Voltage Drop Calculation vs OSG

[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.