New Zealand Volt drop cont.

[Monkeyblaine]

Ok I have another VD question. I'm installing 9 LED floods around my friends garden which totals at just over 100m in length. Obviously for each branch the VD varies so for argument sake I worked It out as if all the cable would be taking all the load (rather than dropping relevant current after each light, worse case scenario) the load is minimal being LEDS but to be safe I'm putting 2.5mm 3c hightuf in to cope with the distance and to help for the zs reading. Now I am also using a wise box control box (wireless switching) now am I right in thinking I can supply this box from the CU with a length of 1mm t+e as this is a very short distance and is my thinking correct that as long as no voltage has been dropped through the 1mm to the wise box then I have a full 230v (give or take) for the garden circuit and as its run in 2.5mm it will stop any VD issues. Basically what I'm asking is, whatever voltage you have at a certain point regardless of cable size, then that's what you have from that point onwards. I.e if I lost 3volts through initial 1mm cable then I'm down to 227v at wise box (for example)

 

[Monkeyblaine]

On top of all this can someone please explain this to me. So you have a lighting circuit which over a long run gets down to 220v from supply to junction box. You have a cable which goes to a light and another cable which carrys on the feed. Now say the next run to the next point drops a further 2 volts. You take the 2 from the already dropped 220v correct? What I'm trying to get at is there will be a further VD from first junction box to the light I mentioned but as we T'd off at junction box both the light and the next feed take a refrence point of 220v to work from correct?? We don't include VD to light in circuit calc as it would be wrong. Because where we T'd off it is at 220v not 220 - the VD to light.... So the feed out will also start at 220 not including VD to first light. I find this all very confusing. Does anyone follow what I'm trying to get at.

 

[telectrix]

the cable to the first joint/light will have a VD depending on the total load of the complete circuit. at the 2nd, the VD is dependent on the load of the 2nd light , 3rd light and so on. at the 3rd, it's lights 3,4, etc. so the further down the circuit you go, the VD decreases.

 

[Monkeyblaine]

I understand that. Haha I'm so confusing. What I'm finding hard to work out is. The voltage at the joint is 220. Then to the light it drops a little more, now thats the total VD for that light. But to the next joint we don't include the first lights VD we take it from supply, to first joint, to second joint. Missing first light. Any clearer?

 

[telectrix]

sketch it out. select a hypothetical load and cable size, then calculate the VD at each light. you will see a progressive fall in voltage at each load.

you work out the first one using the total load and the cable length from the CU. then the next point is the remaining loads and cable length from CU and so on

 

[Deleted member 72666]

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 where you can expect to have no more than 3 or 5% vd, a good example is where you have sub mains.

 

[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.