Question regarding Voltage Drop Calculations

[AReynolds]

When one is calculating the voltage drop of a circuit, one must determine the length of said circuit. However, I am not sure whether the length of the circuit includes the length of both the Live conductor running out from the consumer unit to the furthest circuit AND the Neutral conductor returning to the consumer unit, or just the Live conductor? For example, if one were designing a circuit supplying a cooker, with a twin+earth cable running 15m to reach it, would the circuit length for the purpose of voltage drop be 15m, or 15*2 = 30m to take account of the Neutral conductor?

 

[Ian1981]

The mv/a/m given in the tables have already taken the live and neutral into account so we don't have to work both out

 

[AReynolds]

So if the Live and Neutrals both share the same cross sectional area, then is the length of the circuit for calculating the circuit volt drop, the length of the cable run out to the last circuit load- so referring back to my example, 15m and not 30m?
What if the Live and Neutral conductors have different cross sectional areas- for example a 2.5mm live and 1.5mm neutral? Would you have to use two different mV/A/m values for the live and neutral conductors, and would this make the length of the circuit for the circuit volt drop calculation include the length of the Neutral conductor running back to the consumer unit?

 

[Ian1981]

So if the Live and Neutrals both share the same cross sectional area, then is the length of the circuit for calculating the circuit volt drop, the length of the cable run out to the last circuit load- so referring back to my example, 15m and not 30m?
What if the Live and Neutral conductors have different cross sectional areas- for example a 2.5mm live and 1.5mm neutral? Would you have to use two different mV/A/m values for the live and neutral conductors, and would this make the length of the circuit for the circuit volt drop calculation include the length of the Neutral conductor running back to the consumer unit?

The live and neutral have to be the same CSA in a single phase system.
In 3 phase the neutral can be a reduced CSA above 16mm copper and 25 mm aluminium if there is no danger of overloading the neutral

 

[AReynolds]

You are absolutely right- looks like I am having a stupid moment! Anyway, my question still stands- when determining the length of a cable run in a circuit to calculate the volt drop, is the circuit length taken as the length of the live conductor from the consumer unit to the last circuit load, or the length of the live out to the last load AND the neutral conductor returning to the consumer unit? For example, if a twin+earth cable is run out from a consumer unit to a load for 15m, is the circuit length taken as 15m, or 15*2=30m to account for the returning neutral conductor?

 

[westward10]

You tell him Ian

 

[Ian1981]

You are absolutely right- looks like I am having a stupid moment! Anyway, my question still stands- when determining the length of a cable run in a circuit to calculate the volt drop, is the circuit length taken as the length of the live conductor from the consumer unit to the last circuit load, or the length of the live out to the last load AND the neutral conductor returning to the consumer unit? For example, if a twin+earth cable is run out from a consumer unit to a load for 15m, is the circuit length taken as 15m, or 15*2=30m to account for the returning neutral conductor?

18 mv/a/m is the figure given for 2.5mm twin and earth cable.
So if your circuit length is 15m then 18mv/a/m is what you calculate it with
15meters x 18mv/a/m x design current/ 1000

 

[AReynolds]

Ok, thanks for answering my question.
To use my previous example again, if I used a single core live cable, and a single core neutral cable, and for some reason, the length of the neutral conductor was longer or shorter than the live conductor, then would I have to modify the mV/A/m value to reflect the different conductor lengths, and would the length of the circuit for volt drop calculation purposes now be both the live and neutral lengths?

 

[westward10]

The neutral will generally be shorter using singles especially on lighting circuits where the lines go to and from switches whereas the neutral generally only loops between the lights.

 

[Ian1981]

I'll pass the baton to westward as it's the weekend and I don't want to have to think anymore

 

[AReynolds]

That is true. So, would this change the way that you determine the mV/A/m of the circuit cabling, to take the shorter neutral into account- or can the circuit length for the volt drop calculation still be taken as the length of the live conductor out to the furthest circuit load?

 

[westward10]

No you are over thinking this. I view the length of circuit as being the most extreme point of the earth fault loop path which is line and cpc. If you look at the volt drop tables in BS7671 it gives you reference methods and quantity of conductors for single and three phase circuits just use these figures.

 

[Richard Burns]

In a standard calculation the csa of line and neutral will be the same and in most cases the line and neutral will be the same length, therefore the circuit length is the distance the cable runs from the origin to the furthest load.

If you have different lengths of line and neutral conductors then you can use the tables based on the longest length of cable (say the line conductor for loop in lighting) and if this is compliant then the shorter length of the neutral will also be compliant.
If the circuit is so closely designed that the difference in length makes a difference to the compliance then the circuit is probably badly designed.
If you really wanted to calculate with the different lengths then using half the value in the tables applied to the total length of the line conductor and the neutral conductor would also give you the result.
However if you were doing this then it would be also necessary to apply calculations for the different loads on different parts of the circuit otherwise you are using the worst case scenario and the slight change in cable length will be immaterial.

 

[telectrix]

i'm not going to mention multiple loads along the length of the circuit ( oh bugger, i just did ).

 

[AReynolds]

Many thanks for your answer, it has certainly answered my questions!