Voltage Drop and Regs-how do you do it?

[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....

 

[telectrix]

not as long winded as it looks , though. with the aid of as table kindly posted some time ago as useful info. for form filling, it's just caclulating the length from the resistance. quicker than following a complicated cable run with a tape measure.

 

[JUD]

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.

I'm sorry, what's long winded about it?

I know the voltage drop is calculated using the 'circuit impedance'. However, this has already been calculated for you in the voltage drop tables in the BRB in the form of mV/A/m values.

Therefore the only things you need to know to be able to calculate voltage drop is the length and the design current of the circuit.

The way I explained it was intended for an existing circuit and the fact that you are doing an R1+R2 or R2 test on the circuit anyway, so you can conveniently work out the length of the circuit from the result of the test.

As you rightly say, for new installations voltage drop would be calculated at the design stage when selecting cable sizes etc.

 

[pirhead]

Thanks all this stuff is really straightening my head out have read it three times now it better everytime:smiley2:

 

[ban-all-sheds]

Page 161

Whereabouts on p161 does it say that you can work out voltage drop using Zs or diagrams?

 

[malcolmsanford]

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.

We have all been blinded by JUD's normal excellent skills here and no one as notice the obvious flaws in this calculation.................... he as not taken into account any poorly connected JB, cables twisted together under the floor boards and finally dirty connection points on the 24 yr old socket he tested the R1 + R2, obviously all these will give us a false ohmeric reading and an improper length of the circuit:biggrin:

 

[Des 56]

Whereabouts on p161 does it say that you can work out voltage drop using Zs or diagrams?

Ok I will play your silly games
Where indeed does it say that



Or more to the point and for your benefit only,because you have asked this question

The first question asked by the poster of the thread was
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??

Reply by Teletrix
where exactly in the regs are you referring to?

Reply by Des 56
Page 161

Just for your benefit,because you seem to want make issue with a very concise reply by me

Firstly read and understand the reply, and in what context that reply is given, and secondly please dont associate your interpretation of that reply to the thread subject, with an assumption of my opinion on that subject, then make issue based on that assumption

Any understanding or lack of understanding with the content and question throughut the thread was not in my reply given

Your replies to the topic may have been more appreciated if you gave the thread poster an opinion on the subject, or even information that would have assisted him, rather than take issue with other replies you mis understand

 

[ban-all-sheds]

Ok I will play your silly games
Where indeed does it say that

I'm not playing silly games.

Firstly read and understand the reply, and in what context that reply is given,

The context seemed quite clear to me -

pirhead asked telectrix "just wondering how that works with part six of the regs that talks about diagrams and Zs measurements just wondering how that works with part six of the regs that talks about diagrams and Zs measurements",

telectrix replied "where exactly in the regs are you referring to?"

And you replied "Page 161"

I took the context to be that of this topic, i.e. where pirhead originally said, in relation to calculating voltage drop, "the regs say that this can be worked out using the Zs or using diagrams"

So was not that not the context? Were you not saying that Page 161 is where the regs say that voltage drop can be worked out using Zs or diagrams?

and secondly please dont associate your interpretation of that reply to the thread subject, with an assumption of my opinion on that subject, then make issue based on that assumption

I'm not making any assumptions of your opinion of anything. You said "Page 161", I asked you where.

Your replies to the topic may have been more appreciated if you gave the thread poster an opinion on the subject, or even information that would have assisted him, rather than take issue with other replies you mis understand

I'm sorry, you'll have to excuse me - I'm new here and I hadn't realised that I wasn't allowed to ask you a question if what you said wasn't clear to me.

 

[pirhead]

Whereabouts on p161 does it say that you can work out voltage drop using Zs or diagrams?

612.14 where reqd to verify compliance with section 525 the following options may be used
1. The voltage drop may be evaluated by measuring the circuit impedance
2the voltage drop may be evaluated by using calculations for example diagrams or graphs showing cable lengthv load current for different conductor cross sectional areas with different percentage voltage drops for specific nominal voltages, conductor temperatures amd wiring systems. That is the quote word for word I think...

 

[ban-all-sheds]

Thanks for that.

There must be a reason I didn't read it properly when I looked, but I can't think what it is right now...


But in any event "circuit impedance" doesn't mean Zs, does it....



I can see how you could have a chart which showed the relationship between cable length and load current and % drop graphically. Never seen one but I guess it would have multiple lines on it like a time-current chart, for different csas or temperatures etc (ony one variable unless you want a 3D chart ;-), but the lines would be straight and the scales linear.

It would just be a way to look VD up - you wouldn't draw one whch applied to a specific installation.

 

[Sparky Ninja]

I'm sorry, what's long winded about it?

It's just not necessary when a measurement of R1&Rn will suffice and most likely would have been measured anyway.