Voltage Drop and Regs-how do you do it?

[pirhead]

Hi All wonder if anyone can help just wondering how sorting out voltage drop works in practice the regs say that this can be worked out using the Zs or using diagrams. Any idea how this is done in practice?:6:

 

[mickys86]

Have you not got an onsite guide or regs book?

 

[Amp David]

Its not done in practice, its calculated using the values given in the regs. GN3 is very usefull on all things testing, well worth buying if you're serious about doing any sort of electrical work IMO

 

[telectrix]

nothing to do with Zs or diagrams, in the osg you will find the mV/A/m values for various cable sizes. then yo take that value, say, for ease of understanding we'll call it "mA". then:

vollt drop = (mA x A x L) /1000 , where A is the design current, L is the length of the circuit.

 

[mickys86]

nothing to do with Zs or diagrams, in the osg you will find the mV/A/m values for various cable sizes. then yo take that value, say, for ease of understanding we'll call it "mA". then:

vollt drop = (mA x A x L) /1000 , where A is the design current, L is the length of the circuit.

That's exactly what I thought but was keeping quiet in case there was something I had missed at college haha 

 

[telectrix]

sorry , forgot to say L is the length in meters, not feet.

 

[pirhead]

Have you not got an onsite guide or regs book?

Yes have regs OSDG and GN£

 

[Marnik]

When testing why not use R1+Rn
formula as follows; R1+Rn * 1.2 * Lb

Where R1+Rn = overall resistance of phase and neutral conductor
The 1.2 being the correction factor for operating temperature
Lb being the load

Simples

 

[JUD]

When carrying out the continuity tests on the protective conductor of a circuit you will record either an R1+R2 or an R2.

You can use the result of this test to work out the length of the circuit using the resistance/meter values in Table 9A (page 166 OSG).

Once you have the length and design current of a circuit you can then work out the expected Voltage Drop using tables in the OSG or BS 7671.

As an example, let's say you have a 16A radial circuit wired in 2.5mm² T&E.

You do an R1+R2 test and record a value of 0.98Ω.

Now, using Table 9A in the OSG you can work out the length of the circuit.

From the table the resistance of 2.5mm² line and 1.5mm² cpc (R1+R2) is 19.51mΩ/meter.

So, if you divide the result of the R1+R2 test by this value it will give you the length of the cable.

0.98 ÷ (19.51 ÷ 1000) = 50.2 meters.

Now you have the length and design current you can work out voltage drop.

From Table 6D2 (page 130 OSG) the voltage drop per amp per meter for 2.5mm² cable is 18mV/A/m so total voltage drop would be (18 x 16 x 50.2) ÷ 1000 = 14.45V. The maximum voltage drop for power circuits is 11.5V(5%) so the circuit would not comply.

 

[pirhead]

hi teletrix just wondering how that works with part six of the regs that talks about diagrams and Zs measurements...thats what I dont get??

 

[pirhead]

Hi Marnik can u show me how this works with and example:stooge_curly:

 

[telectrix]

where exactly in the regs are you referring to?

 

[Des 56]

Page 161


Jud has already given an excellent comprehensive reply

 

[pirhead]

I like this. That is very helpful thanks mate, I will need to read if again to take it in though!!!":angel_smile:

 

[telectrix]

yes, faultless.

 

[telectrix]

When carrying out the continuity tests on the protective conductor of a circuit you will record either an R1+R2 or an R2.

You can use the result of this test to work out the length of the circuit using the resistance/meter values in Table 9A (page 166 OSG).

Once you have the length and design current of a circuit you can then work out the expected Voltage Drop using tables in the OSG or BS 7671.

As an example, let's say you have a 16A radial circuit wired in 2.5mm² T&E.

You do an R1+R2 test and record a value of 0.98Ω.

Now, using Table 9A in the OSG you can work out the length of the circuit.

From the table the resistance of 2.5mm² line and 1.5mm² cpc (R1+R2) is 19.51mΩ/meter.

So, if you divide the result of the R1+R2 test by this value it will give you the length of the cable.

0.98 ÷ (19.51 ÷ 1000) = 50.2 meters.

Now you have the length and design current you can work out voltage drop.

From Table 6D2 (page 130 OSG) the voltage drop per amp per meter for 2.5mm² cable is 18mV/A/m so total voltage drop would be (18 x 16 x 50.2) ÷ 1000 = 14.45V. The maximum voltage drop for power circuits is 11.5V(5%) so the circuit would not comply.

an excellent explanation, Jud, but surely, it's R1+Rn you need to use, not R1+ R2. so for example, you would use the figure of 14.82 as for 2.5/2.5mm.

 

[Sparky Ninja]

Verification of voltage drop on an existing installation will be calculated with the 'circuit impedance' value - R1&RN, as Marnik has mentioned (although it's Ib, not Lb).
Calculation of voltage drop on a new installation will be assessed with cable selection at the design stage using the mV/A/m formula similar to as telectrix has mentioned.

JUD's method, while a good example, is transposition on the secondary with a little reverse engineering to discover the resistivity values. So is a little long winded.

 

[JUD]

an excellent explanation, Jud, but surely, it's R1+Rn you need to use, not R1+ R2. so for example, you would use the figure of 14.82 as for 2.5/2.5mm.

R1+R2 is used to determine the length of the cable only Tel, as you're doing an R1+R2 test anyway.

So you would use 19.51 as during the R1+R2 test you've measured the resistance of the line and cpc i.e 2.5mm² (7.41mΩ/m) and 1.5mm² (12.1mΩ/m). 7.41 + 12.1 = 19.51mΩ/m

The voltage drop is worked out as normal (once you've determined the length) using voltage drop values.

 

[telectrix]

ah, see where you are on that. either way ( R1+R2 or R1+Rn ) will give you the cable length depending on which you measure and which value of resistivity you use.

 

[truckster]

Verification of voltage drop on an existing installation will be calculated with the 'circuit impedance' value - R1&RN, as Marnik has mentioned (although it's Ib, not Lb).
Calculation of voltage drop on a new installation will be assessed with cable selection at the design stage using the mV/A/m formula similar to as telectrix has mentioned.

JUD's method, while a good example, is transposition on the secondary with a little reverse engineering to discover the resistivity values. So is a little long winded.

Yeah, thats what I thought....