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?

 

[Tom256]

In answer to your core question that relates to the deviation from the length of standard final circuit designs in the OSG set against your calculations.
The standard final circuits in the OSG assume certain parameters. For a 6A 60898 the Ib of the circuit is based on a load of 5 Amps. You have an Ib that you have used of 2.38 Amps.

Side point: With a lighting circuit with a [reasonably] evenly distributed load the design current is [generally] divided by 2 in the volt drop calculation.
Take into account that your mV/A/m figure for T & E of 29 is based on a conductor operating temperature of 70 deg C . The design current of your circuit (2.38 Amps) is significantly less than the effective current carrying capacity of the 1.5mm cable - hence resistance values will be less than shown.
In this case the 29 mv/A/m figure will give a calculated VD erring very much on the side of caution . The exact value can be calaculated using the factor Ct if required.

[If you don't want to go Ib/2 then simple radial calculation of Ib load at extremity of your circuit]

 

[Watkins88]

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?

Your right, the maths is however more complex to calculate for what you describe and so in method 3 I simplified by summing loads and treating the loops/switched lives at each point as one length to give a worse case. This still gives a better degree of accuracy then method 1 or 2. Measurements will ofcourse give you the real picture but this is a design analysis prior to installation. I take your point on the LED fixtures in that they are available with GU10 holders that could accept upto 50W and allow an occupant to unintendedly increase the load. In this case each fitting is shown at its maximum power rating shown in the wiring diagram. Blue Bulbs represent 75W LED Panels, Green 30W LED strips, Yellow 30W LED Strips and White are 5W integrated downlights.

 

[Watkins88]

In answer to your core question that relates to the deviation from the length of standard final circuit designs in the OSG set against your calculations.
The standard final circuits in the OSG assume certain parameters. For a 6A 60898 the Ib of the circuit is based on a load of 5 Amps. You have an Ib that you have used of 2.38 Amps.

Side point: With a lighting circuit with a [reasonably] evenly distributed load the design current is [generally] divided by 2 in the volt drop calculation.
Take into account that your mV/A/m figure for T & E of 29 is based on a conductor operating temperature of 70 deg C . The design current of your circuit (2.38 Amps) is significantly less than the effective current carrying capacity of the 1.5mm cable - hence resistance values will be less than shown.
In this case the 29 mv/A/m figure will give a calculated VD erring very much on the side of caution . The exact value can be calaculated using the factor Ct if required.

[If you don't want to go Ib/2 then simple radial calculation of Ib load at extremity of your circuit]

Completely agree but there must be a good reason for the chaps that write the books to base the standard values on 70c?
During fault condition you'd expect the protective device to operate within 0.4-5s so hardly worth correcting for and during normal operation much less. Would be interesting to get your view on this?

 

[Simon47]

I think you have the wrong circuit length.
As I read your first table, I think you've included the length of branches in the total - and that's backed up when I get to your volt drop calculation table. Or am I missing something ?
I.e., is the cable length from P1 to CU 7.5m from the loop length column in first table, or is it 18m as in the volt drop table ?

So, the branch length from P1 to the switches and loads is irrelevant for the calculation for P2 onwards, the branch length from P2 to the switches and loads is irrelevant for the calculation for P3 onwards, and so on.
Ah, I see you've stated that under Option 3 "Loads have been totalled with loops and corresponding switched lives treated as one length between each point." That's wrong.
To do it properly, you would treat all the loads off P1 as the total at that branch, but at 7.5m from the CU, and similarly down the line. You would need to separately work out the volt drop in each branch, so for P1, you would have 1 lamp at 2.5m and 1 lamp at 8m. I'd be adding columns to your first table, and for simplicity, just lump all the loads at the end, so P2-S1 would be 4 lamps @ 6m. I think you'll find that doing it that way gives you a lower voltage drop than you've calculated - for example, for P1, you'll have removed 10.5m of cable carrying the full load current !

 

[Watkins88]

The "Loop Length" column shows the cable length from CU to first light switch (P1) and between each light switch thereafter.
The "SL Length" column shows the switched live lengths that branch from each switch (1 or 2 gang) to the downlights.

I think you are describing the method shown in the Example below.

I did follow this through and as you say the drops appear significantly lower.
I'd dismissed it believing the values couldn't be that far off from the standard forumla used in methods 1 & 2.
Provided I haven't mucked up on the arithmetic, the max Vd is 1.65V (0.72%) which I'm struggling to believe, but happy to be schooled.

 

[Simon47]

The "Loop Length" column shows the cable length from CU to first light switch (P1) and between each light switch thereafter.
I think you are describing the method shown in the Example below.
View attachment 119067

Yes

I did follow this through and as you say the drops appear significantly lower.
I'd dismissed it believing the values couldn't be that far off from the standard forumla used in methods 1 & 2.
Provided I haven't mucked up on the arithmetic, the max Vd is 1.65V (0.72%) which I'm struggling to believe, but happy to be schooled.

View attachment 119068

I haven't checked any numbers, especially working on small phone screen, but the approach looks right.

 

[Watkins88]

Its been an interesting excercise that I think has highlighted its not neccassarily the furthest point you'd find the greatest Vd as some have suggested. In this circuit Vd appears to be at its worst 45m from the CU whereas the furthest point is 51.5m.

The question I suppose I'm asking myself at this point is why isn't it always calculated in this detail?
Is it simply time where the alternative is a playing it safe approach with a quick calculation that could result in either an oversized cable or split circuits.

 

[Rockingit]

I’d need to stick my head in a theory book as a quick refresher but it occurs to me that what’s also missing from this discussion (as, if we’re going to have an actual theory discussion on here for a rare refreshing change then let’s have one…) is that the entire load on this circuit/s is LED and therefore all manner of other aspects come into play (theoretically) as well to affect the energy consumption - inrush and reactance / pf being two obvious ones. Add to that mix some good old fashioned conductor induced voltages from a few 2-way strappers and some smatterings of capacitance and the smoke starts coming out of your ears. The standard circuit maths is otherwise straight forward if not tedious as it’s resistances in parallel for every combination of Sum(lb) radial line (plus the feeders in every combination).

(This ^^ is why we have prescribed tables to use instead, stops the ear smoke thing).

 

[littlespark]

Has it already been mentioned that in some lighting configurations, the line takes a much longer route than the neutral….?
Do you take an average, or the maximum?

Especially long corridors with a couple of 2 ways and any number of intermediates inbetween.


My 2 pennies worth is that the tabulated values have all these thoughts already built into them…

 

[Pretty Mouth]

The question I suppose I'm asking myself at this point is why isn't it always calculated in this detail?

I created a voltage drop spreadsheet for this sort of thing. I used to use it to calculate the drop when designing regular 230V lighting circuits for ordinary houses, but I don't bother anymore. After using it a few times, I concluded that with LED lighting, it's not easy to exceed the 3% allowance, and even if you did go over by a little way, the consequences are negligible.