More on the 3 phase question.....

[HappyHippyDad]

So, the actual job is on a farm. I think I'll keep things simple at the moment and just ask about voltage drop. I'll come on to earthing and bonding a bit later or on another thread!

We have a 3 phase supply (100A p/phase). From supply to first barn (we'll call it Barn A) is 70 metres. For example purposes lets say this is 35mm (5 core) leading to a 3 phase board.

In Barn A there will be 4 x temporary offices (1 - 3 years) each with a 32A single phase supply (2 x double sockets, 1 x light, 1 x 3kW heater in each office). There will also be 2 x single phase (7kW) EVCP. Also a 16A (3 phase) commando socket (being used to supply a a charger for an electric tractor, unsure of kW)

From Barn A out to the poly tunnels it will be a 140meter run. I am assuming a max demand at the poly tunnels of 32A. I do not have exact figures yet. For example purposes lets say this is 25mm (3 core)

Lets say it's all in SWA (70°c, table 4D4A).

Working backwards we have a VD between Barn A and polytunnels of (140m x 32A x 1.75) / 1000 = 7.84v

VD between Supply and barn A (assuming no diversity) = (237A x 70m x 1.1) / 10000 = 18.25v

Clearly the voltage drop is too great (26.09V)

Some questions I have are..

1. Is the maths right?
2. Where could I apply diversity in order to avoid using excessively large cables?
3. What is an acceptable VD for 3 phase (is it 4%)?
4. The supply is approx 100m from the transformer so actual voltage will be higher than design voltage of 400V. Can we utilise actual voltage or should we always use design voltage?
5. The polytunnels are actually only 50m from the transformer, could it be a feasible idea to get DNO to fit separate supply for this? This would avoid the long run of cable.

 

[pc1966]

You seem to be putting all current on one phase "(237A x 70m x 1.1)" which is wrong, it would be split somehow between phases and (from last example) you will get a drop of 2-4% for loads of 0-100A per phase, depending on balance to barn A. If your max per phase is around 80A then drop is typically 1.6% when balanced.

Another point about the poly tunnel is where are the load(s) on it? Is the load 32A at the very end, or are there multiple heaters/lights along its length? If loads are distributed (as assumed for lighting circuits in OSG) then the drop is more or less half of the end-loaded case.

Ideally you would aim for no more than 5% drop across the installation. While 3% is the rule for dedicated lighting circuits in these LED days it hardly matters as power used and impact on brightness are small.

There can be an economic case for lowering the volt drop - basically you can work out wasted power over, say, 10 years versus copper cost to install it and see what makes sense.

 

[HappyHippyDad]

You seem to be putting all current on one phase "(237A x 70m x 1.1)" which is wrong, it would be split somehow between phases and (from last example) you will get a drop of 2-4% for loads of 0-100A per phase, depending on balance to barn A. If your max per phase is around 80A then drop is typically 1.6% when balanced.

Another point about the poly tunnel is where are the load(s) on it? Is the load 32A at the very end, or are there multiple heaters/lights along its length? If loads are distributed (as assumed for lighting circuits in OSG) then the drop is more or less half of the end-loaded case.

Oh I see.... i think. Thanks PC.

So lets say in Barn A we have the following...

L1 = 1 x office(32A) and 1 x EVCP(7kW) = 62.4A
L2 = 1 x office (32A) and 1 x EVCP(7kW) = 62.4A
L3 = 2 x offices (64A) and poly tunnels (32A) = 96A

Plus we have the 3 phase (16A) commando sockets) = 16A (assuming no diversity)

Could you show the workings for the VD for the scenario above? Once I've seen a worked example my brain can see where the maths is coming from. I realise you may have to make some assumptions, but at least I'll see the maths.

 

[pc1966]

The EV chargers will max out the rated supply for significant periods so you would have to assume 30A * 2 simultaneously so defiantly spread over phases. The commando socket for tractor charging I would expect fall in to the same full-use category.

The offices are VERY unlikely to hit 32A. Probably you will get the heater on (13A) and some IT equipment (5A?) so I would assume 20A max for load, even though you want 32A for supply breaker rating.

