Voltage Drop Calculation vs OSG

[Watkins88]

Hi All,

Hoping to get opinions on Vd calculations for a 1.5mm2 downstairs lighting circuit with an estimated length of 128m!
Length is due to loops at switches (neutrals required) and switched lives feeding various strings of lights.

Parameters:
Mv/A/m - 29
ib - 2.38
L - 128

Method 1:
Using the standard forumla Mv/A/m x ib x L / 1000 we get 8.82v (3.84%).
This seems unrealistic as it assumes the full load is at the end of the entire circuit length.

Method 2:
An adaptation of the standard forumla is to halve the length provided the load is evenly distributed. Example.
Using Mv/A/m x ib x (L/2) / 1000 we get 4.41v (1.92%).
Given this is a domestic circuit and not a car park, I'm not convinced the load can be assumed to have an even distribution.

Method 3:
Calculate Vd between each point and derive the sum. Example.
Loads have been totalled with loops and corresponding switched lives treated as one length between each point.
The current is adjusted to account for the voltage drop at each point. The result is a small increase from 2.35A to 2.38A.
From the table below, Vd comes out at 4.35V (1.89%).

OSG Table 7.1(i) suggests a max length of 106m for a 1.5mm2 lighting circuit with a 6A Type B CB on a TN-S supply with 30ma RCD.
The question, Is 128m acceptable provided both EFLI & Voltage drop are within max Zs/3%?

For completeness the max Zs is 5.83Ω and calculated Zs at 2.67Ω. See table below:

The circuit could be split but with only 2x 6A MCBs for upstairs/downstairs lights, no spare ways and a new CU out of question, this would inevitably result in two radials off a single MCB. Another option could be to run the loops with 2.5mm2 and the switched lives with 1.5mm2. This could limit choice of accessories as terminal sizes typically accept upto 1.5mm2.

Comments and suggestions appreciated!

 

[DPG]

Hi All,

Hoping to get opinions on Vd calculations for a 1.5mm2 downstairs lighting circuit with an estimated length of 128m!
Length is due to loops at switches (neutrals required) and switched lives feeding various strings of lights.

View attachment 118983

View attachment 118991
Parameters:
Mv/A/m - 29
ib - 2.38
L - 128

Method 1:
Using the standard forumla Mv/A/m x ib x L / 1000 we get 8.82v (3.84%).
This seems unrealistic as it assumes the full load is at the end of the entire circuit length.

Method 2:
An adaptation of the standard forumla is to halve the length provided the load is evenly distributed. Example.
Using Mv/A/m x ib x (L/2) / 1000 we get 4.41v (1.92%).
Given this is a domestic circuit and not a car park, I'm not convinced the load can be assumed to have an even distribution.

Method 3:
Calculate Vd between each point and derive the sum. Example.
Loads have been totalled with loops and corresponding switched lives treated as one length between each point.
The current is adjusted to account for the voltage drop at each point. The result is a small increase from 2.35A to 2.38A.
From the table below, Vd comes out at 4.35V (1.89%).

View attachment 118984

View attachment 118997

OSG Table 7.1(i) suggests a max length of 106m for a 1.5mm2 lighting circuit with a 6A Type B CB on a TN-S supply with 30ma RCD.
The question, Is 128m acceptable provided both EFLI & Voltage drop are within max Zs/3%?

For completeness the max Zs is 5.83Ω and calculated Zs at 2.67Ω. See table below:
View attachment 118998

The circuit could be split but with only 2x 6A MCBs for upstairs/downstairs lights, no spare ways and a new CU out of question, this would inevitably result in two radials off a single MCB. Another option could be to run the loops with 2.5mm2 and the switched lives with 1.5mm2. This could limit choice of accessories as terminal sizes typically accept upto 1.5mm2.

Comments and suggestions appreciated!

Convert it to a ring circuit maybe. Assuming you do actually have excessive voltage drop.

 

[grumpyjohn01]

Hi All,

Hoping to get opinions on Vd calculations for a 1.5mm2 downstairs lighting circuit with an estimated length of 128m!
Length is due to loops at switches (neutrals required) and switched lives feeding various strings of lights.

