Voltage drop again

[JohnW2]

As the sad old get I am I made up an Excel work sheet to experiment with different cable sizes etc using the formula given. It became apparent that using a B32 MCB the cable lengths allowed for ELI were nearly the same as allowed with a 5% volt drop. So if one works as if there is no RCD in circuit and limits the lengths of cable to ensure the ELI will be within limits then in the main the volt drop will also be within limits.

Perhaps I'm as sad and old, because I've also been through a similar mathematical exercise to that in my time, with similar conclusions.

However, that approach is obvioulsy of no use when, like me, one has a TT supply. Because of the way the figures for ELI for RCD operation is derived, any voltage drop less than about 180V (230v-50v) will probably still meet the requirements for RCD operation! ... so, with a TT installation, one actually has to think explicitly about VD limits.

Kind Regards, John.

 

[sparkticus]

If we used a sub-main for both shower and lighting (total 44.4A), the VD, at 7.2V, of 10mm² cable would not be acceptable for the lighting - so we'd have to move up a further notch to a 16mm² sub-main.

Hence, with seperate circuits for shower and lighting, we could use 6mm² and 1mm² cables. If we wanted to run a sub-main to a local CU, we would have to use 16mm² - more than twice the amount of copper (and probably cost) ... which all goes to remind us that one size does not fit all, and that each individual case has to be designed on its own merits.

Exactly, or one size fits all only where it touches which is why I detest the use of terms such as "shower cable" - "cooker cable" - even "lighting cable"

10mm CSA twin & earth is only a "shower cable" after the calculations are done and the cable is installed as far as I am concerned but many, including the sheds tend to "christen" it while it is in it's embryonic stage on the reel misleading many DIYers (and some professionals) into believing that domestic electric design is an off-the-shelf function.

The only comment I would make regarding your long single final circuits VS a sub-main is (and I know I have mentioned it already) that a good comprehensive design will consider all other factors including what other (if any) local circuits need to be installed in the vicinity plus what are the "inconvenience" and/or safety implications of not having a reasonably local way of isolating circuits/dealing with trips etc, etc.

 

[sparkticus]

I'm not too sure why you are worried, unless you're just talking about 'wasted power' again. We are obviously talking about cables which are being used within their specified current-carrying-capacity ratings, and those ratings are based on a knowledge of the mW/metre that will be dissipated in them at the specified currents

Yes, they are not going to catch fire but equally they are not "wattles" suggesting there is an element of energy saving to be considered in fixed wiring design.

 

[sparkticus]

As the sad old get I am I made up an Excel work sheet to experiment with different cable sizes etc using the formula given.

I don't think that is sad at all. I also do it for complex designs and although it can take some time it is well worth the effort ultimately saving you time as when you fine tune one thing you can immediately see the impact on other parameters. I have considered buying an off the shelf design package but they seem awfully expensive and I'm not sure how flexible they really are?

 

[bernardgreen]

and that's really the main reason for my questions. BS7671 seems to make it very clear that the VD limits relate to the total drop from the origin of the installation, including distribution circuits, not just the final circuit.

Which is sensible, even common sense, otherwise one could daisy chain CUs and sub mains and with a "compliant" final circuit still have an accumulated voltage drop that leaves the final circuit delivering far below 230 volts..

 

[bernardgreen]

Yes, they are not going to catch fire but equally they are not "wattles" suggesting there is an element of energy saving to be considered in fixed wiring design.

Ir is not efficient to waste 5 % of the energy. If it is indirectly heatiing the house on cold day then not a waste but on hot day when heating is not needed it is a waste.

The idea of "in wall" heating comes to mind. A newly painted wall where there is an obvious line of paint drying quicker than the rest of the wall could be a sign of a cable in the plaster working on or above it's maximum rating.

 

[sparkticus]

Yes, they are not going to catch fire but equally they are not "wattles" suggesting there is an element of energy saving to be considered in fixed wiring design.

Ir is not efficient to waste 5 % of the energy. If it is indirectly heatiing the house on cold day then not a waste but on hot day when heating is not needed it is a waste.

The idea of "in wall" heating comes to mind. A newly painted wall where there is an obvious line of paint drying quicker than the rest of the wall could be a sign of a cable in the plaster working on or above it's maximum rating.

It's interesting that as electricians/electrical engineers we often want to "engineer" a solution which is just right for a given application. I find that quite achievable in many commercial and industrial applications especially with fixed equipment but ironically it appears that in the domestic world an engineered solution is not quite so readily achievable.

 

[JohnW2]

It's interesting that as electricians/electrical engineers we often want to "engineer" a solution which is just right for a given application. I find that quite achievable in many commercial and industrial applications especially with fixed equipment but ironically it appears that in the domestic world an engineered solution is not quite so readily achievable.

Indeed. As I've said many times, with fixed loads one can undertake a proper design of an installation. The bugbear is socket circuits - which require a crystal ball, some luck and a propensity for gambling when it comes to any attempt at (approximate) 'design'

Kind Regards, John.

 

[JohnW2]

Yes, they are not going to catch fire but equally they are not "wattles" suggesting there is an element of energy saving to be considered in fixed wiring design.

Ir is not efficient to waste 5 % of the energy. If it is indirectly heatiing the house on cold day then not a waste but on hot day when heating is not needed it is a waste.

Agreed, but how far should we go to save how much of that 5% potential wastage (at least, in summer)?

It's easier to think in terms of radials than RFCs, but I imagine that calculations for RFCs would produce similar results. ... Consider a 32A radial circuit fully loaded (with 32A), hypothetically all at the furthest point of the circuit - at 230V that amounts to 7360W.

