OP
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Not just what the max VD is for a circuit. Surely it's what voltage is left at end so lights/equipment work correctly.
Discuss Volt drop cont. in the UK Electrical Forum area at ElectriciansForums.net
There's conflicting information going on here. Outspoken info understand what your saying and just because at the start of the circuit is the max VD as it has the most load upon it. Doesn't mean it's not going to drop more by the time it reaches furthest point and got operation of things working, that's what im interested in. So an accumulation of volts dropped by the time it reaches last point. Which is surely what the regs require us to obtain.
I'm not stupid and I do get what you are saying. I know how to calculate the entire load of a circuit then use VD calcs to obtain total VD. I was always taught that you had to work out each leg individually. As the way your explaining it is assuming the total load is at the end of the circuit rather than it branching off at various points. Do you not see my point. But I will go with everything you say as you clearly have worlds more knowledge/experience. And if in your examples is correct way to
Determine total VD for a branch circuit, then I will leave it as that. As its a simpler way to calculate.
Monkey, Look at this FFS..
View attachment 19415
The maximum voltage dropped it at the START of the circuit because this has the HIGHEST load placed upon it.
Monkey, Look at this FFS..
View attachment 19415
The maximum voltage dropped it at the START of the circuit because this has the HIGHEST load placed upon it.
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?
I'm gonna ask a stupid again obviously. On the lengths, when you mention lengths from MCB why aren't they they the leg length plus the 5m from cu to joint as mentioned on diagram. They're all different. And on the final calc when you have the lengths in brackets. Where does the 11.5 come from. I see the others but I can't see where that's come from. I would of thought it would of been all legs plus 5m from Ccu. I can't work out what's what here.
I'm sorry. But leg 1 is 6m from joint to last fitting. And from ccu to joint it's 5m. So that's 11m. But then it states that from mcb to last fitting its 8m??? Same with all of them. Then like I said before the final calc. (11.5+6+3+5+3)??? Am I really that dumb that I can't understand anything today?
OK I will respond, but I think it will not be worth it, in the information you give above you are jumping from measurement to calculation.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
OK I will respond, but I think it will not be worth it, in the information you give above you are jumping from measurement to calculation.
I agree that if you were to measure volt drop at the junction box with with the loads disconnected you would measure zero.
Then you go on to say that this section with suffer max volt drop, despite it reading zero in measurement.
However I agree that the calculated volt drop would be under the most onerous conditions (i.e. maximum current flow) along that length.
I do not agree that it is the maximum volt drop because if it was a 1m length of cable taking 10A, and the rest of the circuit was a 100m length of the same cable taking 0.5A (assuming the 9.5A remainder were used at the JB) the volt drop proportion would be five times greater over the long part of the circuit.
Could you let me know how you would calculate volt drop where the circuit has differing sizes of cable.
I also notice in your calculations in later posts that on summing the individual volt drops (that you state as incorrect) you miss out the volt drop on the supply cable, presumably you are assigning this as zero?
(This would not affect your "proof", the calculation would still come out with a higher volt drop by your method)
Outspoken. I am looking at drawing 2 and it says 5m to Ccu and the info I gave previously is all I have to go on and it doesn't add up.
Ok but on your picture it clearly states 5m to ccu. Where I would imagine the MCBs are and also they look switch symbols not joint symbols. So I couldn't the reference point of your lengths.
Just one more thing haha. I's the final calc correct? Shouldn't it be (2+6+3+5+3+5) rather than (11.5+6+3+5+3) as stated in your diagram?
This thread's a prime example of why you have to view all postings with a certain amount of suspicion and then make up your own mind.
Outspoken is putting forward a method of calculating voltage drop, based on the total load and total circuit length. This will always give a pessimistic value of voltage drop, and therefore will be 'safe', but will, in some cases lead to installing a larger conductor than necessary.
Richard Burns is describing the accurate analysis, calculating the actual voltage drop for each section of cable based on the current and length for each section and then adding up the voltage drops for the supply route to each load (and then take the maximum drop as the stated value for the circuit). I think most others here agree with this method, and it is the method mentioned by the OP as taught to him previously.
Here's another thread where Richard describes the correct method of calculation, although not a branched circuit, it shows the process:
http://www.electriciansforums.net/e...lectrical-regulations/46784-voltage-drop.html
my brain hurts now.:6:
Handy, to some degree I agree with your comments and those of Richard, ....................................................................................................................................................................., but that is for another discussion altogether.
This thread's a prime example of why you have to view all postings with a certain amount of suspicion and then make up your own mind.
Outspoken is putting forward a method of calculating voltage drop, based on the total load and total circuit length. This will always give a pessimistic value of voltage drop, and therefore will be 'safe', but will, in some cases lead to installing a larger conductor than necessary.
Richard Burns is describing the accurate analysis, calculating the actual voltage drop for each section of cable based on the current and length for each section and then adding up the voltage drops for the supply route to each load (and then take the maximum drop as the stated value for the circuit). I think most others here agree with this method, and it is the method mentioned by the OP as taught to him previously.
Here's another thread where Richard describes the correct method of calculation, although not a branched circuit, it shows the process:
http://www.electriciansforums.net/e...lectrical-regulations/46784-voltage-drop.html
handyspark where your mention, "and then adding up the voltage drops for the supply route to each load" if you can picture a cable route from CU going down to 5 joints with lights coming off each joint, do you mean add up the VD on the cables from joint to joint (NOT including cables to lights) ....
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.
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.
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.
…..also woken from the deadWell this thread is bloody huge isn't it!
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