View attachment 118983

View attachment 118991
Parameters:
Mv/A/m - 29
ib - 2.38
L - 128

Method 1:
Using the standard forumla Mv/A/m x ib x L / 1000 we get 8.82v (3.84%).
This seems unrealistic as it assumes the full load is at the end of the entire circuit length.

Method 2:
An adaptation of the standard forumla is to halve the length provided the load is evenly distributed. Example.
Using Mv/A/m x ib x (L/2) / 1000 we get 4.41v (1.92%).
Given this is a domestic circuit and not a car park, I'm not convinced the load can be assumed to have an even distribution.

Method 3:
Calculate Vd between each point and derive the sum. Example.
Loads have been totalled with loops and corresponding switched lives treated as one length between each point.
The current is adjusted to account for the voltage drop at each point. The result is a small increase from 2.35A to 2.38A.
From the table below, Vd comes out at 4.35V (1.89%).

View attachment 118984

View attachment 118997

OSG Table 7.1(i) suggests a max length of 106m for a 1.5mm2 lighting circuit with a 6A Type B CB on a TN-S supply with 30ma RCD.
The question, Is 128m acceptable provided both EFLI & Voltage drop are within max Zs/3%?

For completeness the max Zs is 5.83Ω and calculated Zs at 2.67Ω. See table below:
View attachment 118998

The circuit could be split but with only 2x 6A MCBs for upstairs/downstairs lights, no spare ways and a new CU out of question, this would inevitably result in two radials off a single MCB. Another option could be to run the loops with 2.5mm2 and the switched lives with 1.5mm2. This could limit choice of accessories as terminal sizes typically accept upto 1.5mm2.

Comments and suggestions appreciated!

No problem with 2 radials from a single MCB

 

[Watkins88]

The concern is the estimated circuit length of 128m is above the maximum of 106m suggested by OSG Table 7.1(i).
I am after confirmation on whether the OSG length can be deviated provided that the vd and Zs is compliant?
If the OSG values should be taken as gospel then your suggestion of a ring may not help in this respect as this could further increase the total circuit length.

 

[grumpyjohn01]

The concern is the estimated circuit length of 128m is above the maximum of 106m suggested by OSG Table 7.1(i).
I am after confirmation on weather the OSG length can be deviated provided that the vd and Zs is compliant?
If the OSG values should be taken as gospel then your suggestion of a ring may not help in this respect as this could further increase the total circuit length.

Pretty sure the OSG guidance is rule of thumb to save you doing calculations, if your circuit design complies with all the requirements, it complies, end of. If it were me I would be thinking about ease of testing, ability to easily break down a circuit in the event of a fault and practicality in terms of actually running cables from point to point. (you can end up 'wasting' metreage if the route from switch A to B is more tortuous than simply running both A & B back to CU or even an accessible JB.) In some circumstances I have been known to run single feeds to individual switches and then group them at a JB sending a single cable per circuit to CU. When using the fed switch method, overcrowding of cores behind the switch can be an issue what with 3c's for 2W etc

 

[WaytoGo]

Pretty sure the OSG guidance is rule of thumb to save you doing calculations, if your circuit design complies with all the requirements, it complies, end of. If it were me I would be thinking about ease of testing, ability to easily break down a circuit in the event of a fault and practicality in terms of actually running cables from point to point. (you can end up 'wasting' metreage if the route from switch A to B is more tortuous than simply running both A & B back to CU or even an accessible JB.) In some circumstances I have been known to run single feeds to individual switches and then group them at a JB sending a single cable per circuit to CU. When using the fed switch method, overcrowding of cores behind the switch can be an issue what with 3c's for 2W etc

Its with testing/fault finding in mind that this is the preferred method with all conductors being easily accessible at the switch. There is an element of future proofing also with some lighting controls requiring neutrals. The smart wifi switches in my house require them to function. The disadvantage as you pointed is overcrowding and cable length. Deep back boxes a must.

 

[WaytoGo]

Convert it to a ring circuit maybe. Assuming you do actually have excessive voltage drop.