I'm guessing separate billing meters per office, so having a garage CU or something in them for lights / sockets / heaters after the meter?

I would try to find out what the poly tunnel load is, and more about the route (i.e. is it 140m in a single line, or are there several tunnels running in parallel each needing power, etc).

 

[plugsandsparks]

Will make some huge generalisations here but based, sadly, on experience.
1. Cable usually get put in the ground first when all information regarding loads is still emerging
2. Clients add to loads progressively as they think of new things they can do with Electricity
3. Net Zero is already impacting cables i installed less than 5 years ago as clients seek to add more and more loads (To them, they cannot see the problem, dont care about 5% and if the lights go out for a short time when a motor kicks in, will call you out to "fix" the problem, lol
4. PFC located at the load end can be a life saver, looks like your loads are not particularly inductive.
5. Most electricians are surprised how big some cables need to be when distances get over 100M. eg. simple electric gate at distances of couple of hundred metres, CB B6, cable 6sqmm

My recommendations is to sit down with client and explain the delta cost of installing larger cables for the future and ask if when the cables have been buried at a cost of £10,000 and a new load needs to be added at the far end, like a big water pump how he would deal with it ? - Generator maybe ??

ATM you have the joys of trying to predict the future, thats best left to the client, no matter what the client can come up with today, building in growth in capacity on the long runs has proved a very good investment

Yes, maths wrong - if you want some more accurate confirmations, draw it out , list all loads, TP, SP KW, PF .
Hope this helps

 

[Lucien Nunes]

As @brianmoooore and @pc1966 mentioned in the other thread, when dealing with 3-phase distribution feeding single-phase loads that can be 100% unbalanced (e.g. one 1-ph office/EVCP full load, another zero load) the worst case is always the fully unbalanced one, as we derive no benefit from reduction in neutral current due to balancing. If the loads and cable lengths for the three phases are different, they can be assessed individually and the worst case taken. You can proceed as though each were a single-phase 230V system and calculate the VD relative to 230V. As usual, your objective will be to spread the available VD along the lengths of each circuit segment (distribution circuit, final circuit) so that none has to achieve an unrealistically low fraction of the total VD.

In systems where some or all of the load is connected line-line (e.g. 400V 3-phase motors) and/or a certain amount of balance can be guaranteed, then the calculation becomes a true 3-phase one. On a larger installation the balance can be quite good, either because some of the loads are themselves balanced 3-phase loads, or because of diversity e.g. 200 separate lights spread randomly amongst the phases, where it's inconceivable that only all the lights on one phase would be in use at once. At this point the advantage of the neutral current reduction is significant and ignoring it would lead to much oversized cables. But in your case with large SP loads that are likely to be all-or-nothing, there's a clear argument for sticking to the worst-case scenario of full unbalance and calculating as though single-phase.

 

[pc1966]

I think @plugsandsparks point is really where you need to start - make a list of the expected loads and the owner's future wish-list of what they might just want to do in the future. try and get an idea of what sort of usage the various things have so you can get a realistic idea of probably power use.

As @Lucien Nunes has said you can start by doing a worst-case SP calculation of the voltage drops and see how the numbers end up. If you meet 5% at furthest load without too much cable cost then job done. If marginal then you might be willing to consider the 3P balance helping out to lower the drop to the Barn A location.

That is also partly behind my question on the poly tunnels. If they are wanting several kW of light for accelerated winter growth then no only will it be distributed along the cable length, but also you might find it cheaper to use smaller 4C cable and run lights of alternating phases to turn that in to a balanced load.

There are many reasons for polyphase supplies and why 3P became the default choice in most places but a fundamental reason was the cheaper cable due to the cancellation of neutral current and related I2R losses (the other being a rotating field for simple AC motors). True, it is most apparent in 3 wire delta loads where there is no neutral, but even with 3+N you still get something like 33% saving in copper for a given loss (3 times the power for double the conductors = 2/3 = 67% copper per Watt).

