Ib X L/1000 is only part of it, there will be correction factors to be applied for all sorts of reasons.

Cheers.............Howard

 

[Simonslimline]

Yeah ok thanks Howard. When we covered cable selection steps at college we would do Ib<In<Iz (Iz was known after correction factors were applied (if any)) Then confirm that the AVD complied with BS7671. Whether it be 3% for lighting or 5% for other use. (we covered this a while ago so hopefully i still remember it properly)

Cheers for the info its appreciated

 

[Silly Sausage]

What about voltage drop on distribution circuits?

 

[Simonslimline]

What about voltage drop on distribution circuits?

Please explain Archy?

 

[Silly Sausage]

Please explain Archy?

I'm asking mate!!!
I'm not so sure about this and seeing as Howard's online, hopefully he'll explain.

 

[SirKit Breaker]

What about voltage drop on distribution circuits?

No difference, VD is what it is. A dist circuit is just the same as any other radial in construction anyway. There are differences but not when it comes to VD.

 

[SirKit Breaker]

Yeah ok thanks Howard. When we covered cable selection steps at college we would do Ib<In<Iz (Iz was known after correction factors were applied (if any)) Then confirm that the AVD complied with BS7671. Whether it be 3% for lighting or 5% for other use. (we covered this a while ago so hopefully i still remember it properly)

Cheers for the info its appreciated

From your original post, it is obvious that you have got some understanding as you doubted the crap you were fed at college and fair play to you for doubting it.

 

[Simonslimline]

One more question I have is volt drop on three phase. What is the maximum voltdrop on a 400v circuit. 5% would be 20volts. On a question from sparks magazine competition this question came up and the answer was 8volts. I am just wondering where that figure comes from.