DPG, just to clarify when you say "assuming" do you disagree with the voltage drop calculation demonstrated above? In other words what do you believe it should be given the parameters above? just curious as i've seen this topic approached in many way on this forum and would like to understand from someone with experince what information to weed out, no offense to OP. thanks

 

[grumpyjohn01]

Its with testing/fault finding in mind that this is the preferred method with all conductors being easily accessible at the switch. There is an element of future proofing also with some lighting controls requiring neutrals. The smart wifi switches in my house require them to function. The disadvantage as you pointed is overcrowding and cable length. Deep back boxes a must.

of course you are right, I'm pointing out a way to overcome those disadvantages and still have all the benefits of the method, with multiple radial 'legs' meeting at a point (CU or JB) individual legs can be isolated for testing / fault finding.

 

[perwoo47]

Hi All,

Hoping to get opinions on Vd calculations for a 1.5mm2 downstairs lighting circuit with an estimated length of 128m!
Length is due to loops at switches (neutrals required) and switched lives feeding various strings of lights.

View attachment 118983

View attachment 118991
Parameters:
Mv/A/m - 29
ib - 2.38
L - 128

Method 1:
Using the standard forumla Mv/A/m x ib x L / 1000 we get 8.82v (3.84%).
This seems unrealistic as it assumes the full load is at the end of the entire circuit length.

Method 2:
An adaptation of the standard forumla is to halve the length provided the load is evenly distributed. Example.
Using Mv/A/m x ib x (L/2) / 1000 we get 4.41v (1.92%).
Given this is a domestic circuit and not a car park, I'm not convinced the load can be assumed to have an even distribution.

Method 3:
Calculate Vd between each point and derive the sum. Example.
Loads have been totalled with loops and corresponding switched lives treated as one length between each point.
The current is adjusted to account for the voltage drop at each point. The result is a small increase from 2.35A to 2.38A.
From the table below, Vd comes out at 4.35V (1.89%).

View attachment 118984

View attachment 118997

OSG Table 7.1(i) suggests a max length of 106m for a 1.5mm2 lighting circuit with a 6A Type B CB on a TN-S supply with 30ma RCD.
The question, Is 128m acceptable provided both EFLI & Voltage drop are within max Zs/3%?

For completeness the max Zs is 5.83Ω and calculated Zs at 2.67Ω. See table below:
View attachment 118998

The circuit could be split but with only 2x 6A MCBs for upstairs/downstairs lights, no spare ways and a new CU out of question, this would inevitably result in two radials off a single MCB. Another option could be to run the loops with 2.5mm2 and the switched lives with 1.5mm2. This could limit choice of accessories as terminal sizes typically accept upto 1.5mm2.

Comments and suggestions appreciated!

Why don't u included cable between switch to lighting?

 

[cireland]

Isn't voltage drop only calculated to the furthest point?

 

[Des 56]

R2 tested at each light to confirm earth continuity
VD at furthest point or points where load is max

 

[Watkins88]

Why don't u included cable between switch to lighting?

Look at the tables above. Switched lives are included with a combined length of 79m, the feeds/loops at a combined length of 49m, bringing the total circuit length estimation to 128m.

 

[Watkins88]

R2 tested at each light to confirm earth continuity
VD at furthest point or points where load is max

Zs we know for certain is from the furthest point, in this case 51.50m.
Are you suggesting Vd should be also be based on this length?
I was led to believe that Vd should account for the entire circuit length.

 

[westward10]

I haven't sat and calculated it but your Method 3 would be perfectly acceptable.
As for Zs if you have RCD protection then that can provide fault protection.

 

[brianmoooore]

VD would have to be calculated incrementally, at each point where a lamp array branches off from the feed to the last lamp, for the calculated load at that point.
Would be easier to measure VD at the furthest lamp, instead of calculating it, since that's what actually matters. Measure the voltage between cpc and live and cpc and neutral and add them together.
On a related point, unless they are all integrated LEDs, what is maximum load? In the days of incandescent it was assumed 100W per fitting (in domestic). What's the assumed load now?