 

[HappyHippyDad]

So what would actually be the VD between Supply and Barn 1?

L3 has the highest demand, but there is still the 3 phase commando socket to deal with.

Again assuming no diversity, just for simplicity of the example, would it be as follows. I realise it wont be this as 112A is larger than the cut out fuse, I just wanted to stick with the figures I have already started with in the example.

Supply to barn = 96A (L3) + 16A (commando socket) = 112A

VD from supply to barn = (112A x 70m x 1.25) / 1000 = 9.8V

VD from Barn A to polytunnels is 7.84V, so a total of 17.64V

Ps.. I'm not deliberately ignoring all the other points raised in the posts, i just want to get my head around the maths first and then I'll start thinking about all the other bits.

 

[pc1966]

Basically yes, you do the drop for the feed to Barn A, and then add the drop to the end of the poly tunnel.

But I think the figures you are using are a bit pessimistic as I doubt the loads will be that high and solely unbalanced. This is where understanding the usage can make a big difference!

 

[pc1966]

In this case you have a long final circuit, so you are going to see significant VD on that segment. As Lucian has said you generally want to spread the VD around (sorry if that phrase has unfortunate medical connotations...) so you don't have the sub-main or the final as being close to 5% and the other unrealistically heavy cable to keep down the rest.

You have a bit of fact-finding to do, but I suspect you are going to be better with a larger sub-main (such as 50mm 4C and parallel 25mm earth) so you have a bit more leeway at the Barn A for any extensions out (maybe even future feed to another building, etc) without needing to depend on decent 3P balance.

Get the facts together first!

 

[HappyHippyDad]

In this case you have a long final circuit, so you are going to see significant VD on that segment. As Lucian has said you generally want to spread the VD around (sorry if that phrase has unfortunate medical connotations...) so you don't have the sub-main or the final as being close to 5% and the other unrealistically heavy cable to keep down the rest.

You have a bit of fact-finding to do, but I suspect you are going to be better with a larger sub-main (such as 50mm 4C and parallel 25mm earth) so you have a bit more leeway at the Barn A for any extensions out (maybe even future feed to another building, etc) without needing to depend on decent 3P balance.

Get the facts together first!

Hooray, I have a basic grasp of VD. Now.... on to harmonics :-(

I'm joking please don't go on to harmonics, baby steps required.

Could I ask why you are suggesting a separate earth? Is it purely for cost, as it would be cheaper than a 5 core? Also, would my new thread make a difference to this? The new thread is talking about TT'ing the installation and whether it is necessary. If it was TT'd then could I just run a 4 core SWA as you suggest and no earth? The armour would be earthed (PME) giving the cable fault protection, and then it would be a TT at the barn.

Ps.. The customer is a nice chap but near impossible to get any facts from. @plugsandsparks post was highly insightful and accurate with regards this client!

 

[pc1966]

Hooray, I have a basic grasp of VD. Now.... on to harmonics :-(

I'm joking please don't go on to harmonics, baby steps required.

Could I ask why you are suggesting a separate earth? Is it purely for cost, as it would be cheaper than a 5 core? Also, would my new thread make a difference to this? The new thread is talking about TT'ing the installation and whether it is necessary. If it was TT'd then could I just run a 4 core SWA as you suggest and no earth? The armour would be earthed (PME) giving the cable fault protection, and then it would be a TT at the barn.

The reason is cost, it only needs to allow fault disconnection and possibly extraneous bonding if TN-C-S (another thread...)

4C cable is easier to find than 5C and cost is dominated by the amount of copper, so you might want to save it that way.

Now having said all of the above, 4C 50mm has 90mm^2 of steel armour so it meets the 10mm Cu equivalent for bonding if you have to. Next issue is the sub-main protection as your values for 4C SWA only are:

• R1 = 0.387 mV/A/m
• R2 = 1.8 mV/A/m

So if you know the Ze (which should be measured, but for TN-C-S ought to be below 0.35 ohms) and your length then you can compute the end of sub-main Zs and see if it allows disconnection on the supply fuse-switch in under 5s. A quick check in the OSG should answer that.