New Zealand Volt drop cont.

[Monkeyblaine]

To use the above example of a feed from a CU with spurs off to individual lights. If the supply to the first light branches off at JB1, second at JB2, etc:

For light 1, total VD = VD from CU to JB1 + VD from JB1 to light 1.
For light 2, total VD = VD from CU to JB1 + VD from JB1 to JB2 + VD from JB2 to light 2.
For light 3, total VD = VD from CU to JB1 + VD from JB1 to JB2 + VD from JB2 to JB3 + VD from JB3 to light 3.
and so on.

If the CU is any significant distance from the origin of the installation, add the VD for the submain (at the expected maximum loading) to each distribution value.

The VD in each section of cable is calculated on the mV/A/m, actual current and length of that particular section.

When you've calculated the total VD at each load point, the VD for the whole circuit is just the maximum value of those figures. It's likely that you can omit calculating the VD for some of the loads by 'guessing' which will have the highest VD. ie one of the loads at the far end is likely to have the highest VD, unless one of the other spurs is particularly long or heavily loaded.

Yes. So I'm right in saying, you take worse case scenario for the circuit. Ie end of line. Or heavily loaded/long spur. Not an accimulation of every spur and supply cable. Just worse case route. And of its sarisfactory there then we can assume all other lengths/loads are fine.

 

[HandySparks]

Yes. So I'm right in saying, you take worse case scenario for the circuit. Ie end of line. Or heavily loaded/long spur. Not an accimulation of every spur and supply cable. Just worse case route. And of its sarisfactory there then we can assume all other lengths/loads are fine.

Yes, but you need to be sure that your are calculating for the 'worst case'. If there's doubt, you need to calculate the VD at more than one (or all) the loads and compare results. Remember, it may not be the far end load that has the greatest VD, depending on length and load on each spur.

Also remember that, for each section calculated, you need to take into account the whole current in that section (based on the loads supplied through that section), not just the current due to the load that you've chosen to calculate the VD for.

 

[Outspoken]

To use the above example of a feed from a CU with spurs off to individual lights. If the supply to the first light branches off at JB1, second at JB2, etc:

For light 1, total VD = VD from CU to JB1 + VD from JB1 to light 1.
For light 2, total VD = VD from CU to JB1 + VD from JB1 to JB2 + VD from JB2 to light 2.
For light 3, total VD = VD from CU to JB1 + VD from JB1 to JB2 + VD from JB2 to JB3 + VD from JB3 to light 3.
and so on.

If the CU is any significant distance from the origin of the installation, add the VD for the submain (at the expected maximum loading) to each distribution value.

The VD in each section of cable is calculated on the mV/A/m, actual current and length of that particular section.

When you've calculated the total VD at each load point, the VD for the whole circuit is just the maximum value of those figures. It's likely that you can omit calculating the VD for some of the loads by 'guessing' which will have the highest VD. ie one of the loads at the far end is likely to have the highest VD, unless one of the other spurs is particularly long or heavily loaded.

So please explain why you're making a simply task so darned complicated when it need not be, one simple calculation covers all that and you are then compliant and safe...

 

[Monkeyblaine]

Handy spark. I understand completely. Sections will contain load to its particular light/fitting and to all loads downstream of it. Outspoken I will definitely be doing it your way because of the simplicity and you know that you are completely safe. Unless I have some crazy designing where by I need to install smaller CSA cables in sections to fine tune installs.

 

[Monkeyblaine]

I think the point were trying to make is that the whole load of the entire circuit is only upon the first section. After that it changes at each point as a load is dropped. So you could possibly get away with Using a smaller cable in places, which is obviously good for costings and ease of installs. When you say the first section will have the total VD across it, it won't because it will keep falling from that point. So where it could be at 227v at first section with the total load upon it, a drop of three volts. Over the next 3 sections (for example) it could drop another 1v per section. Taking it to 224v, the Max VD for the circuit. But like I said before outspoken your way is the way I will aim to do it.

 

[Monkeyblaine]

Ok another question. When you turn on a10kW shower on in some houses and the lights dim, is that due to a poorly designed supply cable from the DNO. In other words in a home with lights and power under a standard load everything is fine so no major drop on incomer, then when shower is under full load, that extra current the supply cable is dealing with causes a volt drop on the entire installation, for every circuit. Including the shower. Because the supply (DNO
Supply) is dropping to a Lower voltage than it should be?

 

[Outspoken]

I think the point were trying to make is that the whole load of the entire circuit is only upon the first section. After that it changes at each point as a load is dropped. So you could possibly get away with Using a smaller cable in places, which is obviously good for costings and ease of installs. When you say the first section will have the total VD across it, it won't because it will keep falling from that point. So where it could be at 227v at first section with the total load upon it, a drop of three volts. Over the next 3 sections (for example) it could drop another 1v per section. Taking it to 224v, the Max VD for the circuit. But like I said before outspoken your way is the way I will aim to do it.

MB, sorry chap, whilst you have grasped one part of this thorny issue, you have totally missed the most important part...the MAXIMUM voltage drop, the biggest drop in electrical potential, occurs where the circuit is under the most pressure, and this occurs where the circuit has the heaviest load placed upon it, thus it will always be at the point nearest to the source of energy for that circuit, such as the MCB/Fuse/RCBO/RCD.

You are correct to think that different parts of the circuit will have differing voltage drops along their length because this is wholly dependent on the load on that section, but if there is a common connection/section for all of the circuit loads on that section, then you will suffer the highest significant drop at this point.

 

[Monkeyblaine]

MB, sorry chap, whilst you have grasped one part of this thorny issue, you have totally missed the most important part...the MAXIMUM voltage drop, the biggest drop in electrical potential, occurs where the circuit is under the most pressure, and this occurs where the circuit has the heaviest load placed upon it, thus it will always be at the point nearest to the source of energy for that circuit, such as the MCB/Fuse/RCBO/RCD.

You are correct to think that different parts of the circuit will have differing voltage drops along their length because this is wholly dependent on the load on that section, but if there is a common connection/section for all of the circuit loads on that section, then you will suffer the highest significant drop at this point.

Outspoken please don't thnk I'm trying to teach my grandmother how to suck eggs (excuse the expression). Like I said before you are worlds apart from me in both knowledge and experience. I was just wondering if we were all on the same page and as I had created this post and people joined along the way, I was just checking we all were up to date. And yes I do understand that. The biggest significant drop will be at that point, and whatever's left after that point continuing to further loads downstream will drop further equal to the factors of length and load. Perhaps we should put this to bed now. I think I've annoyed everyone enough (you mainly) haha.

 

[Dan]

Well this thread is bloody huge isn't it!

 

[Rockingit]

Well this thread is bloody huge isn't it!

…..also woken from the dead