New Zealand Volt drop cont.

[Monkeyblaine]

I'm sorry to be a pain but surely the second measurement isn't from the cu with the remaining loads. It's from the first light. Otherwise you will be measuring the same length of cable from the CU multiple times. Correct me where I'm being stupid please.

 

[Silly Sausage]

You wouldn't calculate vd that way unless you add extra load to an existing cct, vd is calculated for the full load at it's furthest point.

If you've got multiple loads in a circuit, that gives you the worst case.
So if you were close/above to max VD% and had to up cable csa, then you can calculate the VD at each load/sections of the circuit, that will keep you within limits then.

 

[Monkeyblaine]

You wouldn't calculate vd that way unless you add extra load to an existing cct, vd is calculated for the full load at it's furthest point. The vd is calculated from the origin of supply in all cases, a good example is where you have sub mains.

But on lighting circuits your dropping loads at various points and lengths. So you can't measure full load from
CU to furthest point, as it would be incorrect???

 

[Outspoken]

But on lighting circuits your dropping loads at various points and lengths. So you can't measure full load from
CU to furthest point, as it would be incorrect???

The ONLY volt drop you MUST be concerned with is that at the origin of supply, all the rest is stuff and nonsense. It is at the origin of the supply where Volt drop will make it's presence felt and where it will have the biggest impact on the voltage on the circuit.

If you worry about it down the line then your wasting your time and will likely install a complete bodge.

 

[Monkeyblaine]

Again outspoken I truly am sorry for sounding like an idiot but I don't know what you mean. I understood VD as something that occurred in circuits current x length of cable (resistance) = VD. What do you mean when you say at origin of supply.

 

[Outspoken]

Again outspoken I truly am sorry for sounding like an idiot but I don't know what you mean. I understood VD as something that occurred in circuits current x length of cable (resistance) = VD. What do you mean when you say at origin of supply.

OK, a circuit starts in a consumer unit or distribution board, the origin of the supply for an individual circuit will be the circuit protective device (RCD/MCB/RCBO/Fuse). Technically it should be at the true origin, but the volt drop between your head and consumer unit will be negligible due to the conductor size and loading on the system, it is only an issue in commercial/industrial installs and then only when Tommy Clarkes have done the designing

 

[Engineer54]

The ONLY volt drop you MUST be concerned with is that at the origin of supply, all the rest is stuff and nonsense. It is at the origin of the supply where Volt drop will make it's presence felt and where it will have the biggest impact on the voltage on the circuit.

If you worry about it down the line then your wasting your time and will likely install a complete bodge.

OK, a circuit starts in a consumer unit or distribution board, the origin of the supply for an individual circuit will be the circuit protective device (RCD/MCB/RCBO/Fuse). Technically it should be at the true origin, but the volt drop between your head and consumer unit will be negligible due to the conductor size and loading on the system, it is only an issue in commercial/industrial installs and then only when Tommy Clarkes have done the designing

I'm not sure i'm missing something here or not, but ....
I'm not quite following what your saying here?? What has the voltage at the origin (apart from complying with said norms, eg, 240 true/230 make believe) got to do with calculating the volt drop of a branch circuit over given distances and known loads?? We have formula's for calculating VD regardless of what the voltage present at the origin is, ....that voltage will typically remain constant, it's the design of the circuit and the cable/conductors sizes selected, that will determine if a VD anywhere on that branch circuit complies or doesn't comply....

 

[Monkeyblaine]

Sorry outspoken were not communicating properly. My fault. Yeah I understand from origin to furthest point is the VD which needs to comply. What I'm talking about is getting to certain points along branch circuits and deducting a long the way due to different loads being dropped. I know it's alway from origin. Just trying to point out that at a joint in a lighting circuit or feed in and feed out and a cable to the light they will all be at the same voltage, sort of the particular branches origin of supply. Just it will have been reduced to voltage being dropped before said joint.

 

[Richard Burns]

Monkeyblaine I think your method of explaining can leave others wondering what you mean (and possibly me as well!).
However I believe that what you are saying is that if you have a branched radial circuit then the volt drop of one branch is not added to other branches.
In this you would be correct.
If you assume you have a circuit with a supply cable going to a junction box and then splitting to two cables (one going left, one going right) each of which has two luminaires, each at a different point along the cable.
The volt crop calculation would include the cable from the CU to the point of the first branch for the full load of the whole circuit; added to that would be the volt drop to the first luminaire on the left hand branch using the current load for two luminaires; added to that would be the volt drop to the last luminiare on the left hand branch using the current for the last luminaire. This would give the max volt drop for the left hand branch.

Then you would calculate in the same way for the supply cable, the cable to the first luminaire on the right hand branch and the cable to the last luminaire of the right hand branch. This would give you the maximum volt drop on the right hand branch.

The highest of the left hand and right hand calculations would be the maximum volt drop for the whole circuit.

 

[Monkeyblaine]

Ahhhh ok. So that makes sense. You don't need to be Adding all VD of branches. Just worse case scenario at furthest point. Where it's going to be a problem. That's all I was trying to establish. Because in that case it gets mega confusing. I'm sorry for being rubbish at explaining things I'm sure I'm very frustrating. Thank you everyone for being patient and helping me.