4mm² cable results in a power loss at 32A of about 11.26 W/metre. That means that the 'wastage' would become 5% at a circuit length of about 32.7 metres. Bernard seems to feel that 5% loss is too much, so what about 2.5% - that would limit him to a circuit length of about 16.3 metres, and if he wanted to reduce it to 1.5%, he would be limited to about 9.8 metres.

If those figures didn't allow him to reduce the power wastage as much as he wanted with the circuit length concerned, he'd either have to get into the complexities of large CSA distribution to a point closer to the sockets or ask manufacturers to re-design accessories (and use very deep back boxes), so that he could wire his socket circuit with 6mm² or 10mm² cable.

Kind Regards, John.

 

[bernardgreen]

The bugbear is socket circuits - which require a crystal ball, some luck and a propensity for gambling when it comes to any attempt at (approximate) 'design'.

. And the crystal ball, depending on the way you look at it. gives different answers to the same questions.

With the meter in the kitcchen on one side of a very solid two foot thick stone wall and the rest of the cottage on the other side the design is heavily influenced by how many cables can be got through the one hole in the wall. ( bored through many years ago ) Is it one 16 mm submain feeding a dual RCD consumer unit in the cottage or two 10mm submains each feeding one of the two RCDs in the cottage ?. Both options seem to work but which one to choose. The kitchen has its own CU which also serves the outside lights.

 

[sparkticus]

Yes, they are not going to catch fire but equally they are not "wattles" suggesting there is an element of energy saving to be considered in fixed wiring design.

Ir is not efficient to waste 5 % of the energy. If it is indirectly heatiing the house on cold day then not a waste but on hot day when heating is not needed it is a waste.

Agreed, but how far should we go to save how much of that 5% potential wastage (at least, in summer)?

It's easier to think in terms of radials than RFCs, but I imagine that calculations for RFCs would produce similar results. ... Consider a 32A radial circuit fully loaded (with 32A), hypothetically all at the furthest point of the circuit - at 230V that amounts to 7360W.

4mm² cable results in a power loss at 32A of about 11.26 W/metre. That means that the 'wastage' would become 5% at a circuit length of about 32.7 metres. Bernard seems to feel that 5% loss is too much, so what about 2.5% - that would limit him to a circuit length of about 16.3 metres, and if he wanted to reduce it to 1.5%, he would be limited to about 9.8 metres.

If those figures didn't allow him to reduce the power wastage as much as he wanted with the circuit length concerned, he'd either have to get into the complexities of large CSA distribution to a point closer to the sockets or ask manufacturers to re-design accessories (and use very deep back boxes), so that he could wire his socket circuit with 6mm² or 10mm² cable.

My energy saving comment was mainly tongue in cheek as I'm sure you realised and I think the real solution is in reducing the load with low energy lighting and smart power devices. I see many of the wall-warts now include smart power technology which essentially switches off the primary to the PSU when there is no load. despite us living in a more or less 24/7 world these days I still see potential in the diversity aspect by way of taking the dust covers off the likes of E7 / E10 etc. I wonder with current and future trends of battery technology, inverter technology and microgeneration that we may move to "charging" certain appliances off peak - appliances that are needed during the day that is.

 

[JohnW2]

The bugbear is socket circuits - which require a crystal ball, some luck and a propensity for gambling when it comes to any attempt at (approximate) 'design'.

. And the crystal ball, depending on the way you look at it. gives different answers to the same questions.

Of course - there are nearly always many ways to skin a cat.

... Is it one 16 mm submain feeding a dual RCD consumer unit in the cottage or two 10mm submains each feeding one of the two RCDs in the cottage ?

I really wouldn't have thought there was anything much to choose between those two options, is there? Assuming the sub-mains would be very short, even given your interest in efficiency, I don't think the difference in 'power wastage' between 1 x 16mm² and 2 x 10mm² submains would be worth worrying about, would it?

.. and, I suppose I should ask (given other discussions currently going on!) ... if you decided on two submains, would you run one CPC or two for them?

Kind Regards, John.

 

[sparkticus]

.. and, I suppose I should ask (given other discussions currently going on!) ... if you decided on two submains, would you run one CPC or two for them?

Now where did the lid for the can of worms go again

 

[ericmark]

As the sad old get I am I made up an Excel work sheet to experiment with different cable sizes etc using the formula given. It became apparent that using a B32 MCB the cable lengths allowed for ELI were nearly the same as allowed with a 5% volt drop. So if one works as if there is no RCD in circuit and limits the lengths of cable to ensure the ELI will be within limits then in the main the volt drop will also be within limits.

Perhaps I'm as sad and old, because I've also been through a similar mathematical exercise to that in my time, with similar conclusions.

However, that approach is obvioulsy of no use when, like me, one has a TT supply. Because of the way the figures for ELI for RCD operation is derived, any voltage drop less than about 180V (230v-50v) will probably still meet the requirements for RCD operation! ... so, with a TT installation, one actually has to think explicitly about VD limits.

Kind Regards, John.

I have asked many times should we measure Line - Neutral impedance or PFC? One can be calculated from other so which does not matter.

Two things can be worked out from results
1) Volt drop although this is total and includes the DNO bit as well.
2) If the magnetic part of a MCB/RCBO will operate with a short circuit.

However the forms published by IET don't require this to be measured.

What worries me is if we did measure and calculated prospective volt drop how many installations would fail? And would people when made aware of the design fault it that is the case be able to claim non compliance and expect firms to correct FOC?

 

[JohnW2]

I have asked many times should we measure Line - Neutral impedance or PFC? One can be calculated from other so which does not matter.

With a TN-C-S system, it would presumably the the same as the EFLI / PFC - and even with TN-S I would imagine it would be similar. With TT, however, it is something that would have to be measured, since the L-N loop impedance is going to be far far smaller than the EFLI.

Kind Regards, John.