 

[kingeri]

It's one reason why branched radials are that bit more tricky to work with, and why many folk avoid installing them, even if we do have the IET's blessing to install them. Personally, I have a branched 4mm 32amp radial supplying my kitchen sockets, pretty much as Richard has described. I wouldn't have installed it this way if rewiring from scratch, but as I added a bank of sockets to one side of the kitchen and left the existing sockets in place on the other side, it was the only way to do it without making a mess. Richard has described things pretty much bang on, the 'main' supply cable will be subject to all the voltage drop from both branches but as the circuit has two 'ends' I needed to calculate it twice. Actually it wasn't much of an issue in my example, cos it was an extremely short cable run, but you get my gist!

 

[Outspoken]

I'm not sure i'm missing something here or not, but ....
I'm not quite following what your saying here?? What has the voltage at the origin (apart from complying with said norms, eg, 240 true/230 make believe) got to do with calculating the volt drop of a branch circuit over given distances and known loads?? We have formula's for calculating VD regardless of what the voltage present at the origin is, ....that voltage will typically remain constant, it's the design of the circuit and the cable/conductors sizes selected, that will determine if a VD anywhere on that branch circuit complies or doesn't comply....

You have missed it completely. Monkeyblaine is discussing calculating the volt drop on different parts of a circuit (lighting) and then applying these values to those parts of the circuit and sizing the cable accordingly, a clear nonsense because the volt drop must be calculated across the length of the circuit as the maximum VD will be on the leg from the protective device to the first accessory/equipment.

I don't get what you're not getting?

 

[Outspoken]

Duplicate post!!

 

[Outspoken]

Monkeyblaine I think your method of explaining can leave others wondering what you mean (and possibly me as well!).
However I believe that what you are saying is that if you have a branched radial circuit then the volt drop of one branch is not added to other branches.
In this you would be correct.
If you assume you have a circuit with a supply cable going to a junction box and then splitting to two cables (one going left, one going right) each of which has two luminaires, each at a different point along the cable.
The volt crop calculation would include the cable from the CU to the point of the first branch for the full load of the whole circuit; added to that would be the volt drop to the first luminaire on the left hand branch using the current load for two luminaires; added to that would be the volt drop to the last luminiare on the left hand branch using the current for the last luminaire. This would give the max volt drop for the left hand branch.

Then you would calculate in the same way for the supply cable, the cable to the first luminaire on the right hand branch and the cable to the last luminaire of the right hand branch. This would give you the maximum volt drop on the right hand branch.

The highest of the left hand and right hand calculations would be the maximum volt drop for the whole circuit.

Richard, in the example you have given you are wrong i am afraid to say.

Voltage drop can only be calculated along the entire length of a circuit for design purposes. In your example the Voltage drop on the supply to the JB is ZERO because to measure this properly you need to disconnect from the branch to avoid loading by the installed fittings, and as there would be no load then there would be no volt drop.

This section of the circuit will suffer the maximum voltage drop on the circuit, the voltage drop on the branch sections is irrelevant in this example because the circuit should be designed to take into account all the volt drop along it's length.

This is why we use the equation VD=Mv/A/m

 

[KAS1]

The above makes total sense to me

- - - Updated - - -

The total of the cct

 

[Monkeyblaine]

Haha once again totally confused. Oustpoken in your example is that the same if you take a feed in and out numerous times across the length of a circuit in different jbs. And at each JB A few lights branched out. Surely at each JB. You start with a different Voltage as volts have been lost over previous joints. So the last JB to the last light, Adding all those volt drops from origin to each JB (which will be different to each point) then that total is the
Max VD for that circuit. Surely that's right??

 

[Outspoken]

Haha once again totally confused. Oustpoken in your example is that the same if you take a feed in and out numerous times across the length of a circuit in different jbs. And at each JB A few lights branched out. Surely at each JB. You start with a different Voltage as volts have been lost over previous joints. So the last JB to the last light, Adding all those volt drops from origin to each JB (which will be different to each point) then that total is the
Max VD for that circuit. Surely that's right??

Is this a ruddy wind up or do you simply not see it...Calculating the individual legs of a circuit will give an inaccurate final figure, you need to calculate for the whole circuit...see this I have just knocked up for you..

 

[telectrix]

Is this a ruddy wind up or do you simply not see it...Calculating the individual legs of a circuit will give an inaccurate final figure, you need to calculate for the whole circuit...see this I have just knocked up for you..

View attachment 19414

what software do you use for these drawings? i've got visio and am struugling with it. it's too complex for this type of drawing work.

 

[Outspoken]

Tele, I use AutoCad for proper stuff, but this and the others you have seen me post on here are all done in Visio chap. I create them as a drawing, save as a drawing, then save it a second time as a jpg for posting here.

Patience my friend...

 

[Monkeyblaine]

I was taught that way at college. The reason I'm struggling with this is because not the entire length of the circuit is taking all the load stated. So therefore they must be dropping different voltages across certain lengths. That's why I don't get it. Clearly I've been taught wrong. Sorry to frustrate you.